Clouds
Figure 15: Towering cumulus clouds
Stratocumulus clouds can hide embedded thunderstorms, or even terrain such as
mountains.
Lenticular, or lens-shaped, clouds form under a very particular set of circumstances that
are of interest to pilots. The standing lenticular cloud is a stationary cloud that forms on
top of mountains. When strong winds hit the mountain, they are forced upward by the
terrain. The air cools and creates a cloud that caps the ridge. Beautiful lenticular clouds
look very peaceful, but to pilots, they indicate strong wind and turbulence. Pilots know to
avoid flying in these areas (https://aerocorner.com/blog/types-of-clouds-in-aviation/).
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In METARs only Cumulonimbus (Cb) and Towering cumulus (TCU) are reported.
Figure 16 gives an idea of the widespread distribution of clouds in Earth’s atmosphere and
is an example of the unique views of clouds that satellites can provide. Two days of data
from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra
spacecraft were combined to produce this view of clouds over the whole Earth.
Figure 16: Distribution of clouds in Earth’s atmosphere (Image Credit: Reto Stöckli) (source:
https://cloudsat.atmos.colostate.edu/FactSheet_The_Importance_of_Understanding_Clouds.pdf)
Clouds
5.4. Stability and instability of the atmosphere
One way to define atmospheric stability is the resistance of the atmosphere to vertical
motion of an air mass. The air is unstable when the temperature of a small mass or parcel
of air decreases as the air expands upwards. Rising air encounters lower pressures in the
surrounding air, permitting it to expand. The energy required for expansion comes from
the heat energy in the rising air. Consequently, the temperature of the rising air lowers
(https://www.nwcg.gov/publications/pms425-1/atmospheric-stability).
In stable air, vertical motion is inhibited and if clouds are formed, they will have a shallow
and stratiform structure (e.g. stratus, nimbostratus), while turbulent motion is weak. For
these reasons, stable conditions are often characterized by low visibility or even fog.
When the air is unstable vertical motion occurs and often cumuliform clouds are formed
such as Cumulus, Towering cumulus and Cumulonimbus, in this way often associating
unstable conditions with thunderstorms. The atmosphere becomes unstable when a
parcel of air starts freely to rise (Figure 17). The raising of the parcel is determined by air
pressure and temperature. As a parcel of air rises it cools and the moisture content
condenses forming clouds. The rate of temperature change with height is 6.5 °C km-1 (or -
2 °C/1000 ft) known as the standard lapse rate. However, in an unstable air the rate of
temperature change with altitude has higher (absolute) values. On the other hand, when
the normal lapse rate of temperature (environmental lapse rate) is lower than the dry
adiabatic rate (=-9.8°C km-1 or -3°C/1000ft), the air parcel being colder than its
environment descends and becomes stable. Thus, in other words, stability can occur when
dry adiabatic lapse rate of an ascending dry air parcel is higher than the normal lapse rate
and if it is not saturated and does not attain dew point it becomes colder than the
surrounding air at certain height and thus it becomes heavier and descends. This process
causes stability of atmospheric motion due to which vertical motion of air is supressed
(https://www.geographynotes.com/atmosphere/stability-and-instability-of-the-
atmosphere-precipitation-geography/2800).
On the other side instability occurs when normal lapse rate is lower than the dry adiabatic
lapse rate of ascending parcel of air. In this case the rising air continues to rise upward
and expand and thus becomes unstable being in unstable equilibrium
(https://www.geographynotes.com/atmosphere/stability-and-instability-of-the-
atmosphere-precipitation-geography/2800).
Meteorologists measure the stability of the atmosphere by measuring the air
temperature at various heights. If the temperature lapse rate is higher than normal, then
the atmosphere is unstable. But if the lapse rate is small, meaning there's relatively little
change in temperature with heights, it's a good indication of a stable atmosphere. The
most stable conditions occur during a temperature inversion when temperature increases
(rather than decreases) with height (https://www.thoughtco.com/atmospheric-stability-
and-storms-3444170). However, if you don’t have access to such data, the clearest way
to observe the atmospheric stability or instability is to look at the sky: a stable atmosphere
is indicated by the clear sky or sky with flat layers of clouds; opposite, an unstable
atmosphere is indicated by cumuliform clouds. The tallest the clouds are, the instability is
more accentuated. If the cumuliform clouds grow quickly, they often produce heavy rain
showers. If they grow very tall the ice is forming in the upper levels of the clouds giving a
fluffy, bright white appearance to what is called the anvil cloud-head. Thunder and
lightning are often observed when these clouds, known as cumulonimbus, are formed.
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It is the vertical profile of temperature, or lapse rate of the atmosphere, which determines
whether an air mass is stable or not. The temperature can be measured using an
electronic thermometer attached to a helium-filled weather balloon released from the
ground. As it ascends, the readings are transmitted back to earth and, under normal
circumstances, the temperature would be found to fall with height. But it does not always
fall at the same lapse rate. If it falls rapidly with height, then the atmosphere is said to be
unstable; if it falls more slowly (or even temporarily increases with height) then a stable
atmosphere is present (https://www.rmets.org/metmatters/when-air-stable-or-
unstable).
There are several mechanisms by which air starts rising and becomes instable. The main
mechanisms include convection - the air above a heated surface becomes warmer and
less dense which causes it to rise; advection over coastal, hilly or mountainous terrain
which force air to rise; convergence of two air flows with different temperatures from
different directions; and weather fronts.
It is important to monitor the degree of instability of the atmosphere because it warns of
severe weather for aviation.
Instability in the atmosphere is a concept that is intimately connected with
thunderstorms, cumulus development, and vertical motion (Nugent et al., 2020). On the
other hand, stability is associated with suppressed vertical motion and thus ceased
pollutant dispersion as well as low level stratiform clouds, continuous precipitation and
thus low visibilities and fog formation. Moisture in the atmosphere, clouds, precipitation,
and many other weather phenomena are directly related to these adiabatic responses of
air to lifting and sinking (https://www.nwcg.gov/publications/pms425-1/atmospheric-
stability).
Clouds
Figure 17: Simplified scheme of the atmospheric stability (source: (Nugent et al., 2020)
5.5. End of Chapter Questions
How can clouds be classified?
Which clouds are significant for aviation?
What are the main characteristics of Cumulonimbus clouds?
What is nebulosity in aviation meteorology?
What is atmospheric instability?
How can the stability of the atmosphere be determined?
What weather is associated with stable and unstable conditions that are important
for aviation?
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5.6. Chapter bibliography
Nugent Alison, DeCou David, Russell Shintaro, Karamperidou Christina, Griswold Jennifer Small,
2020. Atmospheric Science: ATMO 200. University of Hawaiʻi at Mānoa
Quante, M., Matthias, V. (2006). Water in the Earth’s atmosphere. J. Phys. IV France 139 (2006)
37–61.
https://sites.google.com/site/gitakrishnareach/cloud-formation-and-types
https://scied.ucar.edu/learning-zone/clouds/how-clouds-form
https://cloudsat.atmos.colostate.edu/FactSheet_The_Importance_of_Understanding_Clouds.pd
f
https://courses.lumenlearning.com/geophysical/chapter/weather-and-atmospheric-water/
https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/clouds.h
tm
https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/clouds.h
tm
https://cloudatlas.wmo.int/en/cumulonimbus-cb.html
https://cloudatlas.wmo.int/en/principles-of-cloud-classification.html
https://cloudatlas.wmo.int/en/cloud-classification-summary.html
https://aerocorner.com/blog/types-of-clouds-in-aviation/
https://www.nwcg.gov/publications/pms425-1/atmospheric-stability
https://www.geographynotes.com/atmosphere/stability-and-instability-of-the-atmosphere-
precipitation-geography/2800
https://www.thoughtco.com/atmospheric-stability-and-storms-3444170
https://www.rmets.org/metmatters/when-air-stable-or-unstable
https://www.nwcg.gov/publications/pms425-1/atmospheric-stability
Chapter 6 - Precipitation
6.1. Overview and development of precipitation
Precipitation is water falling from clouds (https://cloudatlas.wmo.int/en/rain.html).Via
precipitation, the water, which originally evaporated at ground level, is brought back from
the atmosphere to the Earth’s surface. Precipitation includes rain and snow (under
different forms and intensities), sleet, graupel, and hail. Although precipitation and its
distribution in space and time is essential for life on Earth, the cloud processes leading to
precipitation size particles are not known in full detail. The description of the relevant
microphysics and related modelling activities are one of the major tasks of cloud physics.
In the center of interest are the growth processes, which lead to particle sizes large
enough to reach the ground before they evaporate. In the case of water droplets, particles
with radii larger than 0.1 mm are formally called rain drops (Quante and Matthias, 2006).
