METEO-BOOK-01

The Fundamentals of

Aviation
MeteorologyInternational Air Force Semester

2020-1-EL01-KA203-079068

ADRIAN PITICAR

2022

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Table of Contents

Table of Figures........................................................................................................................... 5
Table of Tables............................................................................................................................ 6
Chapter 1 - The atmosphere ..........................................................................................7
1.1. Overview of the atmosphere........................................................................................... 7
1.2. The vertical structure and composition of the atmosphere.............................................. 9
1.3. Altitude variation of key meteorological elements (temperature, humidity, pressure,

wind)............................................................................................................................. 13
1.4. End of Chapter Questions.............................................................................................. 16
1.5. Chapter bibliography..................................................................................................... 17
Chapter 2 - Global Observing System...........................................................................18
2.1. The structure of the system involved in weather observation........................................ 18
2.2. The elements and phenomena that are observed in aviation meteorology.................... 21
2.3. Overview of Numerical Weather Prediction Models ...................................................... 22
2.4. End of Chapter Questions.............................................................................................. 24
2.5. Chapter bibliography..................................................................................................... 25
Chapter 3 - Solar radiation and heat exchange.............................................................26
3.1. Solar radiation and the role of radiative processes in heat transfer and

generation/dissipation of weather phenomena............................................................. 26
3.2. Thermal properties of the atmosphere and the earth's surface ..................................... 30
3.3. Temperature and thermal processes ............................................................................. 31
3.4. End of Chapter Questions.............................................................................................. 33
3.5. Chapter bibliography..................................................................................................... 34
Chapter 4 - Water in the atmosphere ..........................................................................35
4.1. Water in the atmosphere and associated processes ...................................................... 35
4.2. The water phase system (liquid, solid, gas).................................................................... 36
4.3. Physical quantities that define the humidity of the air................................................... 38
4.4. Meteorological phenomena associated with water phase transformations ................... 39
4.5. End of Chapter Questions.............................................................................................. 41
4.6. Chapter bibliography..................................................................................................... 42
Chapter 5 - Clouds .......................................................................................................43
5.1. Clouds formation processes and their description ......................................................... 43
5.2. Vertical distribution of clouds........................................................................................ 47
5.3. Significant clouds for flights........................................................................................... 50
5.4. Stability and instability of the atmosphere .................................................................... 53
5.5. End of Chapter Questions.............................................................................................. 55
5.6. Chapter bibliography..................................................................................................... 56
Chapter 6 - Precipitation..............................................................................................57
6.1. Overview and development of precipitation.................................................................. 57
6.2. Types of precipitation.................................................................................................... 59
6.3. Relationship with cloud types........................................................................................ 60
6.4. End of Chapter Questions.............................................................................................. 61
6.5. Chapter bibliography..................................................................................................... 62
Chapter 7 - Atmospheric pressure ...............................................................................63
7.1. General references to atmospheric pressure................................................................. 63
7.2. Isobars; horizontal and vertical variation of atmospheric pressure; atmospheric pressure

systems......................................................................................................................... 65
7.3. Altimetry....................................................................................................................... 67

International Air Force Semester

7.4. The effect of pressure, air temperature and humidity on air density and how this affects
aircraft performances.................................................................................................... 68

7.5. End of Chapter Questions.............................................................................................. 69
7.6. Chapter bibliography..................................................................................................... 70
Chapter 8 - Wind .........................................................................................................71
8.1. Large-scale and local winds ........................................................................................... 71
8.2. Upper atmosphere winds.............................................................................................. 74
8.3. Turbulence.................................................................................................................... 76
8.4. End of Chapter Questions.............................................................................................. 78
8.5. Chapter bibliography..................................................................................................... 79
Chapter 9 - Visibility.....................................................................................................80
9.1. Types of visibility........................................................................................................... 80
9.2. Atmospheric phenomena that reduce visibility.............................................................. 81
9.3. Types of fog .................................................................................................................. 82
9.4. End of Chapter Questions.............................................................................................. 83
9.5. Chapter bibliography..................................................................................................... 84
Chapter 10 - Fronts and air masses................................................................................85
10.1. Description of general circulation of the atmosphere and air masses ............................ 85
10.2. Properties, origin and types of air masses ..................................................................... 87
10.3. Types of fronts and associated clouds and weather....................................................... 89
10.4. End of Chapter Questions.............................................................................................. 91
10.5. Chapter bibliography..................................................................................................... 92
Chapter 11 - Weather hazards in aviation......................................................................93
11.1. Detailing Icing, turbulence, wind shear, wind associated with mountain areas,

thunderstorms, heavy rain, hail, sandstorm, dust storm................................................ 93
11.2. End of Chapter Questions.............................................................................................. 97
11.3. Chapter bibliography..................................................................................................... 98
Chapter 12 - Meteorological Information and codes......................................................99
12.1. METAR/SPECI................................................................................................................ 99
12.2. TAF ............................................................................................................................. 104
12.3. SIGMET ....................................................................................................................... 107
12.4. Analysis of weather charts: significant weather, 500 hPa geopotential height, surface

analysis and prognosis................................................................................................. 109
12.5. End of Chapter Questions............................................................................................ 113
12.6. Chapter bibliography................................................................................................... 114
Chapter 13 - Climatology .............................................................................................115
13.1. Typical weather situations in Europe........................................................................... 115
13.2. High- and low-pressure systems.................................................................................. 118
13.3. Seasonal variations of meteorological parameters ...................................................... 119
13.4. End of Chapter Questions............................................................................................ 121
13.5. Chapter bibliography................................................................................................... 122

Table of Figures

Figure 1: Composition of the atmosphere (source: https://www.mrgscience.com/ess-
topic-61-introduction-to-the-atmosphere.html#) ............................................................9
Figure 2: The thermal vertical structure of Earth’s atmosphere, with associated
landmarks (Source: Palmer, 2017)..................................................................................10
Figure 3: Smoke trapped at the surface by stable atmospheric conditions and
temperature inversion (source: https://courses.lumenlearning.com/sanjac-
earthscience/chapter/atmospheric-layers/)...................................................................14
Figure 4: Temperature variation with altitude (source:
https://courses.lumenlearning.com/sanjac-earthscience/chapter/atmospheric-layers/)
......................................................................................................................................14
Figure 5: Variation of mean pressure and temperature with altitude (source:
http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap2.html) ....................15
Figure 6: Variation of wind speed with altitude (source: Harb et al., 2013) ....................15

Figure 7. Simplified scheme of the Global Observing System .........................................18
Figure 8: Parameters that are typically parameterized in model forecast equations
(source: Macatangay et al., 2016) ..................................................................................23
Figure 9: Overview of Earth’s radiation budget (source:
http://homework.uoregon.edu/pub/class/es202/GRL/radbal.html) ..............................27
Figure 10: Solar energy intercepted by Earth (a) and spatial distribution (b) (source:
(Palmer, 2017) ...............................................................................................................27
Figure 11: Phase diagram of water (source:
https://upload.wikimedia.org/wikipedia/commons/3/33/Phase_diagram_of_water_sim
plified.svg) .....................................................................................................................36
Figure 12: Cloud formation (source: https://scied.ucar.edu/learning-zone/clouds/how-
clouds-form) ..................................................................................................................44
Figure 13: The vertical distribution of the main types of clouds (source:
https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/
clouds.htm)....................................................................................................................49
Figure 14: Cumulonimbus cloud (source: https://cloudatlas.wmo.int/en/cumulonimbus-
cb.html) .........................................................................................................................50
Figure 15: Towering cumulus clouds ..............................................................................51
Figure 16: Distribution of clouds in Earth’s atmosphere (Image Credit: Reto Stöckli)
(source:
https://cloudsat.atmos.colostate.edu/FactSheet_The_Importance_of_Understanding_Cl
ouds.pdf) .......................................................................................................................52
Figure 17: Simplified scheme of the atmospheric stability (source: (Nugent et al., 2020)
......................................................................................................................................55
Figure 18. Horizontal distribution of air pressure in January (Povară, 2004) ...................65
Figure 19. Horizontal distribution of air pressure in July (Povară, 2004) .........................66
Figure 20: Symbols for surface fronts (source: https://www.gleimaviation.com/wp-
content/uploads/2020/09/SurfaceFrontSymbols.jpg) ....................................................90
Figure 21: Significant Weather (SIGWX) Chart symbols (Source: BOM, 2012)...............110

Figure 22: Example of SIGWX chart (source: https://wxcharts.com/) ...........................110
Figure 23: Example of 500 hPa geopotential height (source:
https://www.wetterzentrale.de/) ................................................................................111

International Air Force Semester

Figure 24: Surface pressure and frontal analysis (source: https://www.met.ie/latest-
reports/surface-analysis) .............................................................................................112

Table of Tables

Table 1: The main types of clouds (source:
https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/
clouds.htm)....................................................................................................................47
Table 2: Weather phenomena in METAR code (after Manual on Codes, WMO-No. 306)
....................................................................................................................................101

