Meteorological Information and codes
U when the runway visual range has increased during the 10 min preceding the
observation;
D when the runway visual range has decreased;
N indicates no distinct change in runway visual range;
when it is not possible to determine a tendency, the indicator is omitted (no character will
be displayed after de RVR value) (WMO, 2019; WMO, 2020).
PRESENT WEATHER
Once it has been decided there is a weather phenomenon to be reported, the present
weather is encoded by considering each column in the table below. If more than one
weather phenomenon is observed, separate groups will be encoded. However, for more
than one form of precipitation, these forms of precipitations will be combined in a single
group with the dominant type of precipitation being reported first (WMO, 2019; WMO,
2020).
Table 2: Weather phenomena in METAR code (after Manual on Codes, WMO-No. 306)
QUALIFIER WEATHER PHENOMENA
Intensity or Descriptor Precipitation Obscuration Other
proximity
– Light MI Shallow DZ Drizzle BR Mist PO
Moderate (no BC Patches RA Rain FG Fog Dust/sand
qualifier) PR Partial whirls (dust
(covering part SN Snow FU Smoke devils)
+ Heavy (well of the SG Snow VA Volcanic SQ Squalls
developed in aerodrome) grains ash
the case of PL Ice DU FC Funnel
dust/ sand aerodrome) pellets Widespread cloud(s)
whirls (dust DR Low drifting GR Hail dust (tornado or
devils) and BL Blowing SA Sand waterspout)
funnel clouds)
SH Shower(s) GS Small hail HZ Haze SS
TS and/or snow Sandstorm
Thunderstorm pellets DS
FZ Freezing Duststorm
(supercooled)
UP
Unknown
precipitation
VC In the
vicinity
There are a few restrictions on the weather phenomena, the most significant being:
– Intensity is reported only with precipitation (including showers and thunderstorms with
precipitation), duststorms or sandstorms.
– Smoke, haze, widespread dust and sand (except drifting sand) are reported only when
visibility has been reduced to 5 000 m or less.
– Mist is reported when visibility is reduced by water droplets to 1 000 to 5 000 m.
– Fog is reported when visibility is reduced by water droplets to less than 1 000 m.
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– Hail (GR) should be used only when the diameter of the largest hailstones observed is 5
mm or more. GS shall be used in all other cases.
– VC denotes manifestation between 8 and 16 km of the aerodrome reference point
(WMO, 2019; WMO, 2020).
CLOUD (or vertical visibility if appropriate)
Cloud groups consist of six characters under normal circumstances. The first three indicate
cloud amount with:
1/8 to 2/8 being reported as FEW (few)
3/8 to 4/8 being reported as SCT (scattered)
5/8 to 7/8 being reported as BKN (broken) and
8/8 being reported as OVC (overcast)
Cloud type
Types of clouds other than significant convective clouds are not identified.
Significant convective clouds are:
– Cumulonimbus indicated by CB;
– Cumulus congestus of great vertical extent indicated by TCU.
The contraction TCU, taken from “Towering CUmulus”, is an ICAO abbreviation
used to describe this type of cloud (WMO, 2019; WMO, 2020).
Vertical visibility
When the sky is obscured and cloud details cannot be assessed but information on vertical
visibility is available, the cloud group should be replaced by a five-character group, the
first two characters being VV followed by the vertical visibility in units of 30 m or 100 ft,
as for cloud base. When the sky is obscured but the vertical visibility cannot be assessed,
the group will read VV///.
CAVOK
The code word CAVOK shall be used when the following conditions occur
simultaneously at the time of the observation:
(a) Visibility is 10 km or more;
(b) No cloud below 1 500 m (5 000 ft) or below the higher minimum sector
altitude, whichever is greater, and no CB;
(c) No significant weather phenomena (WMO, 2019; WMO, 2020).
AIR AND DEWPOINT TEMPERATURE
The observed air temperature and dewpoint temperature, each as two figures rounded
to the nearest whole degree Celsius, should be reported as follows: Temperatures below
0 °C will be preceded by M to indicate “minus” (WMO, 2019; WMO, 2020). Example: 16/13
– where 16 is the value of air temperature and 13 is the value of dewpoint temperature;
M02/M01 – where M02 is the value of air temperature (-2 °C) and M01 is the value of
dewpoint temperature (-1 °C).
PRESSURE – QNH
The last group of the main part of the report should indicate the QNH rounded down to
the nearest whole hectopascal. The group starts with the letter Q followed by four figures.
Meteorological Information and codes
In some countries, inches of mercury are used as the unit of QNH. In this case, the
indicator will be A (instead of Q) (WMO, 2019; WMO, 2020). Example: Q1015.
SUPPLEMENTARY INFORMATION
Recent weather
Using the indicator letters RE, information on recent weather shall be reported, up to a
maximum of three groups using the abbreviations given table 2, if the following weather
phenomena were observed during the previous hour, or since the last observation, but
not at the time of observation.