The size distribution and number concentration of cloud particles play an essential role
during the formation process, as does the vertical wind component (updrafts). Also, the
temperature at cloud level plays a crucial role, as it determines the phase of cloud
particles. In pure water clouds, precipitation formation results from coalescence (merging
of water droplets of typically different sizes after collision) which is favored by differing
fall velocities. In mixed phase clouds, consisting of supercooled liquid droplets and ice
crystals, ice crystals acquire water molecules from nearby supercooled water droplets. As
these ice crystals gain mass they may begin to fall, acquiring more mass as coalescence
occurs between the crystal and neighboring water droplets. The resulting precipitation
can reach the ground either in liquid or solid phase depending on the local atmospheric
conditions. Precipitation from pure ice clouds is the result of ice crystal growth by
sublimation of water vapor and by aggregation.
The precipitation efficiency of clouds, on average, is in the order of 30%, thus only the
minor part of the cloud water is transferred to precipitation. It should also be mentioned
that a non-negligible fraction of particles falling from clouds evaporate before they reach
the surface (Quante and Matthias, 2006).
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In general, related to the external drivers supporting the formation of precipitation, this
is distinguished according to convective, stratiform, and orographic precipitation.
Stratiform precipitation compared to convective precipitation is typically covering larger
areas and has a much longer duration and is generally occurring with frontal systems.
Convective precipitation commonly falls as showers, with rapidly change of intensity,
which can reach high values. It occurs over smaller areas, as convective clouds have
limited horizontal extent and lasts for short periods (Quante and Matthias, 2006).
The spatiotemporal distribution of precipitation shows tremendous variation, that is
caused by or largely attributable to the general circulation, the temperature distribution,
the non-uniform land-ocean distribution, and orographic conditions. This has a large
impact on natural environment and social activities (Quante and Matthias, 2006).
The spatial distribution of precipitation occurrence is correlated with the distribution of
cloudiness; however, this is not necessarily true for the amount of precipitation. Although,
large inhomogeneities exist in the annual mean of precipitation spatial distribution this is
generally decreasing from equator towards poles and from higher to lower altitudes.
About two-thirds of global precipitation falls between 30◦N and 30◦S latitudes. The
highest quantities of precipitation are related to the intense convection in the
Intertropical Convergence Zone and the Southern Pacific Convergence Zone (Quante and
Matthias, 2006).
A secondary maximum in precipitation quantities occurs over the moderate latitudes in
both northern and southern hemispheres (23°26'22" and 66°33'39") along the tracks of
the extratropical cyclones, where substantial amounts are produced by their frontal
systems.
Opposite to areas with abundant precipitation, extremely dry regions can be found in the
subtropical latitudes which are under the influence of large, almost permanent,
anticyclones. Huge parts of the subtropical continents, such as Africa, Asia and Australia,
are covered by deserts, where precipitation is very rare. Over the polar regions the water
vapor content of the atmosphere is extremely low and accordingly the amounts of
precipitation are typically very low with annual amounts less than 200 mm per year
(Quante and Matthias, 2006).
Precipitation
6.2. Types of precipitation
As it has been mentioned in the previous sub-section the most common types of
precipitation are rain, hail, graupel, sleet and snow. Rain falls to Earth’s surface in various
forms. Drizzle is a fine sprinkle of tiny water droplets of size less than 0.5 mm and intensity
greater than 1 mm/h. The tiny drops forming a drizzle appear to float in the air.
Occurrence of drizzle is often accompanied by fog.
Rain is the form of liquid precipitation that has the size of the droplets larger than 0.5 mm.
It forms as a result of coalescence of two or more water particles which merge and form
a single droplet. Raindrops rarely exceed 6 mm in diameter because they become
unstable when become larger than this and break up during their fall. When raindrops fall
through a cold layer of air colder than 0 °C and become supercooled, freezing rain can
occur. The drops may freeze on impact with the ground to form a layer of glazed ice that
is difficult to see because it is almost transparent
(https://cloudatlas.wmo.int/en/supercooled-rain.html).
Hail and graupel are a type of solid precipitation in the form of pellets or lumps. The size
of graupel is 5 mm or less, while that of hail is greater than 5 mm. Hail is reported in
METAR code as GR, while graupel as GS.
Sleet occurs when frozen precipitation partially melts as it falls. Thus, the precipitation at
the moment that reaches the earth’s surface is formed both by raindrops and snowflakes.
It is reported in METAR code as RASN.
Snow begins to form when atmospheric water vapor starts to freeze into crystals.
Temperature, humidity, and wind all help to create snow and a wide variety of snowflakes.
Thus, snow forms when the temperature in the clouds is below freezing. If temperatures
are above freezing on the ground, snow will fall, but will quickly melt when it hits the
Earth’s surface. The size of snowflakes varies from one millimeter to a few centimeters
depending on air temperature.
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6.3. Relationship with cloud types
Different types of cloud may generate different types of precipitation. Some clouds do
not generate precipitation. The highest clouds in the troposphere (cirrus, cirrocumulus
and cirrostratus) do not generate precipitation. Middle level clouds which are usually
situated between 2.5 and 6 km can generate precipitation. Thus, altocumulus may
produce light showers. Altostratus can generate precipitation in the form of rain or snow.
Vertical or inclined trails of precipitation (fallstreaks) attached to the under surface of a
cloud that do not reach the Earth’s surface due to evaporation is known as virga.
Nimbostratus usually generates large, but non intense quantities of rain or snow. Low
level clouds (below 2.5 km) can generate drizzle, showers of rain or snow. More
specifically, stratus and stratocumulus clouds can produce drizzle, while cumulus,
cumulus congestus and cumulonimbus can precipitate in the form of showers of rain and
snow. Cumulonimbus clouds can also generate hail and graupel.
Precipitation
6.4. End of Chapter Questions
Why is air temperature important for clouds formation?
Which are the main differences between stratiform precipitation and convective
precipitation?
What are the main causes of the spatiotemporal variability of precipitation?
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6.5. Chapter bibliography
Quante, M., Matthias, V. (2006). Water in the Earth’s atmosphere. J. Phys. IV France 139 (2006)
37–61.
https://cloudatlas.wmo.int/en/supercooled-rain.html
https://cloudatlas.wmo.int/en/rain.html
Chapter 7 - Atmospheric pressure
7.1. General references to atmospheric pressure
Pressure of the atmosphere at any level is defined as the weight of the overlying column
of air per unit area of the surface at that level. Atmospheric pressure can be measured
with a mercury barometer, which indicates the height of a column of mercury that exactly
balances the weight of the column of atmosphere over the barometer. Atmospheric
pressure can also be measured using an aneroid barometer or with a series of sensors
placed on the ground or on balloons and drones.
Atmospheric pressure is expressed in several different systems of units: millimeters (or
inches) of mercury, pounds per square inch, dynes per square centimeter, millibars (mb),
standard atmospheres, or hectopascals.
Standard sea-level pressure equals 760 mm (29.92 inches) of mercury, 14.70 pounds per
square inch, 1013.25 millibars or one standard atmosphere. The small variations in
pressure that do exist largely determine the wind and storm patterns of Earth. (EASA,
2014).
Near Earth’s surface the pressure decreases with height at a rate of about 3.5 millibars for
every 30 meters (approximately 100 feet). However, over cold air the decrease in pressure
can be much abrupt because the air density is greater than is in a warmer air.
The uneven topography of the Earth’s surface imposes difficulties for determination of
mean sea level pressure at high elevation stations. The mean sea level pressure at high
elevation locations is usually obtained by reducing the observed surface pressure to mean
sea level with the aid of the hydrostatic approximation. The main issue lies in finding the
density distribution in the imaginary air column between the station level and the mean
sea level at the location. A mean value of density is usually assumed, based on the values
of pressure and temperature at the station and the nearest mean sea level or low-level
station. The procedure applied may often lead to large errors at high mountain stations.
In aviation, specific terms such as QFE, QNH and QFF are used to describe the air pressure
according to different applications. QFE is the pressure at the station (or aerodrome) level.
QNH is the pressure at mean sea level, reduced from QFE by applying corrections
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according to the standard atmosphere agreed by the International Civil Organization
(ICAO). QFF is the pressure corrected to mean sea level, considering the actual
temperature conditions.
Considerable fluctuations of atmospheric pressure may occur near mountains,
particularly in high winds. This will lead to erroneous readings of an aneroid altimeter.
Consequently, the pressure will be higher on the windward side of the mountain and
lower on the leeward side. This effect is taken into account by the pilots.
Atmospheric pressure
7.2. Isobars; horizontal and vertical variation of atmospheric
pressure; atmospheric pressure systems
Pressure readings measured at different weather stations (after making corrections
according to temperature) at the same time can be plotted on a weather chart.
Meteorologists then draw lines to show places with the same pressure. These lines are
called isobars. Isobars are usually drawn at 2 or 4 hPa intervals. However, to represent
spatial distribution of air pressure on large areas or global scale isobars are drawn at 5
hPa or more intervals. Isobars are useful in revealing the surface pressure pattern.
The distribution of atmospheric pressure over the globe is known as horizontal
distribution of pressure. It is represented on pressure maps with the help of isobars. The
variables which are responsible for variation in the horizontal distribution of atmospheric
pressure are air temperature, the presence of water vapor and the Earth’s rotation. In
figures 18 and 19 is presented the horizontal distribution of air pressure across the Globe
in January and July, respectively.