Chapter 1 - The atmosphere

1.1. Overview of the atmosphere

Atmosphere is the gas and aerosol envelope that extends from the surface of the planet
outward into space. Earth's atmosphere began to evolve 4.5 billion years ago being
subject to a multitude of influences, both exogenous, like the solar wind and meteoritic
impacts, as well as endogenous, such as outgassing and plate tectonics. By studying noble
gas isotope ratios and the rock record, researchers defined how these processes modeled
Earth's atmosphere. Living organisms have also played a key role in its evolution, affecting
the content of main constituents such as oxygen, nitrogen, and various greenhouse gases.
Based on organisms role in the dynamic of atmospheric gases, the Gaia hypothesis
suggests that the biosphere regulates Earth's climate to sustain suitable conditions for
living organisms. According to this hypothesis, Earth and its biological systems have
closely controlled self-regulatory negative feedback loops that keep the atmospheric
settings within boundaries that are favorable to life (Boston, 2008).
The evolution of Earth’s current atmosphere is not completely understood. However,
there are evidences that the current atmosphere resulted from a gradual release of gases
both from the planet’s interior and from the metabolic activities of various forms of life.
Once organisms developed the capability to synthesize oxygen (by photosynthesis
process), this vital gas was produced in large quantities. The increase of oxygen in the
atmosphere also permitted the development of the ozone layer (Pielke, 2020).
Atmosphere is held close to Earth surface by the gravity force which does not permit it
drifting into space. However, light elements and molecules, such as hydrogen and helium,
typically move faster than heavier ones, like oxygen and nitrogen and are more likely to
escape the Earth’s gravitational field. That’s why light molecules are rare in our
atmosphere, in contrast to their abundance in the Universe.
The atmosphere has a height of about 600 km. There are two common definitions for the
height of the atmosphere taken from different disciplines. Thus, in aeronautics is used the
Kármán line, which is roughly 100 km above mean sea level and is defined as the boundary

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International Air Force Semester

between the atmosphere and outer space. this boundary represents the point where the
atmosphere is too thin to support aeronautical lift (Palmer, 2017).
On the other hand, physicists and planetary scientists use as the exterior limit of the
atmosphere the exobase, which is the bottom of the exosphere (600 km above Earth).
This definition is based on thermodynamics principles in which particles are still bound by
gravity but the density of particles is too low for them to behave like a gas and collide with
each other (Palmer, 2017).
The Earth’s atmosphere plays a central role of energy transfers from one region of the
globe to another. Along with other planetary elements, such transfers determine the
Earth’s climates. By redistributing mass, and energy, air motion influences other vital
atmospheric components such as oxygen, nitrogen, carbon dioxide, water vapor and
ozone which has implications in radiative and chemical processes. Such influence makes
the atmosphere a key ingredient of the global energy budget.
Because of its fluid nature, the atmosphere has the ability to hold a wide variety of
motions. These range from turbulent eddies of a few meters to circulations with
dimensions of thousands of kilometers

The atmosphere

1.2. The vertical structure and composition of the atmosphere

The Earth's dry atmosphere is composed primarily of nitrogen and oxygen (almost 99% by
mass and volume), a small quantity of argon and several trace gases (neon, helium,
krypton, xenon and others) (Figure 1). Nitrogen and oxygen vary only little with time and
are considered permanent gases. Other gases which are found in small quantity such as
carbon dioxide and ozone varies with a higher degree in time (Saha, 2008).
Nitrogen - 78%
Oxygen - 21%.
Argon - 0.9%.

Figure 1: Composition of the atmosphere (source: https://www.mrgscience.com/ess-topic-61-introduction-
to-the-atmosphere.html#)

One of the most important components of the atmosphere for meteorology is water
which can be found in all three states of aggregation. A change of phase in water
significantly affects the properties of atmosphere by release or absorption of heat.
Earth’s atmosphere consists of several interconnected layers. They are classified mainly
by the density of air, temperature variation and their relative proximity to Earth’s surface
and outer space (Palmer, 2017).
The total mass of the gas layer that constitutes the atmosphere corresponds to 0.001% of
the total mass of the planet and is practically concentrated in the first 10 km of altitude
(Palmer, 2017).
The first layer of the atmosphere from the Earth’s surface is the troposphere (Figure 2).
This layer extends from the surface to the tropopause at 8–18 km. The thickness of
troposphere is gradually increasing with the increase of temperature from about 8 km at
the poles to 18 km at the equator.
The stratosphere is the layer of the atmosphere between the tropopause and the
stratopause at 50 km. Above the stratopause the atmosphere continues with the
mesosphere which reaches to about 100 km to the mesopause. The thermosphere lies
above the mesosphere and reaches to 500–1000 km. The last layer of the atmosphere,
which is the exosphere begins at the limit of the thermosphere (thermopause) to the near
vacuum of outer space (Palmer, 2017).
The troposphere represents the lower atmosphere and concentrates about 75% of
atmospheric mass. Within the troposphere, atmospheric temperature normally decreases
by 0.65 °C for every 100 meters increase in altitude. To the superior limit of the
troposphere, in the tropopause, temperatures reach approximately -50 °C. Directly above

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International Air Force Semester
the tropopause, in the stratosphere, atmospheric temperature increases with height up
until the stratopause at approximately 50 km, where temperatures can exceed 0 °C
(Palmer, 2017).
Within the stratosphere, there is a natural level of ozone that is determined by ultraviolet
radiation interacting with molecular and atomic oxygen. Heating of the stratosphere is
mainly due to ozone absorbing incoming ultraviolet and visible solar radiation, with
smaller contributions due to carbon dioxide and water vapor absorbing at shorter infrared
wavelengths. The stratosphere contains only 10% to 20% of the total mass of the
atmosphere, but changes in stratospheric composition are important because they affect
the balance of incoming and outgoing radiation (Palmer, 2017).
Within the stratosphere there is a limit of significant importance. Between 18 and 20 km
lies the Armstrong limit that corresponds to an atmospheric pressure at which water boils
at body temperatures. Flying at or above this limit without the aid of a pressurized suit or
container would boil any living organism. Aviators have worn pressure suits since the mid-
1930s to avoid hypoxia at high altitudes, where a reduction in the partial pressure of
oxygen within the body affects brain functioning (Palmer, 2017).

Figure 2: The thermal vertical structure of Earth’s atmosphere, with associated landmarks (Source: Palmer,
2017)

The next atmospheric layer above the stratosphere is the mesosphere. The stratopause
separates the two layers. Together, the stratosphere and mesosphere are called the

The atmosphere

middle atmosphere. The depth of the mesosphere is 40–50 km, ranging from the
stratopause to the mesopause which is at an altitude of 100 km. Temperature decreases
rapidly with altitude in the mesosphere. This is due to decreasing solar heating because
of progressively fewer molecules available to absorb radiation at these wavelengths.
Another factor which contributes to temperature decrease with altitude is cooling from
carbon dioxide, which absorbs energy from scattered light and releases energy in all
directions so that a portion of it will be carried away from the mesosphere. The
mesopause is the coldest part of the atmosphere. The temperature to this level of
atmosphere is less than -100 °C. Scientists use instruments on small rockets to sample the
mesosphere but the scientific knowledge about this area of the atmosphere is still very
limited (Palmer, 2017).
The mesosphere is home to the highest clouds in the atmosphere. This type of clouds is
called noctilucent clouds. These clouds form directly from ice particles at high latitudes
during summer months when temperature falls below -120 °C. When these ice crystals
grow large enough to scatter incoming solar radiation, they become visible from the
ground during twilight, when the Sun is below the horizon but light is being scattered in
the upper atmosphere. The most recent scientific studies have linked the higher
frequency of this type of clouds with climate change (Palmer, 2017).
The thermosphere lies above the mesosphere and mesopause. Here, the atmosphere can
be characterized not only by temperature but also by electric conductivity or electron
density. In the thermosphere gases start to stratify according to their molecular mass,
with the lower mass gases diffusing higher up.
The thermosphere stretches from about 100 km to 500–1000 km above Earth’s surface.
Above the thermosphere is a thin layer called thermopause.
In thermosphere the temperature increases sharply with altitude to values of over 1000
°C. The highest temperatures are dependent of solar activity. The heating of the
thermosphere follows a different mechanism compared with troposphere, stratosphere
and mesosphere. Therefore, the high temperatures in thermosphere are from the
absorption of very high-energy particles of light (photons, usually at ultraviolet and X-ray
wavelengths), by nitrogen and oxygen (as molecules and single atoms), and from collision
with high-energy particles in the Van Allen radiation belts (a layer of charged particles
held in place by Earth’s magnetic field). These high energy particles cause the atmosphere
to ionize.
The progressively decrease of density in gases within the thermosphere leads to two new
phenomena. Even though the temperatures of the thermosphere are extremely high
hypothetically a living body would be very cold. This is because the transfer of heat needs
a specific environment formed of gases in which to travel and in the absence of that the
energy lost by the living body would far exceed the energy acquired by direct contact with
the dilute gases. The thermosphere forms the upper atmosphere.
Atop the thermosphere lies the exosphere. It extends from the thermopause to the near
vacuum that is outer space. At altitudes within the exosphere, the distance covered by
gas molecules between collisions with each other is large enough (hundreds of
kilometers) that it can accelerate to a speed that enables it to escape Earth’s gravitational
pull. This process is called Jeans escape and is partly responsible for the leakage of
hydrogen from Earth’s atmosphere. This leakage is only important on geological
timescales (Palmer, 2017).

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International Air Force Semester

The ionosphere is the region of the middle and upper atmosphere that is ionized by solar
ultraviolet radiation. It extends from the upper mesosphere to the exosphere. The
ionization variability depends on variations in solar activity, and therefore exhibits diurnal,
geographical, and seasonal variations. The ionosphere has a great importance since
reflects electromagnetic radiation. Because of its importance in the transmission of
electromagnetic radiation, understanding, and thereby forecasting, the variability of the
ionosphere is crucial for transmissions via radio waves. The neutral and charged
atmosphere can produce auroras at high latitudes. In rare situations auroras can occur at
lower latitudes as well. They are formed as a result of ionized gases and neutral gases,
altered by incoming high-energy particles, which emit light in the process of returning to
their normal state. The color of the aurora depends on the gases which emit the photons:
green and orange-red is produced by oxygen and blue or red is produced by nitrogen. The
swirling patterns of the aurora are caused by to the incoming electrons being trapped and
spiraling within Earth’s magnetic field (Palmer, 2017).
In respect to the optical properties, the Earth's atmosphere is relatively transparent to
incoming radiation from the Sun and to a certain degree opaque to outgoing radiation
emitted by the Earth's surface. The blocking of outgoing radiation by the atmosphere,
popularly referred to as the greenhouse effect, keeps the surface of the Earth warmer
than it would be in the absence of greenhouse gases. Much of the absorption and
reemission of outgoing radiation are due to air molecules and cloud droplets.
The atmosphere also scatters the radiation of the visible spectrum that passes through it,
giving rise to a wide range of optical effects. Most of these optical effects do not affect
aircraft during flight operations. Due to the presence of clouds and aerosols in the Earth's
atmosphere, about 22% of the incoming solar radiation is backscattered to space without
being absorbed. The backscattering of radiation by clouds and aerosols has a cooling
effect on the Earth‘s climate. This process opposes the greenhouse effect (Boston, 2008).