Where local circumstances so warrant, information on the existence of wind shear
significant to aircraft operations along the take-off or approach paths in the lowest 500 m
(1 600 ft) should be reported using the following groups, as necessary: WS RDRDR or WS
ALL RWY.
The runway state group is expected to be included in METAR and SPECI as received from
airport managers (WMO, 2019; WMO, 2020).
MISSING ELEMENTS
If a meteorological element in METAR or SPECI is temporarily missing, or its value
considered temporarily as incorrect, it is replaced by a solidus (/) for each missing digit
(WMO, 2019; WMO, 2020).
TREND FORECASTS
TREND forecasts are appended to a METAR or SPECI. the information contained in the
TREND is a forecast covering a period of 2 h ahead from the time of observation. The
TREND forecast indicates significant changes in respect of one or more of the elements,
surface wind, prevailing visibility, weather, and cloud. Only those elements for which a
significant change is expected are included. When no significant change is expected to
occur, this is indicated by the abbreviation NOSIG (WMO, 2019; WMO, 2020).
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12.2. TAF
Terminal Aerodrome Forecast (TAF) is a complete description of the meteorological
elements expected at and over the aerodrome throughout the whole of the forecast
period, including any changes considered to be significant to aircraft operations.
TAF describe the forecast prevailing conditions at an aerodrome and cover a period of not
less than 6 h and not longer than 30 h. The period of validity of TAF produced by
meteorological offices should be determined by regional air navigation agreement.
Routine TAF valid for less than 12 h should be issued every 3 h, and those valid for 12 h
up to 30 h every 6 h (WMO, 2019; WMO, 2020).
IDENTIFICATION GROUPS
This section contains eight parts, as follows:
– The aerodrome forecast code name (TAF) included at the beginning of an individual
aerodrome forecast and at the beginning of a bulletin consisting of one or more
aerodrome forecasts;
– The code word AMD if the TAF is amended;
– The code word COR if the TAF is corrected;
– The ICAO location indicator of the aerodrome to which the forecast refers;
– The date and time of issue of the forecast;
– The code word NIL if the TAF is missing;
– The period covered by the forecast;
– The code word CNL if the TAF is cancelled (WMO, 2019; WMO, 2020).
– The report ends at the symbol “=”.
Example of TAF report:
TAF LRTR 250500Z 2506/2606 01010KT CAVOK TEMPO 2507/2515 5000 -SHRA
SCT010 SCT040CB PROB30 TEMPO 2509/2513 3000 TSRA BKN010 BKN035CB
BECMG 2516/2518 VRB04KT TEMPO 2602/2605 3000 BR=
SURFACE WIND
Normally this is a five-figure group followed by an abbreviation to indicate the wind speed
units used. The first three figures indicate the wind direction from true north, and the last
two the mean wind speed. Additionally, if the wind is expected to be gusty and the
maximum gust speed likely to exceed the mean speed by 10 kt (5 m s–1) or more, this gust
must be indicated using the letter G directly after the mean speed, followed by the gust
speed.
The encode VRB is used only when the mean wind speed is less than 3 kt (2 m s–1). VRB
for higher wind speeds shall be used only when the variation of wind direction is 180° or
more, or when it is impossible to forecast a single wind direction, for example during a
thunderstorm. When a wind speed of 100 kt or more is forecast, it should be indicated as
P99KT (WMO, 2019; WMO, 2020).
PREVAILING VISIBILITY
Forecast prevailing visibility is encoded as a four-figure group. As with the METAR code,
the figures are the expected values in meters, except that 9999 indicates a prevailing
visibility of 10 km or greater (WMO, 2019; WMO, 2020).
Meteorological Information and codes
WEATHER
Forecast weather, using the appropriate abbreviations given in table 2, is restricted to the
occurrence of one or more, up to a maximum of three, of the following weather
phenomena, together with their characteristics, which are deemed significant to aircraft
operations:
– Freezing (FZ) precipitation;
– Freezing fog;
– Moderate or heavy precipitation (including showers – SH);
– Low drifting (DR) dust, sand or snow;
– Blowing (BL) dust, sand or snow;
– Duststorm (DS);
– Sandstorm (SS);
– Thunderstorm (TS);
– Squall (SQ);
– Funnel cloud (tornado or waterspout – FC);
– Other weather phenomena given in table 2 which are expected to cause a significant
change in visibility.
If no significant weather, as defined above, is expected to occur, the group is omitted.
However, after a change group, if the weather ceases to be significant, the weather group
w’w’ is replaced by NSW (abbreviation for Nil Significant Weather) (WMO, 2019; WMO,
2020).
CLOUD (or vertical visibility, if appropriate)
Cloud information is presented in the same format as in the METAR. The group usually
consists of six characters, the first three indicating the expected cloud amount, using the
following abbreviations:
FEW – Few – 1/8 to 2/8
SCT – Scattered – 3/8 to 4/8
BKN – Broken – 5/8 to 7/8
OVC – Overcast – 8/8
Only the cloud type cumulonimbus (CB) is indicated.