Figure 18. Horizontal distribution of air pressure in January (Povară, 2004)
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International Air Force Semester
Figure 19. Horizontal distribution of air pressure in July (Povară, 2004)
When the air pressure in a region is lower than its surrounding, the feature is called a
depression, a cyclone, or an area of low-pressure. In the northern (southern) hemisphere,
the air around a depression moves in a counterclockwise (clockwise) direction (looking
from above). Near the Earth's surface, however, friction tends to cause the air to move
slightly inward across the isobars. As it moves into an area of low pressure, the air has
nowhere to go but up. A low-pressure area is usually associated with unstable weather.
When the air pressure in a region is higher than its surrounding, it is called an anticyclone
or an area of high pressure. In the northern (southern) hemisphere, the air around an
anticyclone moves in a clockwise (counterclockwise) direction. In an anticyclone, friction
tends to cause the air to move slightly outward across the isobars near the Earth's surface.
In an area of high pressure, the air is generally stable and is usually associated with fine
weather.
A ridge of high pressure is an elongated area of high pressure. Depending of its
configuration, sometimes it is represented by a ridge line on the weather chart. The
pressure there is higher than that at neighboring points on either side of the ridge line.
A trough of low pressure is an extended area of low pressure. On the weather charts, it is
represented by a trough line. The pressure there is lower than that at neighboring points
on either side of the trough line https://www.hko.gov.hk/en/education/weather/wind-
and-pressure/00117-introduction-to-air-pressure-part-ii.html.
Atmospheric pressure
7.3. Altimetry
Firstly, the definition of the following terms in relation to aviation is necessary to
understand how they operate.
• Height is the vertical distance above a specified datum, usually ground level.
• Elevation is the vertical distance above mean sea level of a point on the earth's
surface.
• Altitude is the vertical distance above mean sea level.
An aircraft’s altimeter uses a similar operating principle to an aneroid barometer. On an
altimeter, the pilot will set a known pressure in the barometric subscale (for a particular
datum), and the pointers will indicate the height above that datum. The QNH pressure
settings are used. The QNH pressure setting is the mean sea level pressure for a location
or area that has been calculated. An altimeter set to QNH while an aircraft is on the ground
will indicate the height of the aircraft above sea level, or in other words, the aerodrome's
elevation. When an accurate local QNH is available at an aerodrome, the pilot is required
to perform a check of the accuracy of the altimeter prior to departure by comparing the
known aerodrome elevation with that displayed on the altimeter.
During a flight the QNH will indicate the aircraft's altitude. The height above mean sea
level is 1013.25 hPa. An altimeter with this pressure setting will indicate the aircraft height
above the 1013.25 hPa level. This is known as pressure altitude. The 1013.25 pressure
setting is sometimes referred to as QNE.
Pressure altitudes above 10,000 feet are generally quoted in hundreds of feet and called
flight levels. For example, the pressure altitude of 12,500 feet is FL125 (read as flight level
one two five); 41,000 feet is FL410 (read as flight level four one zero).
QFE is the station level pressure, or more simply the pressure at ground level. An altimeter
set to QFE will read zero when the aircraft is on the runway. In flight, the QFE setting will
indicate the approximate height of the aircraft above the aerodrome. QFE is generally
used for local operations such as performing low-level aerobatics or crop dusting.
To obtain the correct altitude, an accurate QNH must be entered on the barometric
subscale on the altimeter. An error of just 1 hPa entered on the subscale will result with
a 30 feet error in the height indicated on the altimeter. If the pilot of an inbound aircraft
incorrectly sets a much higher QNH, the altimeter would read higher altitude than the
actual one. If the aircraft is operating close to terrain without visual reference to the
surrounds, the consequences could be disastrous. Thus, accurate pressure information is
essential for the safety of aircraft operations.
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7.4. The effect of pressure, air temperature and humidity on air
density and how this affects aircraft performances
Pressure, air temperature and humidity have all an impact on air density. Air pressure and
temperature determine the air density and in turn determine the lift of the aircraft. An
aircraft must fly faster to maintain the height when density of air is reduced. This faster
speed induces a greater drag that must be equaled by the engine thrust. Lower density
implies grater thrust, then greater will be the fuel consumption.
Another effect of small air density is a decrease in engine power and in the climbing power
of the aircraft. If the air density falls below a certain value, it may be necessary to reduce
gross weight of the aircraft.
Altitude and weather systems can change the air’s pressure. As you go higher, the air’s
pressure decreases from around 1013 millibars at sea level to 500 millibars at around
18,000 feet. At 100,000 feet above sea level the air’s pressure is only about 10 millibars.
Weather systems that bring higher or lower air pressure also affect the air’s density, but
not nearly as much as altitude variations. The air density is smallest at a high elevation on
a hot day when the atmospheric pressure is low. Contrary, the air density is greater at low
elevations when the atmospheric pressure is high, and the temperature is low.
Atmospheric pressure
7.5. End of Chapter Questions
What is the decrease rate of atmospheric pressure with altitude decrease?
What is QFE referring to?
What is QNH referring to?
What is QFF referring to?
What is a cyclone?
What is an anticyclone?
What weather conditions are associated with cyclones?
What weather conditions are associated with anticyclones?
What are the main principles of altimetry? What are the potential errors
associated with temperature and pressure?
An aircraft is flying at 4500 feet indicated with the altimeter subscale set to 1020
mb towards a mountain range with an elevation of 1500 feet. If during the flight
the QNH in the area falls to 989 mb and the altimeter subscale is not reset, the
expected clearance over the mountain range will be: _____ (assume 27 feet = 1
mb)
The altimeter subscale is set to 1027 mbs and the altimeter reads 4300’. QNH is
995 mbs. What is the altitude of the aircraft? (Assume 1 mb = 27 feet)
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7.6. Chapter bibliography
European Aviation Safety Agency (EASA) (2014). ATPL ground training series – Meteorology. CAE
Oxford Aviation Academy.
Povară, R. (2004). Climatologie generală. Editura Fundaţiei România de Mâine, Bucureşti.
https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00117-introduction-to-air-
pressure-part-ii.html
Chapter 8 - Wind
8.1. Large-scale and local winds
Wind is defined as a horizontal movement of air from one place to another. It is caused
by the differences in air pressure within the atmosphere. Air under high-pressure systems
moves towards areas of low-pressure systems. The greater the differences are between
the two pressure systems, the higher the speed of wind is. Thus, wind is strongly related
to horizontal air pressure gradient.
Large scale winds take place over large areas or even on a global scale. The Sun’s rays
reach the Earth’s surface in polar regions at a much more slanted angle than at equatorial
regions. This sets up a substantial temperature difference between the hot equator and
cold poles. Therefore, the heated air rises at the equator leading to low pressure, whilst
the cold air sinks above the poles, leading to high pressure. This pressure difference sets
up a global wind circulation as the cold polar air tends to move southwards to replace the
rising tropical air. However, this is intricated by the Earth’s rotation known as the Coriolis
effect which deviates the movement of the air.
Air that has risen at the equator moves poleward at higher levels in the atmosphere then
cools and sinks at around 30 degrees latitude north and south. This leads to high pressure
belts in the subtropics. The sinking process warms and dries the air forming the largest
desserts around the globe. The sinking air in the subtropics spreads out at the Earth’s
surface – some of it returning southwards towards the low pressure is combined by an
easterly drift at the equator known as trade winds. Another fraction of this air moves
poleward and meets the cold air spreading towards equator from the Arctic and Antarctic.
As the warm air is less dense than the polar air it tends to rise over it; this rising motion
generates low pressure systems which generally bring wind and rain. This part of the
global circulation is known as the mid-latitude cell.
The Coriolis effect from the earth’s rotation deflect the movement of air to the right in
the northern hemisphere and to the left in southern meaning that air does not flow in
straight line from high to low pressure. This gives the prevailing west to southwesterly
winds across the Europe.
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As atmosphere continuously aims to reach a state of equilibrium, large-scale motion is the
main driver which balances the energy deficit at the poles and the excess at the equator.
This is the basis of the global atmospheric circulation.
Local winds however are occurring over limited areas, typically several tens to a few
hundreds of kilometers. They also tend to last for a short period, typically several hours
to a day. They blow between small-scale low- and high-pressure systems. They are highly
influenced by local physico-geographical factors such as proximity to large bodies of water
or mountain ranges. Local winds can significantly affect the weather and climate of a
region. The main types of local winds are sea breezes and land breezes, katabatic winds,
chinook winds, mountain waves, (https://www.metlink.org/resource/local-winds/).
Sea and land breezes are determined by the specific heat difference between land and
water. The ocean’s thermal inertia is much higher than land’s. Thus, the water surface is
cooler than the land in the daytime. It is also cooler than the land in the summer. On the
contrary, the water stays warmer than the land during the night and the winter. These
differences in heating and cooling cause local winds known as land and sea breezes.