The atmosphere

1.3. Altitude variation of key meteorological elements
(temperature, humidity, pressure, wind)

Vertical profiles of temperature, humidity, pressure and wind are investigated by
radiosondes, research aircrafts, unmanned aerial vehicle and satellites. These
observations are mostly used to understand chemistry of the atmosphere and variations
in climatic characteristics (Turgut and Usanmaz, 2016).
These data and their vertical profiles are important for the aviation. For instance, wind
speed and direction strongly affect the fuel consumption and flight time of a large aircraft.
Another example is the combined effect of wind and temperature on fuel consumption
which can be as high as +1.4% and +2.3% for the westbound (east to west) and +1% for
the southbound flights, for a B747-400 (Turgut and Usanmaz, 2016).
Variations in the key meteorological elements from the surface of the Earth to higher
levels of the atmosphere are determined by a series of geophysical factors such as gravity,
geomorphology, Sun radiation and others. The Sun heats the surface of the Earth which
further radiate into the atmosphere warming the air near surface. The heated air is then
diffused or convected up through the atmosphere. Thus, in the troposphere the air
temperature has the highest values near the Earth’s surface and decreases with altitude
(Figure 4). Rock, sand, soil, water and other materials which form the continents and
oceans absorb the Sun’s radiation and return it back into the atmosphere as heat. The
temperature is also higher near the surface because of the greater density of gases and
due to the very low thermal conductivity of the atmosphere. The rate of decrease of air
temperature with increasing altitude is 0.65 °C / 100 m.
The decrease in air temperature with altitude is not always linear in the troposphere.
Because of the dynamic of the air in the troposphere in some circumstances there can
occur temperature inversion – air temperature increases with altitude (Figures 3 and 4).
Such feature is enabled when the atmosphere is calm and the denser and colder air slides
beneath the warmer air. Temperature inversions may last for several days or even weeks
when the atmosphere is very stable. This phenomenon can trap pollutants near the
surface and substantially decrease visibility.

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International Air Force Semester

Figure 3: Smoke trapped at the surface by stable atmospheric conditions and temperature inversion
(source: https://courses.lumenlearning.com/sanjac-earthscience/chapter/atmospheric-layers/)

At the top of the troposphere there is a thin layer in which the temperature remains
constant with height (tropopause). Above this thin layer, the temperature starts to rise
(Figure 4). Thus, the cooler and denser air of the troposphere is trapped beneath the
warmer, less dense air of the stratosphere. Opposite to troposphere, where the surface
heats the air, the stratosphere gets heat directly from the Sun. Solar energy is converted
into heat through ozone absorption-release mechanism. The air temperature increases
with the altitude has a stabilizing effect and does not cause convection.
The mesosphere is characterized by decreasing temperatures with altitude. The lowest
temperatures in Earth’s atmosphere occur at the top of this layer, in the mesopause. In
the mesosphere air pressure is very low as well.
In the thermosphere due to the absorption of intense ultraviolet solar radiation by the
limited amount of remaining molecular oxygen and nitrogen the temperature generally
increases with the altitude reaching to over 1000 °C.

Figure 4: Temperature variation with altitude (source: https://courses.lumenlearning.com/sanjac-
earthscience/chapter/atmospheric-layers/)

Relative humidity has a large degree of variability in troposphere. However, in the middle-
upper level of troposphere is generally decreasing. Absolute humidity is decreasing
progressively with altitude.
In terms of air pressure variations with altitude, the measurements and models showed a
linear decrease. Compared to variation of air temperature, pressure is only decreasing
progressively until the top of the atmosphere (Figure 5). Aerodynamic forces directly
depend on the air density and implicitly on air pressure. With increase in altitude the
density of air decreases and therefore influences the aerodynamics of aircrafts.

The atmosphere

Figure 5: Variation of mean pressure and temperature with altitude (source:
http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap2.html)

The wind speed is generally increasing with height from the surface to the upper
troposphere. The increase of wind speed with altitude is determined by the increase of
pressure gradient, weaker influence of friction with the Earth’s surface and decrease of
air density. In the lower area of the stratosphere the wind speed decreases up to 25-28
km after which starts to increase again (Figure 6).

Figure 6: Variation of wind speed with altitude (source: Harb et al., 2013)

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International Air Force Semester

1.4. End of Chapter Questions

Which gas is present in the atmosphere in the greatest amount?
Which gas makes up approximately 21% of the atmosphere?
Which is the layer of the atmosphere where most of the world's weather occurs?
How is called the boundary between the stratosphere and mesosphere?
The tropopause is highest at the north and south poles compared to that at the
equator or mid latitudes?

The atmosphere

1.5. Chapter bibliography

Boston, P.J. (2008). Gaia Hypothesis, in: Encyclopedia of Ecology, Five-Volume Set.
https://doi.org/10.1016/B978-008045405-4.00735-7.

Hocking, W., Röttger, J., Palmer, R., Sato, T., & Chilson, P. (2016). Atmospheric Radar: Application
and Science of MST Radars in the Earth's Mesosphere, Stratosphere, Troposphere, and
Weakly Ionized Regions. Cambridge: Cambridge University Press.
doi:10.1017/9781316556115.

K. Harb, A. T. Abdalla, M. Mohamed & S. Abdul-Jauwad 2013. "HAPs communication in Saudi
Arabia under dusty weather conditions," IEEE 11th Malaysia International Conference on
Communications (MICC), 2013, pp. 379-380, doi: 10.1109/MICC.2013.6805858.

North, G.R., Zhang, F., Pyle, J. (2015). Encyclopedia of Atmospheric Sciences: Second Edition,
Encyclopedia of Atmospheric Sciences: Second Edition.

Palmer, P. (2017). The atmosphere. A very short introduction, 157 p Oxford University Press, Great
Clarendon Street, Oxford, OX2 6DP, United Kingdom.

Pielke R. et. al. (2020). Atmosphere, Encyclopedia Britannica.
Turgut, E., Usanmaz, Ö. (2015). An analysis of vertical profiles of wind and humidity based on long-

term radiosonde data in Turkey. Anadolu University Journal of Science and Technology A-
Applied Sciences and Engineering, 17 (4), 830-844.
https://www.mrgscience.com/ess-topic-61-introduction-to-the-atmosphere.html# (accessed on
11 May 2020)
https://courses.lumenlearning.com/sanjac-earthscience/chapter/atmospheric-layers/
http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap2.html

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Chapter 2 - Global Observing System

2.1. The structure of the system involved in weather observation

The Global Observing System (Figure 7) is a very complex effort, and perhaps one of the
most ambitious and successful instances of international collaboration of the last 60
years, initiated in support of the world Weather Watch, and then increasingly in support
also of climate monitoring. It consists of a multitude of individual surface- and space-
based systems of observation owned and operated by a plethora of national and
international agencies with different funding lines (WMO, 2021).

Figure 7. Simplified scheme of the Global Observing System

18

Global Observing System

The component observing systems of Integrated Global Observing System are the Global
Observing System of the World Weather Watch Programme, the observing component of
the Global Atmosphere Watch Programme, the WMO Hydrological Observing System of
the Hydrology and Water Resources Programme and the observing component of the
Global Cryosphere Watch, including their surface-based and space-based networks.
The Global Observing System closely operates with other two components: the global
Telecommunication System (GTS) and the Global Data-processing and Forecasting System
(GDPFS). Through the combination of the Global Observing System and Global
Telecommunication System, billions of observations are obtained and exchanged in real
time between WMO Members and other partners every single day. Without the Global
Observing System and Global Telecommunication System, not a single WMO Member
would be able to serve the weather needs of its citizens as well as they do today.
The Global Observing System provides observations on the state of the atmosphere and
ocean surface from the land-based and space-based instruments. These data are used for
the preparation of weather analyses, forecasts, advisories and warnings, and for the
climate monitoring and environmental activities (WMO, 2019).
The main types of stations/platform in the Global Observing System consist in:

• Stations/platforms on land;
• Stations/platforms on the sea surface;
• Airborne stations/platforms;
• Stations/platforms underwater;
• Stations/platforms on ice;
• Stations/platforms on lakes/rivers;
• Satellites.
Stations/platforms on land include observations which are made at a fixed position in
relation to the land surface, a mobile observation on land or those which transfer their
data to a facility on land. Stations/platforms consist of surface meteorological weather
stations, upper-air in situ observations, weather radar observations and other surface-
based remote-sensing observations.
Upper-air in situ observations uses instrumentation attached to meteorological balloons
(radiosondes), or unmanned aerial vehicles (also called drones). The balloon tracking for
the calculation of winds (that is, by radar or radio-theodolite) is also regarded as an upper-
air in situ observation. The radiosonde measurement, often referred to as a sounding,
delivers a complete profile from the launch point to balloon burst. To ensure timely
availability for the data users the sounding is often split into several messages, but the
same metadata are included in all parts of the transmitted messages. Observations such
as those made by dropsondes, rockets and kites are also included in this category (WMO,
2019).
Weather radar observations have been made particularly for the detection of
precipitation, hydrometeor classification and quantitative precipitation estimation.
Doppler wind speed and direction can also be reported from some weather radars (WMO,
2019).

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International Air Force Semester

Other surface-based remote-sensing observations include all observations, excluding
those from weather radars, made using remote-sensing instrumentation located at a fixed
station. These systems are wide ranging in their methods of observation, but primarily
result in a measurement profile representative of the atmosphere above the sites.
Examples of systems in this category are wind profiling radars, lidars, sodars, radiometers,
ground-based GNSS receivers, and high-frequency radars. So, both active and passive
remote-sensing technologies are considered here (WMO, 2019).
Sea-surface observations are taken from a variety of stations/platforms. These include
moored buoys, drifting buoys, ships and off-shore installations. Also, terrestrial-based (on
shore) high-frequency radars (measuring surface current direction and speed) can be
considered as such. Variables most commonly measured in this category of observations
are air temperature, atmospheric pressure, humidity, wind direction and speed, sea-
surface temperature, wave height, wave period, wave direction, sea-level, current speed
and direction, and salinity. Ship observations typically include air and seawater
temperature, atmospheric pressure, humidity, and wind direction and speed. These are
commonly measured automatically. Manual ship observations also include wave height,
wave period, wave direction, ceiling (cloud cover) and visibility (WMO, 2019).
Airborne observations consist in aircraft-based observations which are carried out on
board of a series of aircrafts. Observations include profiles near aerodromes or are
composed of a series of equidistant observations at constant altitude.
Underwater observations include thermistor strings and devices attached to inductive
cabling, expendable bathythermographs, acoustic Doppler current profilers, Argo floats,
and conductivity, temperature and depth devices. Bottom-mounted water pressure
sensors are used to measure variations in the water column, which are indicative of a low-
amplitude wave (tsunami) generated by an underwater disturbance (seismic activity). A
new technology, profiling gliders, which are unmanned underwater vehicles, is becoming
more widespread. The variables observed by these devices include water temperature,
water pressure, salinity, current direction and speed, fluorescence and dissolved oxygen.
All of these variables are measured at depth – as deep as the sensors or gliders are located
(WMO, 2019).
Stations/platforms on ice provide information on the atmosphere above surfaces covered
with ice.
Stations/platforms on lakes/rivers perform direct observation of a staff gauge or by
automatic sensing through the use of floats, transducers, gas-bubbler manometers and
acoustic methods.
Satellite observations provide information from all areas of the world. These observations
deliver information on surface characteristics, as well as atmospheric conditions
depending on the instrument type. Essential information about satellites are orbit and
type of orbit (geostationary or polar orbiting), height of the satellite, local observation
intervals, types of technology applied (active/passive, optical/microwave,
imager/sounder) and instrument characteristics (bands measured, footprint,
measurement approach such as scanning versus push broom or similar, swath size if
applicable, return period etc.) (WMO, 2019).

Global Observing System

2.2. The elements and phenomena that are observed in aviation
meteorology

Although the Global Observing System perform a wide range of measurements, required
observations for aviation are carried out mainly through aerodrome weather stations.
However, other types of observations such as those performed by aircraft, remote-
sensing systems and other are still used.
Notwithstanding the excellent performance of modern aircraft, weather still has a
significant impact on the safe and economic conduct of a flight (WMO, 2014). Thus,
meteorological information is an essential asset for aviation.
The meteorological variables required to be observed at aerodrome meteorological
stations include surface wind, visibility (and RVR on all runways intended for use during
periods of reduced visibility), present weather, cloud (and vertical visibility when the sky
is obscured), air temperature, dewpoint temperature, atmospheric pressure and
supplementary information concerning significant meteorological conditions, particularly
in the approach and climb-out areas (WMO, 2014).
In some areas, low-level wind shear, particularly when associated with microbursts and
downbursts, can be a serious hazard to aviation operations. A number of fatal accidents
have been attributed to this phenomenon. If warranted by the climatology of the area
where an aerodrome is or is to be located, consideration should be given to installation
of a wind shear detection system (WMO, 2014).
Wind observations, for instance, are used for the selection of runways, noise abatement
procedures and for the assurance of the maximum allowable take-off and landing weights.
Reduced visibility has a major impact on aviation safety, and when fog is present, airport’s
disruption causes significantly economic impacts. Temperature is also important and can
impact significantly on aircraft engine performance, required take-off speed and runway
length. For instance, high temperatures are equivalent with lower air density which
reduces lift, resulting in the need for greater take-off speeds and consequently more
runway length. If runway is not long enough, take-off weight must be reduced. High
temperatures may also impose limitations on take-off power. This is particularly
important in the case of high-altitude aerodromes in hot climates (WMO, 2014).
At aeronautical meteorological stations, observations are made throughout the 24 hours
of each day, except as otherwise agreed between the meteorological authority, the
appropriate air traffic service authority and the operators concerned and in accordance
with regional air navigation agreements.
One of the most important tools for the detection of weather information of interest to
aviation is weather radar, which can provide continuous information, in real time, about
conditions over a large area surrounding an aerodrome. Weather radar is particularly
valuable in areas where thunderstorms occur frequently, but is also very useful for the
detection of areas of rain or snow. Doppler weather radar can be used to detect low-level
wind shear, a serious hazard to aviation operations (WMO, 2014).

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2.3. Overview of Numerical Weather Prediction Models

Numerical Weather Prediction Models focus on taking current observations of weather
and processing these data with computer models as part of their input to forecast the
future state of atmosphere. Knowing the current state of the atmosphere is just as
important as the numerical computer models processing the data. Current weather

observations serve as input to the numerical computer models through a process known
as data assimilation to produce outputs of temperature, precipitation, and hundreds of
other meteorological elements from the Earth’s surface to the top of the atmosphere
(NOAA, 2021).
Starting with the first barotropic model experiments of Charney and Von Neumann in
1950, numerical models of the atmosphere have developed into the primary means by
which forecasters are able to predict synoptic-scale weather beyond 6 hours (Ross, 1986).
Even after the design of complex Coupled Numerical Weather Prediction Models, the
errors in prediction have been found to be significant. The forecast results of the
Numerical Weather Prediction (NWP) models are operationally verified against
observations and/or model analyses. The results of the verification are used for the
reference of research, improvement and development of the further NWP models.
NWP models describe the main physical processes in the atmosphere, at the surface and
in the soil interface and take their impact on the temporal evolution of the model
variables like pressure, temperature, wind, water vapor, clouds and precipitation into
account. Many physical processes in the atmosphere or at the surface like the formation
of clouds or the interaction between solar radiation and cloud droplets take place on very
small spatial scales which cannot be resolved explicitly by the NWP models. The impact of
these unresolved processes on the model variables must be included approximately via
so-called parameterization schemes
(https://www.dwd.de/EN/research/weatherforecasting/num_modelling/01_num_weat
her_prediction_modells/num_weather_prediction_models_node.html).
To solve the complex equations of models on computers different numerical methods can
be employed. In grid point models the temporal evolution of the model variables is
calculated in a three-dimensional spatial grid which covers the atmosphere from the
surface up to a given limit above the ground. A very important characteristic of the model
grid is the horizontal distance of neighboring grid points. The smaller the grid spacing, the
more detailed atmospheric structures can be resolved by the numerical prediction model.

To solve the complex set of model equations on computers different numerical methods
can be employed.
Each important physical process that cannot be directly predicted requires a
parameterization scheme based on reasonable physical or statistical representations. The
graphic depicts some of the physical processes and parameters that are typically
parameterized, both because they cannot be explicitly predicted in full detail in model
forecast equations (Figure 8).

Global Observing System

Figure 8: Parameters that are typically parameterized in model forecast equations (source: Macatangay et
al., 2016)

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2.4. End of Chapter Questions

Which are the main types of stations/platforms in the Global Observing System
consist in?
Which elements and phenomena are observed in aviation meteorology?
What variables are generated by radar observations?
What parameters are typically parameterized in forecast models?

Global Observing System

2.5. Chapter bibliography

Macatangay, R., Bagtasa, G., Sonkaew, T. (2016). Finding the Optimum Resolution, and
Microphysics and Cumulus Parameterization Scheme Combinations for Numerical Weather
Prediction Models in Northern Thailand: A First Step towards Aerosol and Chemical
Weather Forecasting for Northern Thailand. International Symposium on Grids and Clouds
2016 13-18 March 2016, Academia Sinica, Taipei, Taiwan.

Ross, B.B. (1986). An Overview of Numerical Weather Prediction. In: Ray P.S. (eds) Mesoscale
Meteorology and Forecasting. American Meteorological Society, Boston, MA.
https://doi.org/10.1007/978-1-935704-20-1_30.

World Meteorological Organization (2014). Guide to Meteorological Observing and Information
Distribution Systems for Aviation Weather Services, WMO-No. 731.

World Meteorological Organization (2019). Guide to the WMO Integrated Global Observing
System, WMO-No. 1165.

https://www.dwd.de/EN/research/weatherforecasting/num_modelling/01_num_weather_predi
ction_modells/num_weather_prediction_models_node.html

https://www.weather.gov/media/ajk/brochures/NumericalWeatherPrediction.pdf
https://www.ncdc.noaa.gov/data-access/model-data/model-datasets/numerical-weather-

prediction
https://www.dwd.de/EN/research/weatherforecasting/num_modelling/01_num_weather_predi

ction_modells/num_weather_prediction_models_node.html

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Chapter 3 - Solar radiation and heat
exchange

3.1. Solar radiation and the role of radiative processes in heat
transfer and generation/dissipation of weather phenomena

The evolution of Earth’s atmosphere is closely related to Sun activity. The Sun emits
electromagnetic radiation in all directions at a wide range of wavelengths. The rate at
which solar flux reaches to a surface per unit area and per unit time, reduces as an inverse
square law, thus for every 2 km from the Sun, solar irradiance drops off by a quarter.
Because Earth is (approximately) spherical and inclined, it receives less solar energy per
unit area at higher latitudes. The midlatitudes receive about 70% of the solar energy
compared to the equator area. The polar regions receive only about 40 % of the energy
that receives the equator (Palmer, 2017).

26

Solar radiation and heat exchange

Figure 9: Overview of Earth’s radiation budget (source:
http://homework.uoregon.edu/pub/class/es202/GRL/radbal.html)

Figure 10: Solar energy intercepted by Earth (a) and spatial distribution (b) (source: (Palmer, 2017)

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International Air Force Semester

The incoming solar radiation to Earth’s atmosphere comprises visible wavelengths and
parts from infrared and ultraviolet wavelengths.
The intensity of the incoming solar radiation is depleted by the processes of scattering,
reflection and absorption by air molecules or other particulate matter that may be
suspended in the atmosphere and by clouds. In scattering, a straight parallel beam of
radiation changes direction either sideways or backwards. However, the degree of
scattering depends upon the size of the molecules or particles compared to the
wavelength of the incident beam (Saha, 2008).
The ultraviolet radiation is mostly absorbed in the middle and upper atmosphere. The
infrared radiation is mostly absorbed by water in the lower atmosphere. The atmosphere
absorbs approximately a quarter of the incoming solar radiation. But only certain
atmospheric gases absorb and emit radiation, and only at specific wavelengths. Gases like
carbon dioxide and water vapor have a whole range of capacity that enables them to
absorb and emit radiation. The radiation which is absorbed will cause atmospheric
molecules to reach an excited state; afterwards they emit radiation that allows them to
return to their initial state. This radiation is absorbed by the atmosphere again, by Earth’s
surface, or is returned to outer space.
A third of all the 340 W/m2 of incoming radiation is reflected to space (Figure 9). This
reflection is defined as albedo. Every surface reflects a certain quantity of radiation. The
whiter is the surface, the more radiation is reflected. For instance, snow can be a perfectly
reflective surface for visible light. However, snow is highly absorbent for longer infrared
wavelengths. In other words, snow is a very complicated material because its albedo also
varies with a number of other factors, including age, wetness, and depth.
About half of the incoming radiation (above 0.3μm) is absorbed by Earth’s surface. The
Earth subsequently warms up with this absorbed radiation. Earth’s low atmosphere is
afterwards heated from the surface. Heat is later transported to higher levels of the
atmosphere from the surface by conduction and convection processes. Atmosphere is
also indirectly warming by the evaporation of water which cools the surface and heats up
the atmosphere when the vapor eventually condenses or ice particles form (Palmer,
2017).
Moreover, incoming solar radiation quantity also obeys to other variables. Earth follows
an elliptical trajectory around the Sun and makes the zenithal position with an oscillation
between 23.5 °N and 23.5 °S during a complete round. The angle at which the sun’s rays
strike the earth’s surface varies with time of the day, season and latitude (Figure 10). Since
the length of a day or night varies with latitude, the insolation varies with latitude and
season only. In the summer hemisphere, the length of the day gets longer and longer with
increasing latitude till near the pole, it is almost 24 h of daylight. The reverse is the trend
in the winter hemisphere where day gets shorter and shorter with increasing latitude, and
it is almost 24 h of night near the pole. Thus, the duration of sunlight is the longest over
the earth’s polar region which receives more radiation from the sun than any other place
on earth at the time of the summer solstice, though the distance from the sun is greater
and the sun’s rays fall at slanting angles (Saha, 2008).
Variations of the incoming solar radiation with season and latitude cause corresponding
changes in surface temperatures which are observed all over the globe. Diurnal variation
of solar radiation determines a similar variation in temperature at a specific location.
However, this linear relationship may suffer disturbances when there is excessive
moisture in the atmosphere in the form of clouds. The increase of the water vapor content

Solar radiation and heat exchange
of the atmosphere suppresses the amplitude of the diurnal cycle by causing the daytime
maximum air temperature to have lower values and the nighttime minimum temperature
to go have higher values, while the dew point rose rapidly (Saha, 2008).

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International Air Force Semester

3.2. Thermal properties of the atmosphere and the earth's
surface

It is estimated that of the total solar energy that is intercepted by the earth at the top of
the atmosphere, only about half reaches the earth’s surface and is absorbed by it as heat
energy. When heated, the surface emits its own radiation. The radiation from bodies at
the temperatures of the earth’s surface and atmosphere lies in the longwave part of the
spectrum, the so-called infrared radiation, with maximum emissions at wavelengths
between about 10μm and 15μm. It so happens that a fraction of this longwave radiation
moving upward is absorbed by some gases present in the atmosphere, such as water
vapor and carbon dioxide, which have strong absorption bands in the infrared part of the
spectrum. The absorption leads to warming of the gases which then emit their
characteristic longwave radiation, a part of which is sent back to the Earth. Thus, the net
radiative heating of the Earth’s surface depends on three factors:
1. heat gained from the incoming solar radiation;
2. heat lost or emitted by longwave (infrared) radiation;
3. heat gained from atmospheric gases as downward longwave (infrared) radiation.
Concurrently, the Earth’s surface loses heat by the exchanges of water with atmosphere.
Thus, the heat is lost through evaporation (Saha, 2008).
About two-thirds of the Earth’s surface consists of water and one-third of land. Though
the major parts of the oceans appear as water surfaces, the polar regions are largely
frozen and appear as solid ice surfaces. Then, there are the warm and cold ocean currents
which cover wide areas and introduce large inhomogeneity in ocean surface
temperatures. The land surface also is highly uneven, in fact more so than the ocean
surface, with high snow-clad mountains over several areas flanked by warm valleys, vast
deserts and lands co-existing with equatorial rainforests and rivers and in-land lakes, and
so on. To date, despite numerous field experiments and laboratory and theoretical
studies, the values of the coefficients of heat exchanges with the environment under
different stability conditions are barely known. So, when we talk of the heat balance of
the earth’s surface, the existing inhomogeneity of the surface and the above-mentioned
uncertainties in the values of the exchange coefficients must be taken in consideration
(Saha, 2008).

Solar radiation and heat exchange

3.3. Temperature and thermal processes

It is well known that the main radiative transfer of heat in the atmosphere is through
vertical movement. However, heat flows both upward into the atmosphere and
downward into the earth surface (Saha, 2008).
Heat transfer by molecular conduction is extremely slow and inefficient thus, heat is
transferred mostly by radiation, turbulent motion and convection (Saha, 2008). The heat
transfer between earth surface and atmosphere can be calculated through a series of
equations which approximate this exchange.
Propagation of radiation and heat in the atmosphere is not the result of a single event or
process. It underlies continuous absorption and reemission processes in the gas, which
are determined by the mean free absorption length and strongly depend on the frequency
of the radiation as well as on the gas density. Radiation also converts into molecular
kinetic energy and via exciting collisions changes back to radiation. This kinetic energy of
the molecules results from absorbed long and shortwave radiation as well as from
sensible and latent heat due to convection and evapotranspiration from the surface to
the atmosphere.
Radiation converts to heat after an absorption, followed by an emission in accordance
with a newly adjusting thermodynamic equilibrium, which only requires that the net
energy transfer is in balance.
Air temperature is one of the most important meteorological elements. Considering the
thermal properties of the Earth’s surface, large diurnal and seasonal temperature values
which are large over land and small over ocean and air masses movements it is important
to measure air temperature with a high spatiotemporal resolution to get accurate
weather forecasts. The diurnal cycle of temperature, because it is controlled by water
vapor and cloud, is not a robust feature of climate simulations (Salby, 2012).
In course of a day air temperature varies reaching a minimum during the nighttime and a
maximum during the daytime.
For flight planning purposes, full details of horizontal and vertical temperature variations
on the route will be required for the calculation of true airspeeds and corrections to
indicated altitude as the instruments are calibrated at standard conditions. Also, the
efficiency of engine depends on the temperature at the cruising altitude. Forecast air
temperatures and heights of the tropopause are therefore important in deciding the
optimum cruising altitude. The engine efficiency is greater with lower outside
temperature. For higher temperature values, more fuel than the normal must be used to
maintain cruising power. The information is required during the planning stage of the
flight when fuel load is being determined. The information on upper air temperature also
tells the pilot the height of the 0 °C isotherm above which icing can occur.
Lift is more with high pressure and lower temperature and hence runway length can be
shorter for the given conditions for take-off (Bhawan, 2013). Thus, monitoring of surface
observations such as METAR and SYNOP will be essential to forecast accurate
temperatures.
Among air temperature observations and forecasts, dew point temperature values are
also essential for flight planning. Air and dew point temperatures are required for
calculating take-off weight, providing information for passengers and others. The air and
dew point temperatures must be representative of all the runways although a single value
for each parameter is used for the aerodrome. Consequently, the measurements must be
taken in an area considered representative of the aerodrome that is not subject to specific

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International Air Force Semester
fluctuations due to the surrounding environment. The measurements must be taken in an
open and naturally ventilated area and the sensors must be protected by a shelter or
screen (ICAO, 2011).
The relationship between dew point and temperature defines the concept of relative
humidity. The dew point, given in degrees Celsius, is the temperature at which the air can
hold no more moisture. When the temperature of the air is reduced to the dew point, the
air is completely saturated, and moisture begins to condense out of the air in the form of
fog, dew, frost, clouds, rain, or snow
(https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/1
4_phak_ch12.pdf).

Solar radiation and heat exchange

3.4. End of Chapter Questions

How much energy do polar regions receive compared to the equator?
Through which type of movement is realized the main radiative transfer of heat in
the atmosphere?
Why dew point temperature values are essential to flight planning?

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International Air Force Semester

3.5. Chapter bibliography

Bhawan, M. (2013). Lecture Notes on Aviation Meteorology. Central Aviation Meteorological
Division, Lodi Road, New Delhi - 110003 India Meteorological Department.

Harde, H. (2013). Radiation and Heat Transfer in the Atmosphere: A Comprehensive Approach on
a Molecular Basis. International Journal of Atmospheric Sciences, vol. 2013, 503727.

International Civil Aviation Organization (2011). Manual on Automatic Meteorological Observing
Systems at Aerodromes. International Civil Aviation Organization 999 University Street,
Montréal, Quebec, Canada H3C 5H7.