When the sky is expected to be obscured and information on vertical visibility is available,
the cloud group is replaced by VVhshshs, where the last three figures for hshshs indicate
the vertical visibility in units of 30 m (100 ft) (WMO, 2019; WMO, 2020).
INDICATION OF SIGNIFICANT CHANGES
When one set of prevailing weather conditions is expected to change significantly and
more or less completely to a different set of conditions, the time indicator group
FMYYGGgg (where FM is the abbreviation for “from”, YY is the date and GGgg is the time
in hours and minutes UTC) is used to indicate the beginning of a self-contained part of the
forecast. All conditions given before this group are superseded by conditions indicated
after the group.
The groups BECMG YYGG/YeYeGeGe indicate a regular or irregular change to the forecast
meteorological conditions expected at an unspecified time within the period YYGG to
YeYeGeGe. This period will normally not exceed 2 h but will never be more than 4 h.
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The groups TEMPO YYGG/YeYeGeGe indicate temporary fluctuations in the forecast
meteorological conditions which may occur at any time during the period YYGG to
YeYeGeGe. The meteorological conditions following these groups are expected to last less
than 1 h in each instance and in aggregate less than half the period indicated by
YYGG/YeYeGeGe.
When the confidence in forecasting alternative values is not high, yet the forecast
element being considered is significant to aircraft operations, the groups PROBC2C2
YYGG/YeYeGeGe are used. C2C2 indicates the percentage probability of occurrence, with
only the values 30 or 40 per cent being used (WMO, 2019; WMO, 2020).
Meteorological Information and codes
12.3. SIGMET
A SIGMET provides concise information issued by a Meteorological Watch Office (MWO)
concerning the occurrence or expected occurrence of specific en-route weather and other
phenomena in the atmosphere that may affect the safety of aircraft operations. The WS
SIGMET provides information on phenomena other than tropical cyclones and volcanic
ash (ICAO, 2016).
The SIGMET structure is as follows:
SIGMET WMO Header contains
Disseminating center
CCCC is the ICAO location indicator of the communication center disseminating the
message (this may be the same as the MWO location indicator).
Transmission time
YYGGgg is the date/time group; where YY is the day of the month and GGgg is the time of
transmission of the SIGMET in hours and minutes UTC (normally this time is assigned by
the disseminating (AFTN) center).
Correction indicator
BBB should only be included when issuing a correction to a SIGMET which had already
been transmitted. The BBB indicator shall take the form CCx for corrections to previously
relayed bulletins, where x takes the value A for the first correction, B for the second
correction, etc., for a specific SIGMET.
Location indicator
CCCC is the ICAO location indicator of the ATS unit serving the FIR or CTA to which the
SIGMET refers.
Message identifier
The message identifier is SIGMET.
Sequence number
The daily sequence number in the form [n][n]n, e.g. 1, 2, 01, 02, A01, A02, T01 restarts
every day for SIGMETs issued from 0001 UTC.
Validity period
The validity period is given in the format VALID YYGGgg/YYGGgg where YY is the day of
the month and GGgg is the time in hours and minutes UTC. The period of validity for a WS
SIGMET shall be no more than 4 hours.
Issuing Office
CCCC- is the ICAO location indicator of the MWO originating the message followed by a
hyphen (ICAO, 2016).
Example of TAF report:
WSRO31 LROM 250953 LRBB SIGMET T04 VALID 250955/251155 LROM-
LRBB BUCURESTI FIR EMBD TS OBS WI N4420 E02230 - N4450 E02340 - N4405 E02425 -
N4345 E02300 - N4420 E02230 TOP FL360 MOV ENE 10KT NC
SIGMET Main Body
FIR/CTA Name
The ICAO location indicator and full name of the FIR/CTA for which the SIGMET is issued
in the form CCCC <name> FIR[/UIR] or CCCC <name> CTA
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Phenomenon Code Description reported in SIGMET messages are as follows:
OBSC TS Obscured thunderstorms
EMBD TS Embedded thunderstorms
FRQ TS Frequent thunderstorms
SQL TS Squall line thunderstorms
OBSC TSGR Obscured thunderstorms with hail
EMBD TSGR Embedded thunderstorms with hail
FRQ TSGR Frequent thunderstorms with hail
SQL TSGR Squall line thunderstorms with hail
SEV TURB Severe turbulence
SEV ICE Severe icing
SEV ICE (FZRA) Severe icing due to freezing rain
SEV MTW Severe mountain wave
HVY DS Heavy dust storm
HVY SS Heavy sandstorm
RDOACT CLD Radioactive cloud
Whether the phenomenon is observed or forecast in the form OBS [AT GGggZ] or FCST [AT
GGggZ] where GG is hours and gg minutes UTC.