A sea breeze blows from sea to land during the day or in summer when air over the land
is warmer than air over the water. When the warm air rises above the land the cool air
over the water flows in to take its place.
A land breeze blows from land to sea during the night or in winter when air over the water
is warmer than air over the land. The warm air above water rises and cool air from the
land flows out to take its place.
Monsoons are comparable to land and sea breezes, but on a larger scale. They occur
because of seasonal changes in the temperature of land and water. In the winter, they
blow from land to water. In the summer, they blow from water to land. In regions that
experience monsoons, the seawater offshore is extremely warm. The hot air absorbs a lot
of moisture and carries it over the land. Summer monsoons bring heavy rains on land.
Monsoons occur in several places around the globe. The main region where monsoons
occur is the southern Asia.
Mountain and valley breezes, known as anabatic winds, are determined by the fact that
the air on a mountain slope warms up more than the air over the nearby valley. The warm
air rises and brings cool air up from below. This is a valley breeze. At night the mountain
slope cools more than the air over the valley. The air flows downhill creating a mountain
breeze.
Katabatic winds move the same way as mountain and valley breezes. However, they are
much stronger. Katabatic winds form over a high plateau that is surrounded by mountains.
In winter, the plateau grows cold. Air sinks through the gaps in the mountains.
Föhn winds occur when air is forced over a mountain range. Warm air rises over mountain
range because it is pushed eastward by the westerly winds. The air cools as it rises and
precipitates becoming dry. Afterwards, it sinks down the far side of the mountains and
may create strong winds. These Föhn winds are relatively warm. If there is snow, the
winds may melt it quickly. The dry sinking air creates a rain shadow effect. Rain shadow
effect is responsible for many of the world's deserts.
As air rises over a mountain it cools and loses moisture. The air warms by compression on
the leeward side. The resulting warm and dry winds are Föhn winds. The leeward side of
the mountain experiences rainshadow effect
(https://k12.libretexts.org/Bookshelves/Science_and_Technology/Earth_Science/10%3A
_Atmospheric_Processes/10.19%3A_Local_Winds).
Wind
Air flowing across a mountain range usually rises relatively smoothly up the slope of the
range, but, once over the top, it pours down the other side with considerable force,
bouncing up and down, creating eddies, turbulence and powerful vertical waves that may
extend for great distances downwind of the mountain range. This phenomenon is known
as a mountain wave. If the air mass has a high moisture content, clouds of very distinctive
appearance will develop. Thus, orographic lift causes a cloud to form along the top of the
ridge starting from the windward side of the mountain. The wind carries this cloud down
along the leeward slope where it dissipates through adiabatic heating. The base of this
cloud lies near or below the peaks of the ridge. The top may reach a few thousand feet
above the peaks. Lenticular clouds form in the wave crests aloft and lie in bands that may
extend to well above 40,000 feet. Rotor Clouds form in the rolling eddies downstream.
They resemble a long line of stratocumulus clouds, the bases of which lie below the
mountain peaks and the tops of which may reach to a considerable height above the
peaks. Occasionally these clouds develop into thunderstorms.
Mountain waves present problems to pilots for several reasons. Vertical currents can
commonly generate downdrafts of 2000 feet per minute and downdrafts as great as 5000
feet per minute have been reported. They occur along the downward slope and are most
severe at a height equal to that of the summit. An airplane, caught in a downdraft, could
be forced to the ground. Turbulence is usually extremely severe in the air layer between
the ground and the tops of the rotor clouds. Wind shear can also form in a mountain wave.
The wind speed varies dramatically between the crests and troughs of the waves. It is
usually most severe in the wave nearest the mountain range. Altimeter can give
erroneous values. The increase in wind speed results in an accompanying decrease in
pressure, which in turn affects the accuracy of the pressure altimeter. Icing can also be
present in mountain waves. The freezing level varies considerably from crest to trough.
Severe icing can occur because of the large supercooled droplets sustained in the strong
vertical currents
(https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/Wx_Terms/Flight_En
vironment.htm).
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International Air Force Semester
8.2. Upper atmosphere winds
All weather forecasts stem from our understanding of the upper air where weather
patterns such as ridges, troughs, upper air disturbances and upper-lows occur and where
they are moving (https://www.weather.gov/jetstream/upperair_intro). The flow of air
around the globe is greatest in the higher altitudes, or upper levels. Upper level airflow
occurs in wavelike currents that may exist for several days before dissipating. Upper level
wind speeds generally occur on the order of tens of meters per second and vary with
height. The characteristics of upper level wind systems vary according to season and
latitude and to some extent hemisphere and year. Wind speeds are strongest in the
midlatitudes near the tropopause and in the mesosphere (EASA, 2014).
Upper level wind systems, like all wind systems, may consist of uniform flow, rotational
flow (with cyclonic or anticyclonic curvature), convergent or divergent flow (in which the
horizontal area of masses of air shrinks or expands), and deformation (by which the
horizontal area of air masses remains constant while experiencing a change in shape).
Upper level wind systems in the midlatitudes tend to have a strong component of uniform
flow from west to east (“westerly” flow), though this flow may change during the summer.
A series of cyclonic and anticyclonic vortices superimposed on the uniform west-to-east
flow make up a wave train (a succession of waves occurring at periodic intervals). The
waves are called Rossby waves after Swedish American meteorologist C.G. Rossby, who
first explained fundamental aspects of their behavior in the 1930s. Waves whose
wavelengths are about 6,000 km or less are called short waves, while those with longer
wavelengths are called long waves. In addition, short waves progress in the same direction
as the mean airflow, which is from west to east in the midlatitudes; long waves retrogress
(move in the opposite direction of the mean flow). Although the undulating current of air
is composed of several waves of varying wavelength, the dominant wavelength is usually
around several thousand kilometers. Near and underneath the tropopause, regions of
divergence are found over regions of rising air at the surface, while regions of convergence
aloft are found over regions of sinking air below. These regions are usually much more
difficult to detect than the regions of rotational and uniform flow. While the horizontal
wind speed is typically in the range of 10–50 meters per second, the vertical wind speed
associated with the waves is only on the order of centimeters per second
(https://www.britannica.com/science/climate-meteorology/Upper-level-winds).
The characteristics of upper-level wind systems are known mainly from an operational
worldwide network of radiosonde observations. Winds measured from Doppler-radar
wind profilers, aircraft navigational systems, and sequences of satellite-observed cloud
imagery have also been used to augment data from the radiosonde network; the latter
two have been especially useful for defining the wind field over data-sparse regions, such
as over the oceans (https://www.britannica.com/science/climate-meteorology/Upper-
level-winds).
Jet streams are of great importance to flying aircraft because they affect the speed and
fuel consumption. Since strong upper-level flow is usually associated with strong vertical
wind shear, jet streams at midlatitudes are accompanied by strong horizontal
temperature gradients. Some regions of high vertical wind shear are marked by clear-air
turbulence (CAT). Jets tend to be strongest near the tropopause where the horizontal
temperature gradient reverses (https://www.britannica.com/science/climate-
meteorology/Upper-level-winds).
Wind
The polar front jet moves in a generally westerly direction at midlatitudes, and its vertical
wind shear which extends below its core is associated with horizontal temperature
gradients that extend to the surface. Consequently, this jet manifests itself as a front that
marks the division between colder air over a deep layer and warmer air over a deep layer.
The polar front jet can break up into waves because of the barometric instability. The
subtropical jet is found at lower latitudes and at slightly higher elevation, because of the
increase in height of the tropopause at lower latitudes. The associated horizontal
temperature gradients of the subtropical jet do not extend to the surface. Thus, a surface
front is not evident. In the tropics an easterly jet is sometimes found at upper levels,
especially when a landmass is located poleward of an ocean, so the temperature increases
with latitude. The polar front jet and the subtropical jet play a role in maintaining Earth’s
general circulation. They are slightly different in each hemisphere because of differences
in the distribution of landmasses and oceans
(https://www.britannica.com/science/climate-meteorology/Upper-level-winds).
Maps and supplementary information can be found here:
https://www.monroecc.edu/weatherweb/upper-air.
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8.3. Turbulence
Turbulence can be defined as small-scale, short-term, random and frequent changes to
the speed and/or direction of air. In other words, when there are rapid changes to either
the air’s velocity or its direction of movement or both, atmospheric conditions are
turbulent. When an aircraft flies through this disturbed air, it will experience turbulence.
Besides convection, wind shear is the second major source for turbulence. According to
basic fluid dynamics any fluid such as the atmosphere can support only a maximum of
shear between laminar flow layers before breaking down into turbulent flow.
Some aircrafts are more susceptible to the effects of turbulence than others. Light
aircrafts are prone to be buffeted and are significantly affected even by light turbulence.
Relatively few reports of turbulence are received from fast military jets which are
designed to give a high degree of tolerance.
The intensity of turbulence is categorized by the International Civil Aviation Organization
as follows:
Light – effects are insignificant and of a low intensity;
Moderate – there may be moderate changes in aircraft attitude and height, but the
aircraft always remains in control. Air speed variations are usually small. Changes in
accelerometer readings of 0.5--1.0 g at the aircraft’s center of gravity. Occupants feel
strain against seat belts. There is difficulty in walking. Loose objects move about.