Palmer, P. (2017). The atmosphere. A very short introduction, 157 p Oxford University Press, Great
Clarendon Street, Oxford, OX2 6DP, United Kingdom.

Salby, M. (2012). Physics of the atmosphere and climate. Cambridge University Press 32 Avenue
of the Americas, New York, NY 10013-2473, USA.

Saha, K. (2008). The Earth’s atmosphere. Its physics and dynamics. Springer-Verlag Berlin
Heidelberg. 10.1007/978-3-540-78427-2.

http://homework.uoregon.edu/pub/class/es202/GRL/radbal.html
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/14_phak_

ch12.pdf

Chapter 4 - Water in the atmosphere

4.1. Water in the atmosphere and associated processes

Solar radiation drives a perpetual exchange of water between the oceans, the atmosphere
and the land surfaces. About 90% of the water in the atmosphere emanates from the
oceans, lakes and other open waterbodies. Via atmospheric transport and relevant
transfer processes a part of the water which evaporated over the oceans reaches the land
areas, where it may precipitate and support the existence of life. In addition, weather and
climate of a region is significantly influenced by water vapor, clouds and precipitation
(Quante and Matthias, 2006).
The weather phenomena are produced mainly by the presence of water in the
atmosphere. Variation in meteorological elements is also determined significantly by the
presence of water in the atmosphere. For example, to formation of wind indirectly
contributes water. Wind blows to equalize differences in air pressure, but those
differences are determined partly by the evaporation and condensation of water which
absorb or release energy into the air, making it expand or contract (Allaby, 2009).
The amount of water vapor present in the air is known as the humidity and there are
several ways to measure and report it. Humidity refers only to water vapor present in the
air and not the droplets of liquid water present in the clouds or fog (Allaby, 2009).
Humidity of the air is measured with hygrometers and psychrometers. After the air
humidity is measured it can be reported in different ways: specific humidity, absolute
humidity, relative humidity, dew point temperature and other variables related to
presence of water vapor in the atmosphere. These variables of humidity will be discussed
later in the subsection “4.3. Physical quantities that define the humidity of the air”.

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International Air Force Semester

4.2. The water phase system (liquid, solid, gas)

Water is the only chemical compound on this planet that occurs naturally in all three
physical states: solid, liquid and vapor.
For a one-component system with one phase, the number of degrees of freedom is two,
and any temperature and pressure, within limits, can be attained. With one component
and two phases (liquid and vapor). For instance, only one degree of freedom exists, and
there is one pressure for each temperature (Wallace and Hobbs, 2006). For one
component and three phases. For example, ice floating in water with water vapor above
it, in a closed container there is no degree of freedom, and temperature and pressure are
both fixed at what is called the triple point (Figure 11).

Figure 11: Phase diagram of water (source: Giorgini, 2016)

Water vapor, which accounts only for roughly 0.25% of the mass of the atmosphere, is a
highly variable constituent in space and time. The inhomogeneous water vapor
distribution is pronounced along the vertical coordinate, its concentration decreases
drastically with the height above the Earth’s surface. But also near the ground the
concentrations vary by more than three orders of magnitude from 10 parts per million by
volume in the coldest regions of the Earth’s atmosphere up to as much as 5% by volume
in the warmest regions. The latter value is only reached in very hot and humid air masses
in the tropics. The tropical atmosphere contains more than three times as much water in
comparison to the extratropical atmosphere. Expressed as specific humidity (mass of
water vapor in g per 1 kg of humid air), the values near the ground vary between 18 to 19
g/kg in the tropics and 1 g/kg in the polar regions. The large-scale distribution pattern of
water vapor principally follows that of the temperature. Since the equilibrium vapor

Water in the atmosphere
pressure strongly increases with temperature (Clausius-Clapeyron-equation), warm air
masses can contain many more water molecules compared to colder ones before
saturation (equilibrium vapor pressure) is reached. The region with highest humidity on
Earth is therefore located over the Western Equatorial Pacific, the area with the highest
observed sea surface temperatures. But there are also exceptions to this rule. Over the
larger scale deserts the water vapor concentration in air is extremely low despite high
temperatures, mainly due to large scale sinking motions over these parts of the
continents. Almost half of the atmospheric water vapor is found below an altitude of 1.5
km. Less than 5% occurs above 5 km and less than 1% in the stratosphere (above
approximately 12 km) (Quante and Matthias, 2006).
Water vapor enters the atmosphere by evaporation. During this process liquid water or
ice at the surface is transferred to the gaseous phase. Evaporation over the oceans is the
dominant source for the atmospheric water budget. The rate of evaporation depends on
several factors, such as the availability of water, the actual vapor pressure, the turbulent
exchange of air near the surface, the surface structure, as well as the natural cover.
Strictly, the term evaporation is used only for the phase change over open water surfaces.
This includes the water on the surface of vegetation (intercepted water). Plants also give
off water vapor to the atmosphere through their leaves or needles (90% through their
stomata), this process is called transpiration. The rate of transpiration depends beside on
meteorological parameters (solar radiation, humidity, temperature, wind) strongly on the
type of plant, the habitat, the season and on soil parameters. Evaporation from the
surface and transpiration are not easy to distinguish above vegetated surfaces, therefore,
often the expression evapotranspiration is used for the sum of land surface evaporation,
interception, evaporation and transpiration. Furthermore, the term potential evaporation
is used in contrast to the actual evapotranspiration to denote the amount of water that
could be evaporated and transpired if there was sufficient water available. For many land
areas, because of an insufficient water supply, the actual evapotranspiration is far below
the potential one (Quante and Matthias, 2006).
In every change of phase, a large amount of heat called the latent heat is either withdrawn
from or released to the atmosphere. A water substance withdraws heat from the
environment when it changes from a solid to a liquid or vapor phase, thereby cooling the
environment. Reversely, it releases heat to the environment when the vapor condenses
into a liquid or sublimes into a solid phase, thereby warming the environment. It is this
cooling or heating effect upon the environment that plays a dominant role in atmospheric
thermodynamics. Further, water vapor plays important roles in radiative and heat balance
processes of the earth-atmosphere system (Saha, 2008).
We, therefore, see that as moist air rises in the atmosphere, the water vapor contained
in it changes into liquid and solid phases and that latent heat is liberated at every change
of phase. The whole process would remain adiabatic and reversible as long as the
products of the phase change are all carried along with the rising current and the heat
liberated remains within the system. But the fact is that when the condensed particles
grow and drop out as rain, snow or hail, they remove some heat out of the system and
the process can no longer be treated as adiabatic and reversible (Saha, 2008).

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International Air Force Semester

4.3. Physical quantities that define the humidity of the air

Specific humidity is the ratio of the mass of water vapor present in the air to a unit mass
of air including the water vapor. Despite comparing the mass of water vapor with moist
rather than dry air, in practice there is little difference between the mixing ratio and
specific humidity. This is because even in very moist air, water vapor seldom accounts for
more than a very small proportion of the total mass, so including or omitting it makes
little difference (Allaby, 2009).
Absolute humidity is the mass of water vapor present in a given volume of air, usually
expressed as grams per cubic meter (g/m2). This takes no account of the fact that changes
in temperature and pressure alter the volume of air, and hence the absolute humidity,
without adding or removing water vapor. This means that absolute humidity is a
somewhat unreliable measure, and it is not used so often (Allaby, 2009).
Relative humidity (RH) is the measure most often used. This variable represents the
amount of water vapor present in the air expressed as the percentage of the amount of
water vapor that would be needed to saturate the air at the prevailing temperature. Since
the amount of water vapor air can hold varies with the temperature (the lower the
temperature the less water vapor air can hold), RH varies according to the temperature.
This makes RH a measure of limited value to meteorologists, who need to know the
amount of water vapor rather than how close the air is to saturation. It is a simple
measure, however, easy to determine and express, and useful in weather forecasting
because it indicates the likelihood of cloud formation and precipitation. RH is reported
either as a percentage or as a decimal fraction (Allaby, 2009).
Partial pressure is the ratio of the partial pressure of water vapor actually present in the
atmosphere at a certain temperature and pressure to the partial pressure of the
maximum amount of water vapor that air can hold at the same temperature and pressure.
It is usually expressed as a percentage. Thus, if the vapor pressure at a given temperature
and pressure is e, the relative humidity is given by (e/es)×100, where es is saturation vapor
pressure at the same temperature and pressure (Saha, 2008).
The dew point is the temperature to which unsaturated air must be cooled at the existing
pressure to produce saturation with the existing amount of water vapor. It is usually
denoted by the symbol Td. The difference between the actual dry-bulb temperature and
the dew-point, which is called the dew point depression, gives a measure of the humidity
of the air. The depression varies inversely with the humidity of the air (Saha, 2008).

Water in the atmosphere

4.4. Meteorological phenomena associated with water phase
transformations

The transformation of water from one phase to another determines various types of
meteorological phenomena. When water passes from gaseous state into the liquid phase
a series of phenomena and transformations can occur in the atmosphere. Thus, when this
transition takes place clouds can form, fog, precipitation in all liquid forms, such as rainfall,
drizzle and others. One may note that there is no difference between fog and clouds other
than altitude. Fog is defined as the suspension of water droplets in the lower atmosphere
that reduces the horizontal visibility to less than 1 km near to the ground. If the visible
moisture begins at or above 50 feet, it is called a cloud. Condensation itself does not cause
precipitation. The droplets of water must become heavy enough to fall on the earth’s
surface.
When water passes from gaseous or liquid state to solid state the associated phenomena
on the ground and the objects of the ground are hoar (vapor to solid), glaze (liquid to
solid) and rime (liquid to solid). Hoar frost forms when water vapor in the air comes into
contact with an object that is below freezing. Rather than the water vapor first condensing
onto the object and then freezing, the water vapor immediately freezes to form ice
crystals. The hoar frost is distinctive due to its feathery structure and the freezing process
is so quick that it traps air, giving it a white or silver opaque appearance
(https://www.rmets.org/metmatters/how-does-hoar-frost-form). Hoar frosts most
commonly attach themselves to the branches of trees, leaves and grasses, but can also be
seen on objects such as gates and flowerpots. Sometimes the deposits can be so thick that
it may even look like a dusting of snow has fallen
(https://www.rmets.org/metmatters/how-does-hoar-frost-form). Such deposits can
form on runaways and aircrafts.
Rime is a deposit of ice formed by the rapid freezing of supercooled water drops as they
impinge upon an exposed object such as tree’s branches and electric wires that are at a
temperature below the freezing point. Rime is composed of small ice particles with air
pockets between them; this structure causes its typical white appearance and granular
structure. Because of the rapid freezing of each individual supercooled droplet, there is
relatively poor cohesion between the neighboring ice particles, and the deposits may
easily be shattered or removed from objects they form on. Thus, rime is not normally a
serious problem when it forms on the wings or other surfaces of aircraft
(https://www.britannica.com/science/rime-weather).
Glaze is formed on exposed objects by the freezing supercooled water deposited by rain,
drizzle or fog. Glaze is denser, harder, and more transparent than either rime or hoarfrost.
The accretion of glaze on terrestrial objects constitutes an ice storm as a type of aircraft
icing it is called clear ice (https://glossary.ametsoc.org/wiki/Glaze).
Hail and ice pellets are solid precipitation in the form of balls or pieces of ice known as
hailstones. Hail forms in thundercloud when drops of water are continuously taken up
and down though the cloud by updrafts and downdrafts. When they go to the top of the
cloud where it is very cold they freeze. As the updrafts in thunderclouds are very big, they
can keep these hailstones for a long time, so they get larger and larger by becoming
coated with more and more ice. When the hailstones get bigger and heavier, the updrafts
in the cloud cannot hold them up anymore and they fall to earth. Hail can only be formed
in this way, in this type of convective clouds, unlike snow which can also be formed in

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International Air Force Semester

weather fronts, and by ascending up hills and mountains, just like rain can
(https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/hail).
Hail has a diameter of 5 mm to more than 15 cm, while ice pellets have a diameter less
than 5 mm. Because the formation of hail usually requires cumulonimbus or other
convective clouds with strong updrafts, it often accompanies thunderstorms (EASA,
2014).
Snow is a form of precipitation composed of white or translucent ice crystals, chiefly in
complex branch hexagonal form and often agglomerated into snowflakes. Snow forms
when temperatures are low and there is moisture in the atmosphere in the form of tiny
ice crystals in clouds which stick together to become snowflakes. When the snowflakes
become heavy enough, they fall to the ground.
One notable phenomenon when water passes from solid state to liquid state is sleet. This
occur when frozen precipitation partially melts as it falls. Thus, the precipitation at the
moment that reach the earth’s surface is formed both by raindrops and snowflakes. It is
reported in METAR code as RASN.
The process when water passes from solid state directly to gaseous state in the absence
of melting is called sublimation. The opposite of sublimation is deposition, where water
vapor changes directly into ice, such as snowflakes and different types of frost.
Sublimation occurs more readily when certain weather conditions are present, such as
low relative humidity and dry winds. Sublimation also occurs more at higher altitudes,
where the air pressure is less than at lower altitudes. Energy, such as strong sunlight, is
also needed. Without the addition of energy (heat) to the process, ice would not
sublimate into vapor. That is where sunlight plays a large role in the natural world
(https://www.usgs.gov/special-topic/water-science-school/science/sublimation-and-
water-cycle?qt-science_center_objects=0#qt-science_center_objects).

Water in the atmosphere

4.5. End of Chapter Questions

What is the main source of water for the atmosphere?
In which ways can air humidity be reported?
What is the potential evapotranspiration?
Why is it important for a pilot to possess knowledge regarding the water phase
change?
What are the main phenomena associated with water phase transformation?

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4.6. Chapter bibliography

Allaby, M. (2009). Atmosphere: A Scientific History of Air, Weather, and Climate. Infobase
Publishing, 132 West 31st Street, New York NY 10001.

European Aviation Safety Agency (EASA) (2014). ATPL ground training series – Meteorology. CAE
Oxford Aviation Academy.

Giorgini, S. (2016). Lecture notes on statistical mechanics. University of Trento.
Quante, M., Matthias, V. (2006). Water in the Earth’s atmosphere. J. Phys. IV France 139 (2006)

37–61.
Saha, K. (2008). The Earth’s atmosphere. Its physics and dynamics. Springer-Verlag Berlin

Heidelberg. 10.1007/978-3-540-78427-2.
Wallace, J., Hobbs P. (2006) Atmospheric Science. An Introduction Survey (Second Edition).

University of Washington
https://glossary.ametsoc.org/wiki/Glaze
https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/hail
https://www.britannica.com/science/rime-weather
https://www.usgs.gov/special-topic/water-science-school/science/sublimation-and-water-

cycle?qt-science_center_objects=0#qt-science_center_objects
https://www.rmets.org/metmatters/how-does-hoar-frost-form
https://upload.wikimedia.org/wikipedia/commons/3/33/Phase_diagram_of_water_simplified.sv

g

Chapter 5 - Clouds

5.1. Clouds formation processes and their description

Clouds can be seen as the connecting link between water vapor and precipitation.
Precipitation is exclusively produced by clouds, but not all clouds lead to precipitation.
Clouds are the visible evidence for the existence of the liquid or solid phase of water in
the atmosphere. Although clouds on average cover more than 60% of the Earth’s surface,
the amount of water they contain is comparatively small. It accounts for only 0.25–0.3%
of the total water in the atmosphere. Despite this relatively small amount of water, clouds
play a crucial role in the global water cycle. The microphysical processes in clouds
eventually form large cloud particles, which may start falling as rain, snow or hail.
Precipitation is an effective path to bring water from the atmosphere back to the oceans
or land surfaces. Beside this vital role, clouds contribute to the vertical and horizontal
redistribution of water vapor in the atmosphere. As a result of their significance in the
radiation and energy budget of the Earth, in many regions of the globe clouds determine
the rates of evaporation and influence regional and local circulation systems through the
release of latent heat or heating and cooling rates associated with radiative processes.
Substantial requirement for the effective formation of clouds are the water vapor
saturation of the environment and the existence of suited cloud condensation nuclei and
ice nuclei, respectively. Water vapor saturation can be reached in several ways. In most
cases of cloud formation, saturation is the result of lifting of air masses with subsequent
(adiabatic) cooling. Corresponding vertical motions are mainly due to thermal convection
(cumulus, cumulonimbus), active and passive lifting in connection with movements of
frontal systems (cirrostratus, altostratus, nimbostratus), and forced lifting by mountain
ranges (orographic lifting) (Quante and Matthias, 2006).
In general, clouds are classified as low-level, mid-level and high-level clouds and by their
form (stratiform and convective clouds). Another way to classify clouds is the distinction
between precipitating and non-precipitating clouds (Quante and Matthias, 2006).

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Clouds form when water vapor rises, then it cools and condenses, as a part of the water
cycle (Figure 12). The result is clouds, which contain water droplets or ice crystals, taking
different shapes and sizes depending on where the cloud is formed.
Orographic uplift- This type of uplift occurs when air is forced rise, then cool down. If the
cooling process is successful, then the water vapor turns into clouds and precipitation.
Orographic lifting can trigger amplified amounts of precipitaion and widespread
cloudiness on higher ground.
Convectional uplifting happens when the surface of the air is heated at the ground then
it will begin to rise and cool. When this is successful, a cloud forms, due to saturation
(https://sites.google.com/site/gitakrishnareach/cloud-formation-and-types).
Frontal uplifting occurs when two masses of air converge. Usually, the two masses of air
have different temperatures and moistness. For example, one air mass is hot and moist
while the other air mass is cool and dry. Then the cool air mass would act as a vertical wall
and the warm air mass would use the wall to rise and form clouds, and also the warm air
mass cools down during that process
(https://sites.google.com/site/gitakrishnareach/cloud-formation-and-types).
Radiative cooling takes place at night when the Sun is no longer shining and providing the
surface of the ground with energy. Thus, the surface of the earth starts to lose energy and
the air above the surface begins to cool. Fog may begins to form as a result
(https://sites.google.com/site/gitakrishnareach/cloud-formation-and-types).

Figure 12: Cloud formation (source: https://scied.ucar.edu/learning-zone/clouds/how-clouds-form)