Location
The location of the phenomenon is provided with reference to geographical coordinates
in latitude and longitude in degrees and minutes. The number of coordinates should be
kept to a minimum and should not normally exceed seven.
Level
The level and vertical extent of the phenomenon: FLnnn or nnnnM or nnnnFT or
SFC/FLnnn or SFC/nnnnM or SFC/nnnnFT or FLnnn/nnn or nnnn/nnnnFT or TOP FLnnn or
ABV FLnnn or TOP ABV FLnnn. Movement or Expected Movement (not included if
‘forecast time’ and ‘forecast position’ are given).
Direction and rate of movement of the phenomenon where the direction is given with
reference to one of the sixteen points of the compass (using the appropriate abbreviation)
and the rate is given in KT (or KMH) in the form MOV <direction> <speed>KT or KMH. The
abbreviation STNR (Stationary) is used if no significant movement is expected.
Changes in Intensity
The expected evolution of the phenomenon’s intensity as indicated by:
INTSF or WKN or NC
Forecast time and forecast position (not included if movement given). The forecast
position of the hazardous phenomena at the end of the validity period of the SIGMET
message in the form FCST AT <GGgg>Z <location> (ICAO, 2016).
Meteorological Information and codes
12.4. Analysis of weather charts: significant weather, 500 hPa
geopotential height, surface analysis and prognosis
Weather charts are graphic charts that depict current or forecast weather. They provide
an overall picture of a specific region and should be used in the beginning stages of flight
planning. Typically, weather charts show the movement of major weather systems and
fronts. Surface analysis, weather depiction, and radar summary charts are sources of
current weather information. Significant weather prognostic charts provide an overall
forecast weather picture.
SIGWX Forecast Charts are valid for 00, 06, 12 and 18 UTC. SIGWX High (SWH) are normally
available sixteen hours before validity; and SWM are normally available twenty hours
before.
Forecast charts are valid for the time point indicated (00, 06, 12 or 18 UTC) but are used
for operations within three hours each side of this time (BOM, 2012).
Airspace:
• FL250 to FL630 – SIGWX High (SWH)
• FL100 to FL250 – SIGWX Medium (SWM)
The following phenomena are shown on the charts:
• tropical cyclones
• moderate or severe turbulence, including clear air turbulence (CAT)
• moderate or severe icing
• surface fronts
• cumulonimbus (CB) cloud associated with thunderstorms and with any of the
above
• non-convective cloud associated with in-cloud moderate or severe turbulence or
icing
• jet streams
• volcanic eruptions
• tropopause heights
• radioactive material
Note that the inclusion of CB on SIGWX charts implies the existence of thunderstorms,
moderate or severe turbulence, moderate or severe icing, and hail.
Cloud amount is indicated by the standard abbreviations:
• ISOL, OCNL, FRQ and EMBD for CB
• FEW, SCT, BKN and OVC for clouds other than CB
A similar convention is employed to indicate the forecast height of icing and turbulence.
Jet streams are indicated by a solid line with pennants to show wind speeds of 50 knots,
feathers to show 10 knots, and half feathers to show 5 knots (Figure 21). A 20 knot change
is indicated by two parallel lines across the jet stream. The tropopause is indicated at
various points by a rectangular box framing the flight level of the tropopause. The location
of the highest and lowest tropopause points may also be shown (BOM, 2012).
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Figure 21: Significant Weather (SIGWX) Chart symbols (Source: BOM, 2012)
Figure 22: Example of SIGWX chart (source: https://wxcharts.com/)
Meteorological Information and codes
The geopotential height of the 500 hPa pressure surface shows approximately how far
from the surface the pressure drops to 500 hPa (i.e. 500 millibars) in the atmosphere. On
average this level is around 5.5 km above sea level, and it is often referred to as a steering
level, because the weather systems beneath, near to the Earth's surface, roughly move in
the same direction as the winds at the 500 hPa level. Height contours are labelled in tens
of meters (=decameters, ="dam") with an interval of 4 dam. The contours effectively show
the main tropospheric waves that largely influences our weather. Low heights indicate
troughs and cyclones in the middle troposphere whilst high heights indicate ridges and
anticyclones. Black contours indicate the geopotential height of the 500 millibar surface,
in tens of meters. Low geopotential height (compared to other locations at the same
latitude) indicates the presence of a storm or trough at mid-troposphere levels. Relatively
high geopotential height indicates a ridge, and quiescent weather.
The color shaded contours indicate vorticity at 500 millibars: Red for positive vorticity,
blue for negative. Positive vorticity indicates counterclockwise rotation of the winds,
and/or lateral shear of the wind with stronger flow to the right of the direction of flow.
Negative vorticity indicates clockwise rotation of the winds, and/or lateral shear of the
wind with stronger flow to the left of the direction of flow. Positive (or negative in the
Southern Hemisphere) vorticity at 500 millibars is associated with cyclones or storms at
upper levels and will tend to coincide with troughs in the geopotential height field.