Severe – abrupt changes in aircraft attitude and height. The aircraft may be out of control
for short periods. Air speed variations are usually large. Changes in accelerometer
readings greater than 1.0 g at the aircraft’s center of gravity (but note, military aviators
regard +4 g/-2 g as severe. Objects are forced violently against seat belts. Loose objects
are tossed about.
Extreme – effects are more pronounced than for severe intensity. However, the only
criterion of this category that is not subjective is that of airborne accelerometer readings.
Converting the standard parameters available to forecasters such as wind speed, gusts,
stability and others to such values would necessarily be very difficult and would require a
specific calculation for each aircraft separately. Forecasters, therefore, must rely upon
more general, empirical rules and relationships (https://community.wmo.int/activity-
areas/aviation/hazards/turbulence).
Turbulence is caused by the relative movement of disturbed air through which an aircraft
is flying. Its origin may be thermal or mechanical and it may occur either within clear or
cloudy atmosphere. The absolute severity of turbulence depends directly upon the rate
at which the speed or the direction of airflow (or both) is changing, although perception
of the severity of turbulence which has been encountered will be affected by the mass of
the aircraft involved (https://www.skybrary.aero/index.php/Turbulence).
Mechanical turbulence results from the passage of strong winds over irregular terrain or
obstacles. Less severe turbulence of low heights can also be the result of convection
occasioned by surface heating (https://www.skybrary.aero/index.php/Turbulence).
Turbulence may also arise from air movements associated with convective activity,
especially in or near a thunderstorm or due to the presence of strong temperature
gradients near to a jet stream. Jet stream turbulence, like turbulence caused by mountain
waves, which can form downwind of ridges, occurs clear of cloud and in the form of CAT.
Very localized, but sometimes severe, wake vortex turbulence may be encountered when
following or crossing behind another aircraft. This turbulence is due to wing tip trailing
vortices generated by the preceding aircraft.
Wind
Air moving over or around high ground may create turbulence in the lee of the terrain
feature. This may produce violent and, for smaller aircraft especially if this is a heavy
aircraft type, potentially uncontrollable effects resulting in pitch and/or roll to extreme
positions.
Relative air movements which involve rapid rates of change in wind velocity are described
as wind shear and, when severe, they may be sufficient to displace an aircraft abruptly
from its intended flight path such that substantial control input is required to compensate.
The consequences of such encounters can be particularly dangerous at low altitude where
any loss of control may occur sufficiently close to terrain to make recovery difficult. The
extreme downbursts which occur below the base of cumulonimbus clouds called
microbursts are a classic example of circumstances conducive to low level wind shear
(https://www.skybrary.aero/index.php/Turbulence).
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8.4. End of Chapter Questions
What is the Coriolis effect?
What are the mechanisms behind sea and land breezes?
What are the mechanisms behind mountain and valley breezes?
What are the problems that mountain waves generate to pilots?
Why are jet streams important for aviation?
How is turbulence generated and does it affect different types of aircraft?
Wind
8.5. Chapter bibliography
European Aviation Safety Agency (EASA) (2014). ATPL ground training series – Meteorology. CAE
Oxford Aviation Academy.
https://www.metlink.org/resource/local-winds/
https://k12.libretexts.org/Bookshelves/Science_and_Technology/Earth_Science/10%3A_Atmosp
heric_Processes/10.19%3A_Local_Winds
https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/Wx_Terms/Flight_Environme
nt.htm
https://www.weather.gov/jetstream/upperair_intro
https://www.britannica.com/science/climate-meteorology/Upper-level-winds
https://www.monroecc.edu/weatherweb/upper-air
https://community.wmo.int/activity-areas/aviation/hazards/turbulence
https://www.skybrary.aero/index.php/Turbulence
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Chapter 9 - Visibility
9.1. Types of visibility
Visibility for aeronautical purposes is of great importance. It can be defined as the greatest
safe distance at which a black object of suitable dimensions, situated near the ground,
can be seen and recognized when observed against a bright background.
Runway Visual Range (RVR) is a form of visibility defined as the range over which the pilot
of an aircraft on the center line of a runway can see the runway surface markings or the
light delineating the runway or identifying its center line, 15 ft (4.6 m) above the runway.
Visibility is usually measured in meters (or kilometers) or in statute miles, depending on
the country concerned.
Ground visibility is the highest distance that can be seen from the ground. When
identifying objects from the ground a set of known distances for different landmarks is
normally used as to properly assess the visible distance from a predetermined point.
Prevailing visibility is the greatest distance that can be visibly seen in a horizontal direction
throughout at least half of the horizon circle, and which does not need to be seen
continuously.
Flight visibility is the average forward distance that can be seen from the cockpit in a
horizontal distance when trying to identify prominent lighted or unlighted objects. These
objects can be other planes, landmarks, mountains, buildings, radio towers, lighting
towers, or any other type of object that can be clearly seen from a distance while in the
air on a standard atmospheric day with unrestricted visibility.
Vertical visibility is reported when the sky is expected to remain or become obscured. The
vertical visibility is forecast to improve and change to or pass through one or more of the
following values, or when the vertical visibility is forecast to deteriorate and pass through
one or more of the following values: 30, 60, 150 or 300 m (100, 200, 500 or 1 000 ft), the
trend forecast in METARs shall indicate the change.
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Visibility
9.2. Atmospheric phenomena that reduce visibility
The main phenomena that reduce visibility consist in precipitation, fog and mist, haze,
smoke and incoastal areas sea spray under the right conditions.
Rain, snow or any other form of precipitation reduces visibility which depends on the
intensity. However, it depends a bit on how heavy the precipitation is (drops/snow flake
size and the intensity). A light drizzle will not hinder VFR operations (although commercial
operations usually will have higher limits) but heavy precipitation in Cumulonimbus (Cb)
or Towering Cumulus (TCu) can reduce visibility to 100 m or even less accompanied with
effects like wind shear and turbulence
(https://www.experimentalaircraft.info/wx/weather-visibility.php).
Fog is visibility less than 1000 meters and mist, by definition, is visibility between 1000
and 5000 meters. Both have their origins in light suspended cloud droplets with a nearly
100% relative humidity and an abundance of condensation nuclei for the condensation
process to produce.
When visibility is reduced to 5000 meters or less by the presence of dust particles it is
called haze. It is not related with cloud forming factors as is the case with fog or mist.
When dust or sand particles are blown off and visibility reduces to less than 1000 meters
it is referred to as a dust or sandstorm, with altitudes usually not higher than around 150
- 200 ft. In desert areas and with unstable air conditions fine dust particles can go up to
8000 ft or higher and this condition can last for hours and have their effects on long
distances from the place of formation.
In Europe it is not uncommon to experience sand dust from the Sahara carried by high
altitude winds from the south and eventually raining down well into the mid and northern
parts of Europe, leaving yellowish dust traces all over.
Activity from industrial districts and fires in residential areas add soot and carbon to the
environment reducing visibility even more. This adds enough condensation nuclei to the
air so that condensation will take place before reaching a relative humidity of 100%.
Commonly seen in coastal areas, sea spray adds salt particles to the air thus increasing
the amount of condensation nuclei and condensation can take place with a RH lower than
100% and thereby reducing visibility greatly. With strong onshore winds, reduced visibility
can be experienced many kilometers inland limiting VFR flights for coastal airports
(https://www.experimentalaircraft.info/wx/weather-visibility.php).
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International Air Force Semester
9.3. Types of fog
Radiation fog forms when all solar energy exits the earth and allows the temperature to
meet up with the dew point. The best condition to have radiation fog is when it had rained
the previous night. This help to moisten up the soil and create higher dew points. This
makes it easier for the air to become saturated and form fog. However, the winds must
be light less than 13 knots to prevent moist and dry from mixing.
Precipitation fog forms when rain is falling through cold air. This is common with warm
fronts but it can occur with cold fronts as well. Cold air and dry at the surface while rain is
falling through it evaporates and causes the dew point to rise. This saturation forms fog.
Advection fog forms from surface contact of horizontal winds. This fog can occur with
windy conditions. Warm and moist air blows in from the sea and if there is snow or cool
moisture on the ground it will come in contact with the warm and moist air advected by
the wind. Then dew point rises and creates high humidity and forms fog.
Steam fog is commonly seen in large bodies of water such lakes. This forms during the fall
season. As summer ends, the air temperature falls steeply than the water temperature.
As a mass of dry, cold air moves over a warmer lake, which is evaporate continuously the
heat is transferred to the air in contact to the water’s surface. Steam fog does not become
very deep but enough to block some of the sunlight.
Upslope fog forms as moist air carried by the wind which blows toward a mountain, it up
glides and this causes the air to rise and cool. The cooling of the air from rising causes to
meet up with the dew point temperature. Fog forms on top of the mountains.