Clouds

There are an infinity variety of clouds forms. Luckily, they are not hard to recognize
because they are categorized in a very logical order using a classification scheme. The
scheme is organized into genera, species and varieties
(https://cloudatlas.wmo.int/en/principles-of-cloud-classification.html). Clouds can be
identified based on their appearance, shape, and altitude. The two primary cloud
categories are cumuliform and stratiform clouds.
Cumuliform clouds develop because of vertical motions by atmospheric instability. They
are convective clouds meaning that they form in air parcels that are buoyant and are
undergoing convection, which is the transfer of heat or mixing within a fluid due to warm
air rising and cool air sinking. Examples of cumuliform clouds include cumulus, cumulus
congestus, and cumulonimbus (Nugent et al., 2020).
Stratiform clouds are horizontally layered clouds. They tend to spread into wide regions
and take on an appearance of a sheet or blanket. They typically form when a layer of air
is brought to saturation but is thermodynamically stable, or when a convective cloud
meets a stable layer and spreads out in a layered fashion. Examples of stratiform clouds
include nimbostratus, stratus, altostratus, cirrostratus, and cirrus (Nugent et al., 2020).
By water phase - clouds composed of only liquid water droplets are called “warm clouds”,
and typically have clearly defined edges. Low altitude clouds are usually warm clouds.
Clouds made up of only ice crystals are called “cold clouds” and typically have fuzzy
looking edges. The edges look fuzzy and not well defined because it takes longer to go
from the ice phase to the vapor phase as compared to warm clouds. This transition time
scale results in a larger cloud to dry air transition region. High altitude clouds are always
cold clouds. Clouds composed of liquid water and ice crystals are called “mixed phase
clouds”. It is difficult to distinguish by eye whether a cloud is mixed phase. Often the
decision comes down to the height of the cloud, knowing that clouds that extend from
the surface up high in the atmosphere likely have a mixture of both liquid droplets and ice
crystals (Nugent et al., 2020).
By altitude - clouds at a high altitude have the prefix “cirro” or “cirrus”. Due to the high
altitude, the cirrus and cirrostratus are made of ice crystals. Clouds at mid-altitude have
the prefix “alto”. They are usually made of liquid droplets, but can be a mixture between
liquid droplets and ice crystals. Low level clouds don’t have a particular prefix (Nugent et
al., 2020).
By Characteristics - The prefix “nimbo” or suffix “nimbus” indicates a precipitating cloud.
Nimbostratus usually have light to moderate precipitation, whereas cumulonimbus or
thunderstorm clouds have heavy precipitation and sometimes even hail (Nugent et al.,
2020).
Other types of clouds - the above clouds represent the primary cloud types, but many
other cloud names and cloud types are used in atmospheric sciences. A few of the more
common cloud names are given below.
Lenticular clouds are stationary lens-shaped clouds with a smooth appearance that
usually forms over the summit of a mountain or on the lee wave crest. They are also
referred to as lee-wave clouds or mountain-wave clouds (Nugent et al., 2020).
Mammatus clouds are formed from downward motion, typically in the anvil portion of a
cumulonimbus. They have a distinctive look that makes them easy to spot, especially at
sunset. They are made up of hanging pouches that look a little bit like udders (Nugent et
al., 2020).

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International Air Force Semester

Description of the main types of clouds is as follows:
Cirrus - Cirrus clouds are white, very wispy looking and usually mean fair weather. They're
completely made of ice (Figure 13).
Cirrocumulus - These clouds are white and sometimes gray. They look like small round
puffs similar to cotton balls. They are usually seen in the winter, or during cold weather.
Cirrostratus- These are transparent white fibrous or smooth veil that completely or
partially covers the sky.
Altocumulus- These clouds appear to have one part of the cloud darker than the other,
so they are grayish-white. You will have thunderstorms in the afternoon if you see the
clouds on a humid morning.
Altostratus - These clouds cover the whole sky. They are grayish-blue and form before
storms that are continuous. The Sun (or Moon) can easily shine through an altostratus
cloud. If rain falls from an altostratus cloud and strikes the ground, the cloud will be known
as nimbostratus.
Stratus - Stratus clouds are gray and look like fog. They completely or partially cover the
sky. Some mist or light drizzle usually comes with stratus clouds.
Stratocumulus - Stratocumulus clouds are gray and lumpy. They are low in the sky,
sometimes they will spread out and sometimes they line up in rows. A light rain or drizzle
is associated with these clouds.
Nimbostratus- These are grayish cloud layers, which usually cover the entire celestial
dome. They consist of raindrops, ice crystals or snowflakes
Cumulus - These clouds are puffy white and are low in the sky. They also have flat
bottoms. Cumulus clouds could mean bad of fair weather. It depends on what they look
like.
Cumulonimbus - These clouds have an anvil- like shape. They are linked to heavy
thunderstorms, which is why they are known as "thunderstorm clouds"
(https://sites.google.com/site/gitakrishnareach/cloud-formation-and-types).

Clouds

5.2. Vertical distribution of clouds

Clouds are classified in several ways. The most common classification used today divides
clouds into four separate cloud groups, which are determined by their altitude and if
precipitation is occurring or not.
High-level clouds form from ice crystals where the air is extremely cold and can hold little
water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds.
Cirrocumulus clouds are small, white puffs that ripple across the sky, often in rows. Cirrus
clouds may indicate that a storm is coming
(https://courses.lumenlearning.com/geophysical/chapter/weather-and-atmospheric-
water/).
Middle-level clouds, including altocumulus and altostratus clouds, may be made of water
droplets, ice crystals or both, depending on the air temperatures. Thick and broad
altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean
a large storm, bearing a lot of precipitation, is coming
(https://courses.lumenlearning.com/geophysical/chapter/weather-and-atmospheric-
water/).
Low-level clouds are nearly all water droplets. Stratus, stratocumulus and nimbostratus
clouds are common low clouds. Nimbostratus clouds are thick and dark that produce
precipitation.
Clouds with the prefix cumulo- grow vertically instead of horizontally and have their bases
at low altitude and their tops at high or middle altitude. Clouds grow vertically when
strong unstable air currents are rising upward. Common clouds include cumulus humilis,
cumulus mediocris, cumulus congestus, and cumulonimbus
(https://courses.lumenlearning.com/geophysical/chapter/weather-and-atmospheric-
water/).
The main types of clouds and their vertical distribution are presented in table 1 and figure
13.

Table 1: The main types of clouds (source: https://cloudatlas.wmo.int/en/cloud-classification-
summary.html)

Species Varieties Supplementary Accessory Mother-clouds and special clouds
features clouds (most commonly occurring
Genera
mother-clouds are listed in the
same order as genera)

(listed by frequency of observation) Genitus Mutatus
Cirrocumulus Cirrostratus
Cirrus fibratus intortus mamma Altocumulus
Cirrocumulus Cumulonimbus Homo
Cirrostratus uncinus radiatus fluctus Homo

spissatus vertebratus

castellanus duplicatus

floccus

stratiformis undulatus virga - Cirrus
Cirrostratus
lenticularis lacunosus mamma Cirrocumulus Altocumulus
Cumulonimbus Homo
castellanus cavum Cirrus
Cirrocumulus
floccus

fibratus duplicatus -

nebulosus undulatus

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International Air Force Semester

Altocumulus stratiformis translucidus virga Cumulus Altostratus
lenticularis perlucidus mamma Cumulonimbus Homo
Altostratus castellanus opacus cavum pannus Cirrocumulus
Nimbostratus floccus duplicatus fluctus Altostratus
Stratocumulus volutus undulatus asperitas pannus Nimbostratus
radiatus Stratocumulus
Stratus - lacunosus virga
translucidus praecipitatio Altocumulus Cirrostratus
- opacus mamma Cumulonimbus Nimbostratus
duplicatus pileus
stratiformis undulatus praecipitatio velum Cumulus Altocumulus
lenticularis radiatus virga pannus Cumulonimbus Altostratus
castellanus - pannus Stratocumulus
floccus virga pileus Altostratus Altocumulus
volutus translucidus mamma velum Nimbostratus Nimbostratus
perlucidus praecipitatio flumen Cumulus Stratus
nebulosus opacus fluctus Cumulonimbus
fractus duplicatus asperitas
undulatus cavum Nimbostratus Stratocumulus
radiatus Cumulus
lacunosus praecipitatio Cumulonimbus Stratocumulus
opacus fluctus Homo Stratus
translucidus Silva Cumulus
undulatus Cataracta
Altocumulus
Cumulus humilis radiatus virga Stratocumulus
mediocris praecipitatio Flamma
congestus arcus Homo
fractus fluctus Cataracta
tuba Altocumulus
Cumulonimbus calvus - praecipitatio Altostratus
capillatus virga Nimbostratus
incus Stratocumulus
mamma Cumulus
arcus Flamma
murus Homo
cauda
tuba

Clouds

Figure 13: The vertical distribution of the main types of clouds (source:
https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/clouds.htm)

In aviation meteorology, nebulosity is the extent to which the sky is covered with clouds
and it is expressed in octas. For example, when the sky is totally covered with clouds the
expression 8/8 is used and coded as OVC in METARs.

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International Air Force Semester

5.3. Significant clouds for flights

By far, cumulonimbus are the most significant clouds for any type of flight (Figure 14).
They are associated with icing, thunder, hail, turbulences, heavy rain or snow, showers,
strong winds, wind shear, strong updrafts and downdrafts, and even tornadoes. This type
of cloud should always be avoided when flying. Updrafts and downdrafts and the
associated turbulences may result in dangerous situations such as difficulty in maintaining
altitude, airspeed, and attitude control of the aircraft. The top of these clouds represents
the greatest threat and potential for icing, even short encounters may result in large ice
build on the exterior aircraft components.

Figure 14: Cumulonimbus cloud (source: https://cloudatlas.wmo.int/en/cumulonimbus-cb.html)

Cumulonimbus are heavy and dense clouds, with a considerable vertical extent, in the
form of a mountain or huge towers. They can grow up to 10 km high. At least part of its
upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this
part often spreads out in the shape of an anvil or vast plume. Under the base of this cloud,
which is often very dark, there are frequently low, ragged clouds, either merged with it or
not, and precipitation sometimes in the form of virga
(https://cloudatlas.wmo.int/en/cumulonimbus-cb.html).
Cumulus clouds are associated with turbulence, icing and wind shear. Flying beneath a
cumulus cloud, aircraft may experience turbulence associated with downdrafts and wind
shear. These hazards are particularly exacerbated in the case of developing towering
cumulus from cumulus clouds. Generally, the bigger and taller a cumuliform cloud is, the
more danger poses to flights. A continuous weather frame will sometimes reveal the
transformation from Cumulus to Towering Cumulus and to Cumulonimbus.
Towering cumulus clouds have a considerable vertical extent (Figure 15). Their bulging
upper part frequently resembles a cauliflower. The sunlit parts of these clouds are mostly
brilliant white due to sunlight reflection; their base is relatively dark due to sunlight
blocking, and nearly horizontal. Associated hazards to these clouds are showers of rain,
snow, or snow pellets. In the tropics, they often release abundant rain in the form of
showers.
These clouds have the potential to turn into stormy weather as they can be a precursor
to cumulonimbus clouds.