Negative (positive in SH) vorticity is associated with calm weather and will tend to coincide
with ridges in the geopotential height field (http://wxmaps.org/fcstkey.php).
Figure 23: Example of 500 hPa geopotential height (source: https://www.wetterzentrale.de/)
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Supplementary information can be found at:
http://www.atmo.arizona.edu/students/courselinks/spring16/atmo336/lectures/sec1/i
nfo500mb.html.
Surface analysis charts show the surface pressure pattern over large areas. They display
isobars and indicate areas of high and low pressure along with their central pressure
value. High pressure is usually associated with mainly dry, settled weather with light winds
while low pressure is normally associated with unsettled weather which is often wet and
windy. The lower the pressure generally the stronger the winds and the heavier the rain
are. Among pressure pattern, fronts are represented as well on these charts. Usually, a
cold front is blue, a warm front is red, and an occlusion is purple.
Surface pressure charts showing pressure and weather fronts are provided up to five days
ahead for Europe and the Northeast Atlantic.
Figure 24: Surface pressure and frontal analysis (source: https://www.met.ie/latest-reports/surface-
analysis)
Meteorological Information and codes
12.5. End of Chapter Questions
What types of clouds are reported in a METAR?
When VRB is reported in a METAR?
Decode the following METAR: METAR LRAR 280600Z 17009KT 6000 -SHSN SCT003
SCT030CB 01/01 Q1005.
Decode the following TAF: TAF LROP 040500Z 0406/0506 VRB04KT 9999 SCT015
BKN030 TEMPO 0406/0411 5000 -SHRA BKN010 SCT025CB OVC030 PROB40 TEMPO
0411/0418 3000 TSRA BKN010 SCT030CB BKN045 PROB30 TEMPO 0502/0506 5000
-SHRA BKN007 OVC030.
Decode the following SIGMET: WSSN31 ESWI 080206 ESAA SIGMET M01 VALID
080210/080610 ESSAESAA SWEDEN FIR SEV MTW FCST WI N6839 E02140 - N6611
E01722 - N6626 E01514 - N6835 E01818 - N6907 E02025 - N6839 E02140 SFC/FL200
STNR
How are different types of fronts represented on weather charts?
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12.6. Chapter bibliography
BOM (2012) Australian Bureau of Meteorology. Significant weather (SIGWX) charts.
www.bom.gov.au/aviation/knowledge-centre
ICAO (2016) SIGMET quick reference guide, e-documents.
https://www.icao.int/APAC/Documents/edocs/WS-SIGMET.pdf
WMO (2019) Manual on Codes, WMO- No. 306, CH-1211 Geneva 2, Switzerland
WMO (2020) Aerodrome Reports and Forecasts A Users’ Handbook to the Codes, 2020 edition,
WMO-No. 782, CH-1211 Geneva 2, Switzerland
https://wxcharts.com/
http://wxmaps.org/fcstkey.php
https://www.wetterzentrale.de/
http://www.atmo.arizona.edu/students/courselinks/spring16/atmo336/lectures/sec1/info500m
b.html
https://www.met.ie/latest-reports/surface-analysis
Climatology
Chapter 13 - Climatology
13.1. Typical weather situations in Europe
European continent has a mild climate with few extreme events yearly. However, in the
context of climate change the frequency of extreme events increased. The mild climate is
primarily due to the Atlantic Ocean’s warm Gulf Stream current, which exerts a
moderating effect on a significant portion of the continent, particularly its western half.
The highest quantity of precipitation falls on the western Europe, on mountain ranges on
the altitudes higher than 1000 m and on the western coasts of Great Britain, Norway,
Italy, and Balkan Peninsula.
Patterns of some permanence controlling air mass circulation are created by belts of air
pressure over five areas. They are the Icelandic low, over the North Atlantic; the Azores
high, a high-pressure ridge; the (winter) Mediterranean low; the Siberian high, localized
over Central Asia in winter but extending westward; and the Asiatic low, a low-pressure
summertime system over southwestern Asia. Given those pressure configurations,
westerly winds generally prevail over Europe. The winter westerlies, often from the
southwest, transport warm tropical air; in summer, by contrast, they veer to the
northwest and bring in cooler Arctic or subarctic air. In Mediterranean Europe the rain-
bearing westerlies affect the western areas, but only in winter. In the same season, the
eastern Mediterranean basin experiences bitter easterly and northeasterly winds derived
from the Siberian high. Those winds occasional projection westward explains unusually
cold winters in western and central Europe, while exceptionally warm winters in that
region result from the sustained flow of tropical maritime air masses. In summer the
Azores high moves 5° to 10° of latitude northward and extends farther eastward,
preventing the entry of cyclonic storms into the dry Mediterranean region. The eastern
basin, however, experiences the hot and dry north and northeast summer winds called
etesian by the ancient Greeks, formed by the relative location of the semi-permanent high
at the Balkan Peninsula and the low-pressure system over Cyprus. In summer too, the
Siberian high gives place to a low-pressure system extending westward, so that westerly
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air masses can penetrate deeply through the continent, making summer generally a wet
season.
Because of the interplay of so many different air masses, Europe experiences highly
changeable weather. Winters get sharply colder eastward, but summer temperatures
relate fairly closely to the normal of each latitude. Northwestern Europe, including
Iceland, enjoys some amelioration because of warm Gulf Stream waters of the Atlantic,
which, for example, keep the Russian port of Murmansk open throughout the year
(https://www.britannica.com/place/Europe/Climate).
It is worth mentioning that westerly winds bring a significant contribution to weather
configuration in Europe. Humid air flows in a stretched westerly high drift from the
Atlantic towards Europe. Embedded in this, Polar-Front waves connected to low-pressure
cells drift over Central Europe at an interval of one or two days. During west-wind
influence the weather in different parts of Europe is quite changeable (Burri, 2011).
Anticyclonic situations bring calm weather over Europe with almost no winds and
precipitation inside the anticyclone unless local topographical effects become important
in specific seasons. In this type of circulation, the air masses are descending slowly and on
a large scale (subsidence). The descending air becomes warmer by compression,
therefore relative humidity decreases (so it is bound to be below 100%), and clouds break
up. Anticyclonic areas are therefore regions of fair weather which move only very slowly
away and generate good weather to aviation from a few days to several weeks. In winter,
the earth's surface cools down significantly in an anticyclonic situation. It cools down the
ground layer in which extensive fogbanks develop; these cannot dissolve during the
daytime between November and January. The average thickness of these fogbanks is 200
m (Burri, 2011).
Flat-pressure gradient configuration is typical to summer. In contrast to an anticyclonic
situation, there is no subsidence in flat-pressure gradient configuration, thus the uplift of
warm air is not slowed down, and cumuliform clouds form (Burri, 2011).
Portuguese mainland is in the south-western edge of continental Europe, it spans over an
area between 37° and 42°N, and by -6.5° and -9.5°W and shares the Iberian Peninsula with
Spain. According to the Köppen-Geiger classification (Peel et al. 2007), most of mainland
Portugal lies in the temperate climate zone, which is conditioned by large scale circulation
patterns, driven by sub-polar transient low-pressure systems during the winter, carrying
frontal systems, and by a mid-latitude semi-permanent anticyclone during the summer,
which keeps a persistent northern wind regime along the shore. In this typical dry season,
climate contrasts arise between inland hot weather and mild temperatures along the
coast (Cunha et al. 2011). Another climate factor regards the geography which is
characterized by gentle landforms and mountain relief of typical elevation values between
1000 and 1500 m, and a maximum height of 1993 m at Serra da Estrela, embedded in a
northeast-southwest mountain range in the central region of Portuguese mainland. The
precipitation distribution is characterized by large spatial variations, with annual
precipitation ranging between 400 mm at southeast inland, and 2200 mm in the north-
western region (Soares et al. 2012). The fog as an impacting phenomenon on aviation is
one of the most important weather limitations. Its occurrence varies greatly. The
frequency of fog occurrence decreases from north to south and has distinct
characteristics, being a winter phenomenon at north and a summer one at south
(Guerreiro et al. 2020). Despite the 10% of the annual frequency of the daily fog events,
Climatology
the most common last one hour, but some of them are very persistent and last up to 3
days.
The Portuguese Air Force Weather Group’s mission is to provide meteorological
information for the planning and development of air operations, in Portugal and abroad,
by integrating joint and combined forces. The development of the Weather Information
System over the last 30 years has achieved the desired interoperability in the European
and NATO military contexts, with tactical and control skills. National and deployment
Weather Group responsibilities ensure weather observation, forecast, weather watch and
risk assessment with permanent local meteorological observations and report,
continuously, 24/7.
Romania is located in Southeastern Europe. The Romanian territory extends from 43°40ʹN
to 48°11ʹN and from 20°19ʹE to 29°66ʹE. The country's landform types are relatively
equally distributed between mountains (the Carpathians, 31% of the country's area), hills
and tablelands (33%) and lowlands (plains and meadows, 36%), and range altitudinally
from 0 to 2544 m a.s.l. (Prăvălie et al., 2019). It has a temperate climate with prevailing
continental influences in the eastern and southern regions and moderate oceanic
conditions in central and western regions (Croitoru et al., 2016). Precipitation ranges from
300 to 700 mm in East and South and generally above 500 mm in central and western
regions. In the Carpathian Mountains, precipitation rises to values higher than 1000 mm.
These differences are mainly determined by the presence and position of Carpathian
Mountains (Croitoru et al., 2016). The regime of air temperatures follows a particular
pattern with excessive conditions during summer and winter in eastern regions and milder
conditions in western regions. Southern regions are dominated by hot conditions in
summer and mild temperatures in winter.
Several other features of a smaller scale can also be distinguished. Mediterranean
influences are present in southwestern region, while Baltic and Scandinavian influences
are specific to northwestern region, which is generally colder and wetter than other
regions of Romania.
The main climatic parameters describe very well the particular features of the Romanian
climate. Thus, mean multiannual temperatures ranges between 1 °C in mountainous
regions and 11-12 °C in the south. Precipitations values range from 300-400 mm in the
east and southeast and to over 1000 mm in the Carpathian Mountains (Prăvălie et al.
2019).
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International Air Force Semester
13.2. High- and low-pressure systems
Atmospheric pressure has a great influence on weather and climate of a region. Standard
pressure at sea level is defined as 1013.25 hPa, but we can see large areas of either high
or low pressure. These areas are all relative to each other, thus what defines a high will
change depending on the areas around it. On a weather chart, atmospheric pressure is
expressed through isobars. Their configuration shows features such as anticyclones,
cyclones, or other types of systems of high or low pressure.
In general, low pressure leads to unsettled weather conditions and high pressure leads to
settled weather conditions. In an anticyclone (high pressure) the winds tend to be light
and blow in a clockwise direction in the northern hemisphere. Also, the air is descending,
which reduces the formation of cloud and leads to light winds and settled weather
conditions (https://www.metoffice.gov.uk/weather/learn-about/weather/how-weather-
works/high-and-low-pressure).
In a depression (low pressure), air is rising and blows in an anticlockwise direction around
the low in the northern hemisphere. As it rises and cools, water vapor condenses to form
clouds and perhaps precipitation. This is why the weather in a depression is often
unsettled, there are usually weather fronts associated with depressions
(https://www.metoffice.gov.uk/weather/learn-about/weather/how-weather-
works/high-and-low-pressure).
In winter the clear, settled conditions and light winds associated with anticyclones can
lead to frost and fog. The clear skies allow heat to be lost from the surface of the earth by
radiation, allowing temperatures to fall steadily overnight, leading to air or ground frosts.
Light winds along with falling temperatures can encourage fog to form; this can linger well
into the following morning and be slow to clear. If high pressure becomes established over
Northern Europe during winter this can bring a spell of cold easterly winds over Western
Europe (https://www.metlink.org/resource/weather-systems/).
In summer the clear settled conditions associated with anticyclones can bring long sunny
days and warm temperatures. The weather is normally dry, although occasionally, very
hot temperatures can trigger thunderstorms
(https://www.metlink.org/resource/weather-systems/).
Climatology
13.3. Seasonal variations of meteorological parameters
Climate is typically described by the summary statistics of a set of atmospheric and surface
variables such as temperature and precipitation. The official average value of a
meteorological element for a specific location over 30 years is defined as a climate normal.
Temporal climate variations are emphasized in normal rhythm from a season to another.
The amplitude of seasonal variability is generally larger than that of the diurnal cycle at
high latitudes and smaller at low latitudes (WHO, 2003).
Seasons are regulated by the amount of solar energy received at Earth’s surface. This
amount is determined primarily by the angle at which sunlight strikes the surface, and by
how long the sun shines on any latitude (daylight hours). More daylight hours, mean that
more energy is available from sunlight. In a given location, more solar energy reaches
Earth’s surface on a clear, long day than on a day that is clear but much shorter. Hence,
more surface heating takes place. From a casual observation, we know that summer days
have more daylight hours than winter days. Also, the noontime summer sun is higher in
the sky than is the noontime winter sun. Both situations occur because our spinning
planet is inclined on its axis (tilted) as it revolves around the sun.
Climatological seasons are defined by natural breakpoints in time series of climate
variable. In regions with less extreme seasonal shifts than those affected by the monsoon,
climatological seasons have been defined by intra-annual changes in air masses or
synoptic map types (Cannon, 2005). In Europe, traditionally exist four seasons such as
astronomical seasons and meteorological seasons are recognized as standard seasonal
cycles. Socio-economic activities during a year are shaped by the seasonal rhythm. Inter-
annual fluctuations of seasonal onsets and durations cause climatic stress on humans and
changes in flight operations.
In a traditionally way, it has been regarded those seasonal onsets recur on the same date
year to year, anywhere in the world, based on calendar months or astronomical
benchmarks. Four three-month fixed seasons, which are demarcated based on calendar
months, are called meteorological seasons. In northern hemisphere the onset and offset
of meteorological seasons are fixed on the same dates regardless of geographical
locations and years: spring (March, April and May), summer (June, July and August), fall
(September, October and November) and winter (December, January and February).
However, seasonal onset/offset and duration are not fixed spatially and temporally as
opposed to conventionally used meteorological data which support the calculation of
three-month fixed interval averages and other types of statistics. The analyses of data at
a global scale highlighted the dynamical characteristics of floating climatological seasons.
Satellite data play a crucial role in determining the “real” onset and offset of seasons.
Intra-annual northward or southward fluctuations of snow cover, air temperature, snow
cover, and vegetation condition are regarded as essential indicators that can be used in
defining such floating climatological seasons (Choi, 2007).
Air temperature is one of the most important parameters of atmospheric state. The
atmospheric temperature is controlled principally by the incoming solar radiation. Thus,
its distribution depends largely on the latitude. Also, it is affected by the nature of the
surface of the earth especially by the differences between water and land, by the altitude,
and by the prevailing winds. During January, the northern hemisphere has its winter. The
temperature decrease from the equator toward either pole is considerably greater in
winter than in summer due to the seasonal variation of radiation. The greatest surplus of
radiation during January occurs at about 30° south, while the temperature maxima are
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International Air Force Semester
found at the equator. This is because of the fact that the temperature does not depend
on the radiative balance alone (Tathe, 2012).
The annual range of temperature is the difference between the mean temperature of the
warmest and of the coldest month. It is notices that at the equator and at 10° N latitude
the range is not equal to the difference between the mean temperatures of July and
January because at these latitudes the monthly maxima and minima are not reached in
July and in January (Tathe, 2012).
The unequal heating of water and land produces considerable modifications of the
pressure and wind distribution. These deviations are more noticeable during the extreme
months of January and July (Tathe, 2012).
During July, the large continental mass of the northern hemisphere is much warmer than
the surrounding oceans. Hence a lower pressure is found over the continents, which
interrupt the high-pressure belt. The continental low is in direct connection with the
equatorial low-pressure zone. The southern hemisphere has winter in July so that here
the land is colder than the water. But the disturbance of the pressure distribution is much
less because of the smaller land area in the southern hemisphere.
During January in the northern hemisphere, the continent is much colder than the
adjoining oceans where a strong high is developed. On the south side of the continental
anticyclone, the winds are northeasterly. The centers of low pressure over the oceans are
considerably intensified during the winter so that the cyclonic activity is now at a
maximum. On the north side of the continental anticyclone, the winds are westerly. Thus,
the belts of westerlies surround the northern hemisphere completely, but are displaced
more northward over the continents (Tathe, 2012).
Climatology
13.4. End of Chapter Questions
How would you describe the climate of Europe?
Which winds are prevailing over the European continent?
How does a cyclone rotate in the northern hemisphere?
What are the main drivers of seasons and how are they generated?
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13.5. Chapter bibliography
Burri, Klaus (2011) Climatology and Meteorology Weather Situations in Switzerland,
Kantonsschule Enge Zürich.
Cannon, A. (2005) Defining climatological seasons using radially constrained clustering.
Geophysical Research LettersVolume 32, Issue 14, L14706
Choi, Gwangyong (2007) Climatology and variability of northern hemisphere seasons, Doctoral
thesis, Rutgers, The State University of New Jersey
Croitoru AE, Piticar A, Ciupertea AF, Roşca CF. Changes in heat waves indices in Romania over the
period 1961–2015. Global and Planetary Change 2016, 146, 109–121
Cunha S, Silva A, Herráez C, Vanda P, Chazarra A, Mestre A, Luis N, Mendes M, Neto J, Marques J,
Mendes L (2011) Atlas Climático Ibérico – Iberian Climate Atlas. AEMET, IM. ISBN:978-84-
7837-079-5
Guerreiro, P.M.P.; Soares, P.M.M.; Cardoso, R.M.; Ramos, A.M. An Analysis of Fog in the Mainland
Portuguese International Airports. Atmosphere 2020, 11, 1239.
Peel MC, Finlayson BL, McMahon TA (2007) Updated world map of the Köppen-Geiger climate
classification. Hydrology and Earth System Sciences, 11(5), 1633–1644. doi:10.5194/hess-
11-1633-2007
Prăvălie R., Piticar A., Roșca B., Sfîcă L., Bandoc G., Tiscovschi A., Patriche C. Spatio-temporal
changes of the climatic water balance in Romania as a response to precipitation and
reference evapotranspiration trends during 1961–2013, 2019, CATENA 172, 295-312
Soares PM, Cardoso RM, Miranda PM, Viterbo P, Belo-Pereira M (2012) Assessment of the
ENSEMBLES regional climate models in the representation of precipitation variability and
extremes over Portugal. Journal of Geophysical Research: Atmospheres, 117(D7)
Tathe, A.D., (2012) Lecture notes on climatology. India Meteorological Department
WHO (2003) Climate change and human health. Risks and responses, Switzerland, Geneva
https://www.britannica.com/place/Europe/Climate
https://www.metoffice.gov.uk/weather/learn-about/weather/how-weather-works/high-and-
low-pressure
https://www.metlink.org/resource/weather-systems/
Σχολή Ικάρων Força Aérea Portuguesa Academia Fortelor Aeriene European Security and Defence College
“Henri Coanda”
International Air Force Semester
2020-1-EL01-KA203-079068
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