Valley fog forms in the valley when the soil is moist from previous rainfall. As the skies
clear solar energy exits earth and allow the temperature to cool near or at the dew point.
This forms deep fog, so dense it's sometimes called tule fog.
Freezing fog occurs when the temperature falls at 0 °C or below. This type of fog produces
drizzle and these tiny droplets freeze when they contact an object. But at the same time
there is sublimation going on.
Ice fog is only seen in the polar and arctic regions. Temperatures of -10 °C are too cold for
the air to contain super-cooled water droplets so it forms small ice crystals
(https://www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/fog_definitions/F
og_definitions.html). Supplementary information can be found at:
https://aerocorner.com/blog/types-of-fog-in-aviation/.
Visibility
9.4. End of Chapter Questions
What is the Runway Visual Range (RVR)?
What are the main phenomena that reduce visibility?
What is radiation fog?
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International Air Force Semester
9.5. Chapter bibliography
https://www.experimentalaircraft.info/wx/weather-visibility.php
https://www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/fog_definitions/Fog_defini
tions.html
https://aerocorner.com/blog/types-of-fog-in-aviation/
Chapter 10 - Fronts and air masses
10.1. Description of general circulation of the atmosphere and air
masses
Even with disruptions like weather fronts and storms, there is a consistent pattern to how
air moves around our planet. This pattern, called atmospheric circulation, is caused
because the Sun heats the Earth more at the equator than at higher altitudes or at the
poles. It is also affected by the spin of the Earth (https://scied.ucar.edu/learning-
zone/how-weather-works/global-air-atmospheric-circulation).
Over the major parts of the Earth's surface there are large-scale wind circulations present.
The global circulation can be described as the worldwide system of winds by which the
necessary transport of heat from tropical to polar latitudes is accomplished.
In each hemisphere there are three cells (Hadley cell, Ferrel cell and Polar cell) in which
air circulates through the entire depth of the troposphere.
Hadley cell is the largest cells extend from the equator to between 30 and 40 degrees
north and south, and are named Hadley cells, after English meteorologist George Hadley.
Within the Hadley cells, the trade winds blow towards the equator, then ascend near the
equator as a broken line of thunderstorms, which forms the Intertropical Convergence
Zone (ITCZ). From the top of these storms, the air flows towards higher latitudes, where
it sinks to produce high-pressure regions over the subtropical oceans and the world's hot
deserts, such as the Sahara Desert in North Africa
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/global-
circulation-patterns).
In the middle cells, which are known as the Ferrel cells, air converges at low altitudes to
ascend along the boundaries between cool polar air and the warm subtropical air that
generally occurs between 60 and 70 degrees north and south. The circulation within the
Ferrel cell is complicated by a return flow of air at high altitudes towards the tropics,
where it joins sinking air from the Hadley cell
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International Air Force Semester
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/global-
circulation-patterns).
The Ferrel cell moves in the opposite direction to the two other cells (Hadley cell and Polar
cell) and acts rather like a gear. In this cell the surface wind would flow from a southerly
direction in the northern hemisphere. However, the spin of the Earth induces an apparent
motion to the right in the northern hemisphere and left in the southern hemisphere
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/global-
circulation-patterns).
The smallest and weakest cells are the Polar cells, which extend from between 60 and 70
degrees north and south, to the poles. Air in these cells sinks over the highest latitudes
and flows out towards the lower latitudes at the surface.
An air mass is a large body of air with generally uniform temperature and humidity. The
area over which an air mass originates is what provides its characteristics. The longer the
air mass stays over its source region, the more likely it will acquire the properties of the
surface below. As such, air masses are associated with high pressure systems
(https://www.weather.gov/jetstream/airmass).
There are two broad overarching divisions of air masses based upon the moisture content.
Continental air masses, designated by the lowercase letter 'c', originate over continents
are therefore dry air masses. Maritime air masses, designated by the letter 'm', originate
over the oceans and are therefore moist air masses.
Each of the two divisions is then divided based upon the temperature content of the
surface over which they originate. In the following subchapter will discuss properties,
origin and types of air masses.
Fronts and air masses
10.2. Properties, origin and types of air masses
There are six main types of air masses that affect the Europe weather and climate. We
classify these air masses primarily by the area in which they originate.
They are classified as continental or maritime – dependent on whether they originate over
land or sea – and arctic (or Antarctic, the corresponding type in the southern hemisphere),
equatorial, tropical, or polar – depending on the region in which they form.
Tropical continental air mass originates over North Africa and the Sahara (a warm source
region). It is most common during the climatological summer months (June, July and
August) although it can occur at other times of the year. The highest temperatures usually
occur under the influence of tropical continental air. Visibility is usually moderate or poor
due to the air picking up pollutants during its passage over Europe and from sand particles
blown into the air from Saharan dust storms
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/air-
masses/types).
The source region for Tropical maritime air mass is warm waters of the Atlantic Ocean
between the Azores and Bermuda Islands. Tropical maritime air is warm and moist in its
lowest layers and, although unstable over its source region, during its passage over cooler
waters becomes stable and the air becomes saturated. Consequently, when a tropical
maritime air mass reaches Western Europe, it brings with it low cloud and drizzle, perhaps
also fog around windward coasts and across hills. To the lee of high ground though, the
cloud my breakup and here the weather, particularly in the summer months, can be fine
and sunny. This is a mild air stream and specifically during the winter month, can raise the
air temperature several degrees above the average
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/air-
masses/types).
Polar continental air mass has its origins over the lands covered with snow of Eastern
Europe and Russia and manifests itself only in cold months (November to April). During
the warm and summer months with the land mass considerably warmer, this air mass
would be classed as a tropical continental (https://www.metoffice.gov.uk/weather/learn-
about/weather/atmosphere/air-masses/types). The weather characteristics of this air
mass depend on the length of the sea trajectory during its passage from Eastern Europe
to Western Europe. The lowest temperatures across the Europe usually occur in this air
mass, lower than -10 °C at night, and sometimes remaining below freezing all day
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/air-
masses/types).
Polar maritime air mass has its origins over northern Canada and Greenland and reaches
the northeastern Europe on a northwesterly air stream.
Polar maritime air mass is commonly affecting northwestern Europe. This air mass starts
very cold and dry but during its long passage over the relatively warm waters of the North
Atlantic its temperature rises rapidly, and it becomes unstable to a great depth. This air
mass is characterized by frequent showers at any time of the year. In the winter months
when instability (convection) is most vigorous over the sea, hail and thunder are common
across much of the western and northwestern side of the Europe
(https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/air-
masses/types).
An arctic maritime air mass has similar characteristics to a polar maritime air mass, but
the air is colder and less moist. Arctic air is uncommon during the summer, but when it
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International Air Force Semester
does occur it may bring heavy showers or thunderstorms and unseasonably low
temperatures. Between October and May, the air is cold enough to produce hail showers
or snow. An arctic maritime air mass has its origins over the North Pole and the Arctic
Ocean. Polar low-pressure systems forming in this air mass can sometimes lead to
widespread and heavy snowfall, but otherwise inland areas remain free of cloud in the
winter months (https://www.metoffice.gov.uk/weather/learn-
about/weather/atmosphere/air-masses/types).
Fronts and air masses
10.3. Types of fronts and associated clouds and weather
A weather front has specific temperature and humidity characteristics. Often there is
turbulence in a front, which is the borderline of two different air masses. Instead to
generate clouds and storms, some fronts just cause a change in temperature. However,
some storm fronts start very large storms. Tropical waves are fronts that develop in the
tropical Atlantic Ocean off the coast of Africa. These fronts can develop into tropical
storms or hurricanes if conditions allow. There are four types of weather fronts: warm
fronts, cold fronts, stationary fronts and occluded fronts.
A warm front forms when a warm air mass pushes into a cooler air mass. Warm fronts
often bring stormy weather as the warm air mass at the surface rises above the cool air
mass, forming clouds and storms. Warm fronts move more slowly than cold fronts
because it is more difficult for the warm air to push the cold, dense air across the Earth's
surface. Warm fronts often form on the east side of low-pressure systems where warmer
air from the south is pushed northward. Often are formed high clouds like cirrus,
cirrostratus, and middle clouds like altostratus ahead of a warm front. These clouds form
in the warm air that is high above the cool air. As the front passes over an area, the clouds
become lower, and rain is likely. There can be thunderstorms around the warm front if
the air is unstable.
On weather maps, the surface location of a warm front is represented by a solid red line
with red, filled-in semicircles along it, like in the figure 20. The semicircles indicate the
direction that the front is moving. They are on the side of the line where the front is
moving (https://scied.ucar.edu/learning-zone/how-weather-works/weather-fronts).
A cold front forms when a cold air mass pushes into a warmer air mass. Cold fronts can
produce significant changes in the weather. They move up to twice as fast as a warm front.
As a cold front moves into an area, the heavier and more dense cool air pushes under the
lighter and less dense warm air, forcing it to climb into the troposphere. Lifted warm air
ahead of the front produces cumuliform clouds and thunderstorms.
As the cold front passes, winds become gusty. There is a sudden drop in temperature, and
heavy rain, sometimes with hail, thunder, and lightning can form. Atmospheric pressure
changes from falling to rising at the front. After a cold front passes an area, the
temperature is cooler, the rain has stopped, and the cumulus clouds are replaced by
stratus and stratocumulus clouds or clear skies (https://scied.ucar.edu/learning-
zone/how-weather-works/weather-fronts).
On weather maps, a cold front is represented by a solid blue line with filled-in triangles
along it (Figure 20). The triangles are like arrowheads pointing in the direction that the
front is moving.
A stationary front forms when a cold front or warm front stops moving. This happens
when two masses of air are pushing against each other, but neither is powerful enough
to move the other. Winds blowing parallel to the front instead of perpendicular can help
it stay in place. A stationary front may stay still for days. If the wind direction changes, the
front will start moving again, becoming either a cold or warm front. Or the front may break
apart. The weather is often cloudy along a stationary front, and rain or snow often falls,
especially if the front is in an area of low atmospheric pressure
(https://scied.ucar.edu/learning-zone/how-weather-works/weather-fronts).
On a weather map, a stationary front is shown as alternating red semicircles and blue
triangles (Figure 20). Notice how the blue triangles point in one direction, and the red
semicircles point in the opposite direction.
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Sometimes a cold front follows right behind a warm front. A warm air mass pushes into a
colder air mass (the warm front), and then another cold air mass pushes into the warm
air mass (the cold front). Because cold fronts move faster, the cold front is likely to
overtake the warm front. This is known as an occluded front.
At an occluded front, the cold air mass from the cold front meets the cool air that was
ahead of the warm front. The warm air rises as these air masses come together. Occluded
fronts usually form around areas of low atmospheric pressure.
There is often precipitation along an occluded front from cumulonimbus or nimbostratus
clouds. Wind changes direction as the front passes and the temperature either warms or
cools. After the front passes, the sky is usually clearer, and the air is drier.
On a weather map, shown to the left, an occluded front looks like a purple line with
alternating triangles and semicircles pointing in the direction that the front is moving
(Figure 20) (https://scied.ucar.edu/learning-zone/how-weather-works/weather-fronts).
Figure 20: Symbols for surface fronts (source: https://www.gleimaviation.com/wp-
content/uploads/2020/09/SurfaceFrontSymbols.jpg)
Fronts and air masses
10.4. End of Chapter Questions
What is Hadley cell?
What characteristics would have an air mass that has been generated over
Greenland?
What is an occluded front?
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10.5. Chapter bibliography
https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/global-circulation-
patterns
https://scied.ucar.edu/learning-zone/how-weather-works/global-air-atmospheric-circulation
https://www.weather.gov/jetstream/airmass
https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/air-masses/types
https://scied.ucar.edu/learning-zone/how-weather-works/weather-fronts
https://www.gleimaviation.com/wp-content/uploads/2020/09/SurfaceFrontSymbols.jpg
Weather hazards in aviation
Chapter 11 - Weather hazards in
aviation
11.1. Detailing Icing, turbulence, wind shear, wind associated with
mountain areas, thunderstorms, heavy rain, hail, sandstorm,
dust storm
Icing can occur if an airplane passes through a cloud when the air temperature is at or
below freezing. This can happen during widespread rain, scattered showers, or even in a
thunderstorm. This is hazardous to flight because it can increase drag and weight on the
aircraft and, therefore, decrease lift. All these effects can degrade the ability of the
aircraft to maintain altitude (https://www.weather.gov/zme/safety_ice). Icing may occur
in flight and at the surface.
Common types of icing are as follows:
• Rime icing occurs when tiny supercooled water droplets freeze onto a surface that
is below freezing (typically an airplane wing or pitot tube). This tends to be
nuisance rime that becomes a rough surface, although accumulation on the pitot
tube can lead to instrument failure. It is the most common type of icing.
• Clear Icing occurs as a result of Supercooled Large Drops (SLD) in a cloud. These
are large raindrops that have cooled to below 0 °C, but are still liquid. When they
contact with a solid object (like an airplane wing), they can accumulate rapidly as
large sheets of ice. This can be very hazardous because they can disrupt the air
flow and reduce lift on an aircraft. More importantly, it can spread beyond the
reach of de-icing equipment on the aircraft and can be difficult for a pilot to see
(https://www.weather.gov/zme/safety_ice).
Mixed Icing is when both clear and rime ice occur at the same time.
Freezing rain icing can also occur when a plane is on or near the ground, as a result of
freezing rain. During the winter season, the atmosphere can set up such that a warm layer
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(about 3 °C and about 5000 ft thick) can ride over top of a shallow cold layer (-1 °C and
3000 ft thick) near the ground. This creates a very strong inversion. Ice crystals forming in
the upper atmosphere fall through the warm layer and melt to become liquid. Finally, the
liquid falls through the cold temperatures and re-freezes on contact with surfaces on the
ground (such as an airplane wing).
High altitude ice crystals are a rare type of ice that can be hazardous if absorbed at high
altitudes. It usually occurs in or near convective weather, such as thunderstorms, in a
tropical environment at high altitudes (FL250-FL350). Unlike supercooled drops or
droplets, ice crystals do not accumulate on the airframe. Rather, they can partially melt
and stick to relatively warm internal engine surfaces. This can cause engine flameouts, a
surge or engine damage. If it occurs, it should be reported as "ice crystal icing", so as not
to be confused with typical ice accumulation (https://www.weather.gov/zme/safety_ice).
Turbulence is an irregular motion of the air characterized by strong currents. It can be
classified in a scale that is divided into light turbulence, moderate turbulence, severe
turbulence and extreme turbulence (details can be found in Chapter 8.3). Before entering
a severe turbulence, the pilot will try to divert the route slightly, to avoid the area and
minimize the inconvenience to aircraft occupants. Whenever strong turbulence is
experienced, the information will be reported via radio or ATC communications, to warn
the traffic flying in the same area. Turbulence generated by a thunderstorm can cause the
aircraft to violently “shake” when flying up to 24 to 32 km ahead from the storm center
(CAE Oxford Aviation Academy, 2014). Turbulence can be a serious hazard, especially
during take offs and landings, and it can cause serious damages to occupants of the
aircraft. (http://havkar.com/en/blog/view/meteorological-hazards-for-an-aircraft/101).
Wind shear is nothing more than a change in wind direction, and/or wind speed over the
distance between two points. If the points are in a vertical direction, then it is called
vertical shear, if they are in a horizontal direction than it is called horizontal shear.
In the aviation, the major concern is how abruptly the change occurs. If the change is
gradual, a change in direction or speed will result in nothing more than a minor change in
the ground speed. If the change is abrupt, however, there will be a rapid change of
airspeed or track. Depending on the aircraft type, it may take a significant time to correct
the situation, placing the aircraft in peril, particularly during takeoff and landing (Hudson
et al., 2001).
Significant shearing can occur when the surface wind blowing along a valley varies
significantly from the free-flowing wind above the valley. Changes in direction of 90° and
speed changes of 25 knots are reasonably common in mountainous terrain.
Updrafts and downdrafts also induce shears. An abrupt downdraft will cause a brie
decrease in the wing’s attack angle resulting in a loss of lift. An updraft will increase the
wing’s attack angle and consequently increase the lift, however, there is a risk that it could
be increased beyond the stall angle (Hudson et al., 2001).
Shears can also be encountered along fronts. Frontal zones are generally thick enough
that the change is gradual, however, cold frontal zones as thin as 200 feet have been
measured. Significant directional shears across a warm front have also been observed
with the directional change greater than 90 degrees over several hundred feet. Pilots
doing a take-off or a landing approach through a frontal surface that is just above the
ground should be wary (Hudson et al., 2001).
Hills and valleys substantially distort the airflow associated with the prevailing pressure
system and the pressure gradient. Strong up and down drafts and eddies develop as the
Weather hazards in aviation
air flows up over hills and down into valleys. Wind direction changes as the air flows
around hills. Sometimes lines of hills and mountain ranges will act as a barrier, holding
back the wind and deflecting it so that it flows parallel to the range. If there is a pass in
the mountain range, the wind will rush through this pass as through a tunnel with
considerable increased speed. The airflow can be expected to remain turbulent and
erratic for some distance as it flows out of the hilly area and into the flatter countryside
(https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/Wx_Terms/Flight_En
vironment.htm).
In mountainous areas, local distortion of the airflow is even more severe. Rocky surfaces,
high ridges, sheer cliffs, steep valleys, all combine to produce unpredictable flow patterns
and turbulences.
Air flowing across a mountain range usually rises relatively smoothly up the slope of the
range, but, once over the top, it pours down the other side with considerable force,
bouncing up and down, creating eddies and turbulence and creating powerful vertical
waves that may extend for great distances downwind of the mountain range. This
phenomenon is known as a mountain wave. Note the up and down drafts and the rotating
eddies formed downstream
(https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/Wx_Terms/Flight_En
vironment.htm).
The ingredients needed for thunderstorms to occur are moisture, instability and lift.
Moisture can come from any nearby body of water, or by moist ground conditions.
Instability occurs when air pockets rise faster than the environment around them. Lift can
be achieved by a frontal passage, thermals, and/or convergence.
Thunderstorms can produce several hazards to aviation. These include, but are not limited
to: lightning, large hail, turbulence, icing, tornadoes, and downbursts. Often, multiple
hazards can develop quickly. As a result, pilots should avoid flying in or near
thunderstorms (https://www.weather.gov/zme/safety_ts).
Lightning is not as much of a hazard to aviation as people think it is, because aircraft are
equipped with static discharge arrestors. In fact, very few crashes have occurred due to
lightning striking an aircraft. However, a plane does not need to be in or right near a
thunderstorm for lightning to occur. Lightning can come out of the anvil and strike an
object several miles away (https://www.weather.gov/zme/safety_ts).
Hail occurs when chunks of ice develop from strong thunderstorm updrafts reaching
above the freezing level. When hail becomes too large and heavy for the updraft to
support it, it falls to the ground. Hail is much larger in the thunderstorm cloud than the
ground, which can produce a major hazard to aircraft
(https://www.weather.gov/zme/safety_ts).
Tornadoes, which are violently rotating columns of air below a thunderstorm base, can
be hazardous not only near the ground, but also aloft. This is because the tornadic
circulation can extend well up into the thunderstorm cloud
(https://www.weather.gov/zme/safety_ts).
Downbursts occur beneath the base of a thunderstorm when rain and rain-cooled air
drops rapidly down to the ground. When it reaches the ground, it spreads out violently
in all directions, causing localized vortices and wind shear to develop. This is especially
hazardous to aircraft because a plane may experience lift at one moment, followed by
rapid descent during the next (https://www.weather.gov/zme/safety_ts).
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Multicell clusters occur when a cluster of airmass cells dissipate and spawn new cells
nearby or adjacent to their location. Individual cells can last 20-30 minutes, but the entire
cluster can last for several hours as long as new cells develop. Oftentimes, these can block
major airways or intersections and should be avoided. These often produce locally strong
wind gusts and severe-extreme turbulence at altitude
(https://www.weather.gov/zme/safety_ts).
Multi-cell lines (also known as squall lines) can span hundreds of kilometers and
effectively block airways. They can last for several hours and move rapidly. These can
produce strong downbursts with severe or even extreme turbulence. Sometimes a
thunderstorm line will appear to have breaks in and look safe to fly through. However,
these breaks can fill in rapidly, and have been responsible for numerous aircraft accidents.
If a squall line is blocking a pilot's flight path, it is advised that they alter their route or
land and allow the storm to pass (https://www.weather.gov/zme/safety_ts).
Supercells typically last four to six hours and can produce large tornadoes, hail, and
damaging downbursts. Although they are known as the king of thunderstorms, pilots can
usually deviate around these storms if they are spread out. However, several supercells
that are close to each other can eventually merge into a squall line
(https://www.weather.gov/zme/safety_ts).
Dust storms and sandstorms are regions of raised dust and sand. The dust and sand are
essentially raised by the wind and are lofted to various heights dependent upon
turbulence and instability and persistency of the flow that lifted the particles.
The size of dust and sand particles ranges from slightly sub-micron to several hundreds of
microns. Smaller and lighter particles will be lifted more readily and to greater heights,
and take longer to settle out, while the larger particles may remain airborne only for short
distances of a few hundred meters (https://community.wmo.int/activity-
areas/aviation/hazards/dust-sand).
Drastic reductions in visibility are likely to accompany dust and sandstorms. Effective
visibility may very likely be close to zero in some circumstances. Dust and sand ingestion
into aircraft engines may cause reductions in power to the extent of complete engine
failure. If dust and sand find their way into cockpits, then problems with electrical
equipment may occur (https://community.wmo.int/activity-
areas/aviation/hazards/dust-sand).
Weather hazards in aviation
11.2. End of Chapter Questions
Why is icing still poses a danger to flights?
What are the factors generating wind shear?
Is lightning dangerous to aircraft flights?
Is it safe to fly an aircraft through a break in a squall line?
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11.3. Chapter bibliography
CAE Oxford Aviation Academy. ATPL Ground Series – Meteorology. 2014. KHL Printing Co. Pte Ltd,
Singapore
Hudson, E., Aihoshi, D., Gaines, T., Simard, G., Mullock, J. (2001). The Weather of Nunavut and the
Arctic, NAV CANADA.
https://www.weather.gov/zme/safety_ice
http://havkar.com/en/blog/view/meteorological-hazards-for-an-aircraft/101
https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/Wx_Terms/Flight_Environme
nt.htm
https://www.weather.gov/zme/safety_ts
https://community.wmo.int/activity-areas/aviation/hazards/dust-sand
Meteorological Information and codes
Chapter 12 - Meteorological
Information and codes
12.1. METAR/SPECI
METAR is the name of the code for an aerodrome routine weather report. A METAR is
issued at hourly or half-hourly intervals. SPECI is the name of the code for an aerodrome
special weather report. A SPECI can be issued at any time when certain criteria are met.
Both METAR and SPECI have the same code form, and both may have a TREND forecast
appended (WMO, 2019; WMO, 2020).
METAR or SPECI contains the following information in the order shown:
IDENTIFICATION GROUPS
- The report code name (METAR or SPECI).
- The ICAO location indicator of the reporting station, for example, LRTR.
- The day of the month and the time of observation in hours and minutes UTC
(coordinated universal time), followed by the letter Z.
- The code words COR and NIL are inserted after the code name and the time group,
respectively, as appropriate.
- The code word AUTO is inserted when the report contains a fully automated
observation, that is without human intervention (WMO, 2019; WMO, 2020).
- The report ends at the symbol “=”.
Example of METAR report:
METAR LRTR 250800Z 33006KT 300V360 9999 -RA FEW018 15/15 Q1015=
SURFACE WIND
The surface wind direction and speed shall be reported in steps of 10° true and 1 kt,
respectively. Any observed value which does not fit the reporting scale in use shall be
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rounded to the nearest step in the scale. Additionally, if, during the 10 min preceding the
observation, the maximum gust speed has exceeded the mean speed by 10 kt (5 m s-1) or
more, this gust will be reported by inserting the letter G followed by the gust speed
directly after the mean speed. Example: 30025G35KT.
If, during the 10 min immediately preceding the observation, the wind direction has varied
by 60° or more but less than 180° and the mean wind speed is 3 kt (2 m s–1) or more, the
two extreme directions should be indicated in clockwise order, with the letter V inserted
between the two directions. Example: 300V360.
The wind reported should be the mean over the 10 min preceding the observation. If
during this period there has been a marked discontinuity lasting at least 2 min, the mean
values should be assessed over the period after the discontinuity. A marked discontinuity
occurs when there is a wind direction change of 30° or more with a wind speed of 10 kt
(5 m s–1) or more, before or after the change, or a wind speed change of 10 kt (5 m s–1) or
more. The averaging period for measuring variations from the mean wind speed (gusts)
should be 3 s. The wind direction is encoded as VRB only if one of the following conditions
is met: the wind speed is less than 3 kt; the wind speed is higher and wind direction is
varying by 180° or more and a single direction is impossible to determine, for example
when a thunderstorm is over the aerodrome. Example: VRB04KT.
When a wind speed is less than 1 kt (0.5 m s–1), the group is encoded as 00000 followed
by the abbreviation for the wind speed units. Speeds of 100 kt (50 m s–1) or more: The
wind speed shall be preceded by the letter indicator P and reported as P99KT (WMO,
2019; WMO, 2020).
PREVAILING VISIBILITY
The group V V V V shall be used to report prevailing visibility. When horizontal visibility is
not the same in different directions, and when visibility is fluctuating rapidly and the
prevailing visibility cannot be determined, the group V V V V shall be used to report the
lowest visibility.
Example: Prevailing visibility of 3 000 m is encoded as 3000.
When visibility sensors are used in such a manner that no directional variations can be
given, the abbreviation NDV shall be appended to the visibility reported. When the
visibility is not the same in different directions and when the minimum visibility is
different from the prevailing visibility, and less than 1 500 m or less than 50 % of the
prevailing visibility, the group VNVNVNVNDv shall also be used to report the minimum
visibility and its general direction (WMO, 2019; WMO, 2020).
RUNWAY VISUAL RANGE (if available)
Runway visual range (RVR) is reported when meteorological optical range is less than 1500
m. Where the RVR can be determined and when it is reported, the group starts with the
letter R followed by the runway designator DRDR and / followed by the RVR in meters.
When the RVR is assessed to be more than 2 000 m, it should be reported as P2000. When
the RVR is below the minimum value that can be assessed, the RVR should be reported as
M followed by the appropriate minimum value that can be assessed.
Where there is the required instrumentation capable of assessing and displaying 1-, 2-, 5-
and 10-min mean values, the variations and the tendency of the change are required. The
tendency is indicated as follows immediately after the RVR value: