Jeppesen Meteorology

Meteorological Observations and Meteorological Services Chapter 21

ACCURACY OF METEOROLOGICAL MEASUREMENT OR OBSERVATION

The accuracies listed refer to assessment by instruments (except cloud amount). They are not
usually attainable in observations made without instruments.

Element Accuracy of Measurement or Observation

Mean Surface Wind Direction: ± 5°
Speed: ± 1 kt up to 20 kt, ± 5% above 20 kt

Variations from the mean ± 1 kt
surface wind

Visibility ± 50 m up to 500 m
± 10% between 500 m and 2000 m
± 20% above 2000 m up to 10 km

RVR ± 25 m up to 150 m
± 50 m between 150 m and 500 m
± 10% above 500 m up to 2000 m

Cloud amount ± 1 okta in daylight, worse in darkness and during
atmospheric obscuration

Cloud height ± 33 ft up to 3300 ft
± 100 ft above 3300 ft up to 10 000 ft

Air temperature and dew point ± 0.2° C
temperature

Pressure value (QFE, QNH) ± 0.3 mb

MARKED TEMPERATURE INVERSION

At certain aerodromes a Warning of Marked Temperature Inversion is issued whenever a
temperature difference of 10° C or more exists between the surface and any point up to 1000 ft
above the aerodrome. This warning is broadcast on departure and arrival ATIS at aerodromes so
equipped, or in the absence of ATIS passed by radio to departing aircraft before take-off, and to
arriving aircraft as part of the report of aerodrome meteorological conditions.

AERODROME WARNINGS

Aerodrome warnings are issued as appropriate when one or more of the following occurs or is
expected to occur:

a. Gales or strong winds agreed to locally agreed criteria (Gales — mean surface
wind >33 kt or gusts >42 kt).

b. Squalls, hail or thunderstorms.
c. Snow, including the expected time of beginning, duration and intensity of fall; the

expected depth of accumulated snow, and the time of expected thaw.
Amendments or cancellations are issued as necessary.
d. Frost warnings when any of the following are expected to exist.
i. A ground frost with air temperatures not below freezing point.
ii. The air temperature above the surface is below freezing (Air frost).
iii. Hoar frost, rime or glaze deposited on parked aircraft.
e. Fog (normally when visibility is expected to fall below 600 m).
f. Rising dust or sand.
g. Freezing precipitation.

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Chapter 21 Meteorological Observations and Meteorological Services

SPECIAL FACILITIES

WINDSHEAR ALERTING

Forecasters at airports review the weather conditions on an hourly basis and monitor any aircraft
reports of windshear experienced on the approach or climb-out. Whenever a potential low level
windshear condition exists, a Windshear alert, based on one or more of following criteria, is
issued:

a. Mean surface wind speed at least 20 kt.
b. The magnitude of the vector difference between the mean surface wind and the gradient

wind (an estimate of the 2000 ft wind) of at least 40 kt.
c. Thunderstorm(s) or heavy shower(s) within approximately 5 nm of the Airport.

Note: Alerts are also issued based on recent pilot reports of windshear on the
approach or climb-out.

The Alert message is given in the arrival and departure ATIS in one of three formats:

a. Windshear Forecast (WSF)
When the meteorological conditions indicate that low level windshear on the approach
or climb-out (below 2000 ft) might be encountered.

b. Windshear Forecast and Reported (WSFR)
As above, supported by a report from at least one aircraft of windshear on the approach
or climb-out within the last hour.

c. Windshear Reported (WSR)
When an aircraft reports windshear on the approach or climb-out within the last hour, but
insufficient meteorological evidence exists for the issue of a forecast of windshear.

WINDSHEAR REPORTING CRITERIA

Pilots using navigation systems providing direct wind velocity readout should report the wind and
altitude/height above and below the shear layer, and its location. Other pilots should report the
loss or gain of airspeed and/or the presence of up or down draughts or a significant change in
crosswind effect, the altitude/height and location, their phase of flight, and aircraft type. Pilots not
able to report windshear in these specific terms are to do so in terms of its effects on the aircraft,
the altitude/height and location and aircraft type (e.g. Abrupt windshear at 500 ft on finals,
maximum thrust required, B747).

Pilots encountering windshear are requested to make a report even if windshear was previously
forecast or reported.

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Meteorological Observations and Meteorological Services Chapter 21

OBSERVING SYSTEMS AND OPERATING PROCEDURES — SURFACE
WIND

Surface wind sensors are positioned to give the best practical indication of the winds which an
aircraft encounters during take-off and landing within the layer between 6 and 10 m above the
runway. The surface wind reported for take-off and landing by ATS units supporting operations by
aircraft whose MTWA is less than 5700 Kg is usually an instantaneous wind measurement with
direction referenced to Magnetic North. At other designated airports the wind reports for take-off
and landing are averaged over the previous 2 minutes. Variations in the wind direction are given
when the total variation is 60° or more and the mean speed above 3 kt. The directional variations
are expressed as the two extreme directions between which the wind has varied in the past 10
minutes. In reports for take-off, surface winds of 3 kt or less include a range of wind directions
whenever possible if the total variation is 60° or more. Variations from the mean wind speed
(gusts) during the past 10 minutes are only reported when the variation from the mean speed
exceeds 10 kt. Variations are expressed as the maximum and minimum speeds attained.

Note: Surface wind measurement in a METAR and SPECI are referenced to true
north.

CLOUD HEIGHT

Information on cloud height is obtained by the use of:

a. Ceilometers
b. Cloud searchlights
c. Alidades
d. Balloons
e. Pilot reports or observer estimation

At some aerodromes an additional cloud ceilometer is installed on the approach. The cloud
heights reported from an approach ceilometer are:

a. The most frequently occurring value during the past 10 minutes if the value is 1000 ft or
less.

b. If cloud is indicated at heights 100 ft or more below that indicated at (a) above then the
height of the lowest cloud is reported, prefaced by OCNL.

c. If the most frequently occurring value is above 1000 ft but the lowest value is 1000 ft or
below, then only the lowest value is reported.

TEMPERATURE

Temperature is reported in whole degrees Celsius, M indicates a negative value.

HORIZONTAL SURFACE VISIBILITY

Horizontal surface visibility is assessed by human observer. Visibility is reported in increments of:

a. 50 m up to 500 m
b. 100 m up to 5000 m

In METAR, SPECI or TAF the maximum value is "10 km or more."

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Chapter 21 Meteorological Observations and Meteorological Services

When the lowest visibility is less than 1500 m and visibility in another direction is more than
5000 m, additionally, the maximum visibility and the direction in which it occurs is reported.

RUNWAY VISUAL RANGE (RVR)

RVR assessment is made by either human observer or by an Instrument RVR system (IRVR).
For the UK the standard RVR reporting increments are:

a. 25 m between 0 and 200 m.
b. 50 m between 200 and 800 m.
c. 100 m from 800 m.

Assessment and reporting in RVR begin when the horizontal visibility, or the RVR, is observed at
less than 1500 m.

RVR is passed to aircraft before take-off and during the approach to landing. Changes to the
RVR are passed throughout an aircraft's approach.

Where multi-site IRVR systems are installed, the procedure is for touchdown, mid-point and stop
end values of RVR to be given (e.g. RVR 600, 500, 550). Where only touchdown and one other
value is given, the RVR is given as RVR 500 stop end 500. Where a single transmitter fails and
the remainder of the system is serviceable RVR readings are not suppressed (e.g. RVR
touchdown missing, 600, 500). If two out of the transmissions fails, then the remaining value is
given provided that it is not the stop end value.

Aerodromes suppress mid-point and/or stop-end values when:

a. They are equal to or higher than the touchdown zone value unless they are less than
400 m.
Example: 300 350 350 All values are reported.
or

b. They are 800 m or more.
Example: 1000 900 900 Only the touchdown value is reported.

21-10 Meteorology

INTRODUCTION

A variety of weather messages are originated by Meteorological Observers at aerodromes. These
are collated and broadcast in text form to stations around the world. Aviators are usually able to
distinguish between the various message types and their uses.

AERODROME METEOROLOGICAL REPORT

Aerodrome Meteorological Reports (METAR) contain observations on the conditions that actually
exist at a station and are made every 30 minutes throughout the day.

¾ Short term landing forecasts, valid for two hours (TREND), may be added to
METARS.

¾ Information on runway condition is added to METAR when appropriate, until these
conditions cease.

SPECIAL AERODROME METEOROLOGICAL REPORTS

Special Aerodrome Meteorological Reports (SPECI) are issued when conditions change
significantly. Selected Special Reports (SPECI) are defined as Special Reports disseminated
beyond the aerodrome of origin.

TERMINAL AERODROME FORECASTS

Terminal Aerodrome Forecasts (TAF) are normally provided only for those aerodromes where
official meteorological observations are made. For other aerodromes, Local Area Forecasts are
made. Amended TAFs or Local Area Forecasts are issued when forecast conditions change
significantly.

ACTUAL WEATHER CODES

The content and format of an actual weather report are shown in the following table.

Report Location Date/Time Wind Visibility RVR
Type Identifier 291250Z
31015G30KT 1400SW R24/P1500
METAR EGSS 6000N
Recent Wind
Present Cloud Temp/ QNH Weather Shear Trend Rwy
Weather Dew Pt Q0999 State
FEW005 RETS WS
SHRA SCT010CB 10/05 RWY25 NOSIG 88290592

BKN025

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Chapter 22 Meteorological Messages

IDENTIFIER

The identifier has three components:

Report Type: METAR or SPECI.
ICAO Indicator:
This is a four-letter group indicating the airfield
Date/Time UTC:
(e.g. EGPL, LFPB).

In a METAR or SPECI this is the date and time of the
observation in hours and minutes UTC (e.g. 091250Z).

Example: METAR EGDL 211020Z.

Note: If a meteorological bulletin consists of a set of reports from one or more airfields the
codename METAR or SPECI is replaced by SA (Actual Report), or SP (Special Report) followed
by a bulletin identifier, date, and time of the observation.

SURFACE WIND VELOCITY

The first 3 figures indicate the wind direction (T) to the nearest 10°, followed by two figures
(exceptionally 3 figures) giving the mean windspeed during the previous 10 minutes. The
permitted units of speed are:

¾ KT indicating knots
¾ KMH for kilometres per hour, or
¾ MPS for metres per second.

Example: 30015KT.

These may be followed by a letter G and two more figures if the maximum gust speed exceeds
the average speed by 10 kt or more.

Example: 30015G30KT.

Variations in wind direction of 60° or more in the 10 minutes preceding the observation are shown
as 3 figures then the letter V followed by another 3 figures, but only if the speed is more than 3 kt.

Example: 270V330 meaning, the wind is varying in direction between 270°T and
330°T.

00000 indicates calm conditions, a variable wind direction is shown by VRB followed by the
speed.

HORIZONTAL VISIBILITY

When there is no marked variation in direction the minimum visibility is given in metres. The
minimum visibility with the direction is given when there is a marked variation with direction.

Example: 2000NE.

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Meteorological Messages Chapter 22

When the minimum visibility is less than 1500 metres and the visibility in any other direction is
greater than 5000 metres the maximum visibility and its direction is also shown.

Example: 1200NE 6000SW.

9999 indicates a visibility of 10 kilometres or more, 0000 indicates a visibility of less than 50
metres.

RUNWAY VISUAL RANGE (RVR)

Runway Visual Range is reported when the meteorological visibility falls below 1500 m. It has the
form R, followed by the runway designator, a diagonal and then the Touchdown RVR. If more
than one runway is in use, the RVR group is repeated. Parallel runways are distinguished by
adding C, L, or R to the runway designator.

Example: R24L/1200R24R/1100.

When RVR is greater than the maximum assessable value the prefix P is added followed by the
maximum value.

Example: R15/P1500.

The prefix M indicates the RVR is less than the minimum value that can be assessed.

Example: R15/M0050.

Tendencies are indicated by U for up, D for down, or N for no change. They show a significant
change (100 m or more) from the first 5 minutes to the second 5 minutes in the 10 minute period
prior to the observation.

Example: R25/1000D.

Variations are reported if the RVR changes minute by minute during the 10 minute period prior to
the report. The 1 minute minimum and maximum separated by V are reported instead of the 10
minute mean.

Example: R15L/0850V1000.

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Chapter 22 Meteorological Messages

WEATHER

Each weather group may consist of the appropriate intensity indicators and abbreviations, making
groups of two to nine characters from the table below.

SIGNIFICANT PRESENT AND FORECAST WEATHER CODES

QUALIFIER WEATHER PHENOMENA

Intensity or Descriptor Precipitation Obscuration Other
Proximity

1 2 3 45

- Light MI DZ BR PO
Shallow Drizzle Mist Dust/sand

whirls

Moderate BC RA FG
(no qualifier) Patches Rain Fog

PR SN FU SQ
Snow Smoke Squalls
Partial (Covering
part of Aerodrome)

+ Heavy DR SG VA FC
Drifting Snow Grains Volcanic Ash
'Well developed Funnel
in the case of FC Cloud(s)

and PO (tornado or
water-spout)

VC BL IC DU SS
Blowing Ice Crystals Widespread Sandstorm
In the vicinity (Diamond Dust)
(within 8 km of Dust

aerodrome
perimeter but

not at
aerodrome)

SH PE SA DS
Shower(s) Ice-Pellets Sand Duststorm

TS GR HZ
Thunderstorm Hail Haze

FZ GS
Freezing
Super-Cooled Small hail

(<5 mm diameter)

and/or snow
pellets

A mixture of weather is reported using up to three groups to indicate different weather types.

Examples: MIFG, VCBLSN, +SHRA, -DZHZ.

Note: BR, HZ, FU, IC, DU and SA will not be given in METAR or TAF when the
visibility is above 5000 m.

22-4 Meteorology

Meteorological Messages Chapter 22

CLOUD

The cloud group usually consists of 3 letters and 3 figures. These show the cloud amount
followed by the height of the cloudbase, above airfield level, in hundreds of feet. The cloud
groups are given in ascending order of height.

Example: SCT015 or OVC080.

These groups are:

FEW indicating 1 − 2 oktas SCT (scattered) indicating 3 − 4 oktas
BKN (broken) indicating 5 − 7 oktas OVC (overcast) indicating 8 oktas.

The cloud group may have a suffix for significant convective cloud, CB for Cumulonimbus, or
TCU for Towering Cumulus. No other cloud types are reported.

Example: BKN015CB.

Layers are reported as:

First Group: Lowest individual layer of any amount.
Second Group: Next individual layer of more than 2 oktas.
Third Group: Next higher layer of more than 4 oktas.
Additional Group: Significant convective cloud not already reported.

SKC indicates no cloud to report when CAVOK does not apply.

Sky obscured is shown by VV followed by vertical visibility in hundreds of feet. When the vertical
visibility is not assessed the group reads VV///.

Example: VV003.

CAVOK

CAVOK is for use in place of groups 4, 5, 6, and 7 when all of the following conditions apply:

a. Visibility is 10 km or more.

b. There is no cloud below 5000 ft or below the highest Minimum Sector Altitude (MSA),
which ever is greater, and no CB.

c. No significant weather phenomenon at or in the vicinity of the aerodrome.

Minimum Sector Altitude is the lowest altitude that is allowed for use under emergency conditions
which provides a minimum clearance of 1000 ft above all objects located in an area contained
within a sector of a circle of 25 nm radius centred on a radio navigation aid. A sector is not less
than 45°.

AIR TEMPERATURE AND DEWPOINT

Air Temperature and Dewpoint are reported in degrees Celsius. M indicates a negative value.

Examples: 10/08, 01/M01.

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Chapter 22 Meteorological Messages

SEA LEVEL PRESSURE (QNH)

QNH is reported in the form Q followed by a four-figure group. If the QNH is less than 1000 mb
the first figure is 0. QNH is rounded down to the nearest whole millibar.

Example: Q0995.

The pressure, when given in inches of mercury, is reported as A followed by the pressure in
hundredths of inches.

Example: A3037.

SUPPLEMENTARY INFORMATION

RECENT WEATHER (RE)
This is operationally significant weather observed since the previous observation (or in the last
hour, whichever is the shorter) but not occurring now. Up to three groups are used to indicate the
former presence of more than one weather type.

Example: RETS REGR.

WINDSHEAR (WS)
Windshear is inserted, if reported in the lowest 1600 ft of the take-off or approach paths.

Example: WS RWY27, WS ALL RWY.

TREND
A forecast of significant changes in weather expected within 2 hours of the observation time is
added to the end of a METAR or SPECI, if a qualified Forecaster is present.

Change Indicator:
BECMG (becoming) or TEMPO (temporary) which are followed by a time group in
hours and minutes UTC, and possibly followed by FM (from), TO (until), or AT (at)
followed by a four figure time group.

Weather:
Standard codes are used in this section. NOSIG is used when no significant changes
are expected to occur during the trend forecast period.

Example: BCMG FM1100 25035G50KT or, TEMPO 0630 TL 0830 3000 SHRA.

Only those elements of the above in which a change is expected are included. When no change
is expected, the term NOSIG is used.

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Meteorological Messages Chapter 22

RUNWAY STATE GROUP

An eight-figure Runway State Group is added to the end of the METAR or SPECI (following any
TREND) when there is lying precipitation or other runway contamination. The student requires the
ability to decode the first two digits (Runway designator) and last two digits (Braking Action). The
complete group consists of:

Runway Designator (First Two Digits) 77 = Runway 27R (50 added to the
27 = Runway 27 or 27L designator to indicate 'right' Runway)

88 = All runways 99 = A repeat of last message because no
new information received

Runway Deposits (Third Digit) 5 = Wet Snow
0 = Clear and dry 6 = Slush
1 = Damp 7 = lce
2 = Wet or water patches 8 = Compacted or rolled snow
3 = Rime or frost covered 9 = Frozen ruts or ridges
(depth normally less than 1 mm)
4 = Dry Snow

/ = Not reported (e.g. due to runway clearance in progress)

Extent of Runway Contamination (Fourth Digit)

1 = 10% or less 2 = 11% to 25%

5 = 26% to 50% 9 = 51% to 100%

/ = Not reported (e.g. due to runway clearance in progress)

Depth of Deposit (Fifth and Sixth Digits)

The quoted depth is the mean number of readings or if operationally significant the greatest depth
measured.

00 = less than 1 mm 01 = 1 mm through to 90 = 90 mm
91 = not used 92 = 10 cm
93 = 15 cm 94 = 20 cm
95 = 25 cm 96 = 30 cm
97 = 35 cm 98 = 40 cm or more

// = Depth of deposit operationally not significant or not measurable

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Chapter 22 Meteorological Messages

Friction Coefficient or Braking Action (Seventh and Eighth Digits)

The value, transmitted is the mean or, if operationally significant, the lowest value.

28 = Friction coefficient 0.28 35 = Friction coefficient 0.35
or
92 = Braking action: Medium/Poor
91 = Braking action: Poor 94 = Braking Action: Medium/Good
93 = Braking action: Medium
95 = Braking action: Good

99 = Figures unreliable (e.g. if equipment used does not measure satisfactorily in slush or loose
snow)

// = Braking action not reported (e.g. runway not operational, closed, etc.)

If contamination conditions cease to exist, the abbreviation CLRD is used.

Examples: 24CLRD93 = Rwy 24 cleared: Braking action; Medium.
88CLRD95 = All runways cleared: Braking action; Good.

'AUTO' AND 'RMK'

Where a report contains fully automated observations with no human intervention, it is indicated
by the code word 'AUTO', inserted immediately before the wind group.

The indicator 'RMK'(remarks) denotes an optional section containing additional meteorological
elements. It is appended to METARs by national decision, and is not disseminated internationally.

MISSING INFORMATION

Information that is missing from a METAR or SPECI is replaced by diagonals.

EXAMPLES OF METARS
SAUK02 EGLY 301220Z METAR
EGLY 24015KT 200V280 8000 -RA SCT010 BKN025 OVC080 18/15 Q0983 TEMPO
3000 RA BKN008 OVC020=

EGPZ 30025G37KT 270V360 1200NE 6000S +SHSN SCT005 BKN010CB 03/M01
Q0999 RETS WS LDG RWY27 BECMG AT 1300 9999 NSW SCT015 BKN100=

The METARs above are for 1220 UTC on the 30th day of the month. The decode in plain
language is:

EGLY: Surface wind: mean 240°True, 15 kt; varying between 200° and 280°
minimum visibility 8 km; slight rain; cloud: 3 − 4 oktas base 1000 ft, 5 − 7 oktas 2500
ft, 8 oktas 8000 ft; Temperature +18°C, Dew Point +15°C; QNH 983 mb; Trend:
temporarily 3000 m in moderate rain with 5 − 7 oktas 800 ft, 8 oktas 2000 ft.

EGPZ: Surface wind: mean 300°True, 25 kt; maximum 37 kt, varying between 270°
and 360°; minimum vis 1200 m (to northeast), maximum visibility 6 km (to south);
heavy showers of snow, Cloud: 3 − 4 oktas base 500 ft, 5 − 7 oktas CB base 1000 ft;
Temperature +3°C, Dew Point -1°C; QNH 999 mb; thunderstorm since previous
report; windshear reported on approach to runway 27; Trend: improving at 1300 UTC
to 10 km or more, nil weather, 3 − 4 oktas 1500 ft, 5 − 7 oktas 10 000 ft.

22-8 Meteorology

Meteorological Messages Chapter 22

AERODROME FORECASTS (TAF) CODES

TAF describe the forecast of conditions at aerodromes and usually cover periods of not less than
9 hours, and not more than 24 hours. Those valid for less than 12 hours are issued every 3 hours
and those valid for 12 to 24 hours issued every 6 hours. TAFs prefixed FC are valid for periods of
less than 12 hrs. TAF's prefixed FT are valid for periods of 12 to 24 hours. An 18 hour forecast
normally starts 8 hours after the time of issue and normally accompanies a 9 hour TAF.

TAF CONTENTS AND FORMAT

The TAF uses the same code system as the METAR, with the following differences:

Validity Period
In the validity period the first two numbers indicate date of issue. The next 4 figures
the forecast period in whole hours UTC. If the TAF bulletin consists of forecasts for
one or more airfields, the codename TAF is replaced by FC or FT, followed by the
date and time of origin and neither codename nor time/date group appears in the
forecast.

Visibility
Same as METAR with only the minimum visibility forecast.

Weather
If no significant weather is expected, the group is omitted. After a change group if the
weather becomes insignificant then NSW (No Significant Weather) is used.

Cloud
If clear sky is forecast the cloud group is replaced by SKC (Sky Clear). If CAVOK and
SKC are not appropriate then NSC (No Significant Cloud) is used.

SIGNIFICANT CHANGES

FM followed by the time to the nearest hour and minute UTC, is used to show the
beginning of a self contained part in the forecast. All conditions given before this
group are superseded - they no longer apply.

Example: FM1220 27017KT 4000 BKN010.

BCMG followed by a four figure time group indicating the earliest and latest start
hours of an expected permanent alteration to the meteorological conditions. This
change can occur at a regular or irregular rate during the forecast change period. The
change does not start before the first time and it is complete by the second time given.

Example: BECMG 2124 1500 BR.

TEMPO followed by a four figure time group indicates the hours of a period of
changes in the conditions of a temporary nature which may occur at any time during
the period. These changes are expected to last less than one hour in each case and
in total for less than half of the forecast period indicated.

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Chapter 22 Meteorological Messages

PROBABILITY of the occurrence of alternative forecast conditions are given as a
percentage but only 30% or 40% is used.

Example: PROB30 0507 0800 FG BKN004.
PROB40 TEMPO 1416 TSRA BKN010CB.

OTHER GROUPS

Three additional TAF groups are often used in overseas and UK military TAF. They are used to
forecast temperature (Group indicator T), Icing (Group indicator 6), and turbulence (Group
indicator 5).

Example 9 hr TAF:

FCUK33 EGGY 300900Z
EGGW 301019 23010KT 9999 SCT010 BKN018 BECMG 1114 6000 -RA BKN012
TEMPO 1418 2000 RADZ OVC004 FM1800 30020G30KT 9999 -SHRA BKN015CB=

Decode:
Nine hour TAF issued at 0900 UTC on the 30th of the month at Luton, Valid from
1000 to 1900 UTC. Wind from 230°T at a speed of 10 kt. Visibility 10 kilometres or
more. Cloud amount 3 − 4 oktas, base 1000 ft, second cloud layer 5 − 7 oktas, base
1800 ft. Between 1100 − 1400 UTC a permanent change occurs. Visibility becomes 6
km in slight rain. with 5 − 7 oktas of cloud base 1200 ft. There are short term changes
between 1400 − 1800 UTC. Visibility decreases to 2000 metres in moderate rain and
drizzle and overcast at 400 ft. From 1800 UTC there is another permanent change.
Wind velocity becomes 300°T at 20 kt gusting to 30 kt. Visibility improves to 10 km or
more with slight rain showers and the cloud is 5 − 7 oktas of cumulonimbus base 1500
ft.

Example 18 hr TAF:

FTUK31 EGGY 102300Z
EGLL 110624 13010KT 9000 BKN010 BECMG 0608 SCT015 BKN020 PROB30
TEMPO 0816 17025G40KT 4000 TSRA SCT010 BKN015CB BECMG 1821 3000 BR
SKC=

Decode:
Eighteen hour TAF issued at 2300 UTC on the 10th for London Heathrow, valid from
0600 − 2400 UTC on the 11th. Wind from 130°T at 10 kt. Visibility 9 km. Cloud 5 − 7
oktas base 1000 ft. A permanent change occurs between 0600 − 0800 UTC to 3 − 4
oktas of cloud base 1500 ft and 5 − 7 oktas of cloud base 2000 ft. There is a 30%
probability that for short periods between 0800 − 1600 UTC the wind velocity
becomes 170°T speed 25 kt maximum to 40 kt with visibility of 4000 m in
thunderstorms with rain, cloud becoming 3 − 4 oktas base 1000 ft and 5 − 7 oktas
cumulonimbus base 1500 ft. A permanent change occurs between 1800 − 2100 UTC
the visibility becoming 3000 m in mist with clear skies.

22-10 Meteorology

Meteorological Messages Chapter 22

VOLMET BROADCASTS

These are aerodrome weather reports, METARS, which are transmitted on VHF frequencies in
plain language in the following order:

1. Surface Wind Velocity (degrees True)
2. Visibility
3. RVR if applicable
4. Weather
5. Cloud
6. Temperature
7. Dewpoint
8. QNH
9. TREND if applicable, or NOSIG
10. The spoken word SNOCLO is added at the end of the aerodrome report when

the aerodrome is unusable for take-offs and landings due to heavy snow on
runways or runway clearance operations.

Meteorology 22-11

Chapter 22 Meteorological Messages

22-12 Meteorology

The Synoptic Chart Chapter23

INTRODUCTION

The Synoptic Chart, Chart 1, is one of the tools used by the Meteorology Forecaster. For the
Meteorology exam there is a required working knowledge of:

a. Chart symbology
b. The synoptic situation
c. The likely future development of the situation shown on the chart
d. The implications of the weather situation to the pilot

Refer to Chart 1 for paragraphs a to f:

a. The chart is a Lambert Projection which covers both the UK and Northern
Europe.

b. To the bottom right of the chart is the Date/Time for which the chart is valid. In
this case January, 1200 UTC.

i. The major synoptic charts are issued at 0000, 0600, 1200, and
1800 UTC.

ii. The minor synoptic charts are issued in the intermediate hours 0300,
0900, 1500, and 2100 UTC.

c. To the bottom left of the chart are two scales:

i. The upper scale is a range scale in nautical miles. This scale is usually
for use when distances require measuring on the chart.

ii. The lower scale is a Geostrophic Wind Scale which is explained later.

d. The pressure pattern is shown by sea level isobars. The isobars join together
points of equal mean sea level pressure (QFF). The isobars are always plotted
as whole even hectopascalic pressure values, typically 2, 4, or 8 hPa depending
on the pressure gradient. How to give numeric values to the isobars is shown
later.

e. Fronts are shown in the standard way:

i. In the warm sector the isobars are nearly straight and parallel and give a
good indication of the Geostrophic winds at 2000 ft (even though the
isobars are MSL values).

ii. Around the depression (55°N 012°30'W) the isobars are curved and the
value derived from the geostrophic wind scale need correcting to
produce a value for the gradient wind at 2000 ft. The direction is
measured directly from the chart.

Rule of thumb corrections to be applied for both depressions and anticyclones
are:

Around a low pressure -5 kt

Around a high pressure + 5 − 10 kt

Meteorology 23-1

Chapter 23 The Synoptic Chart

f. Observations are given for different reporting stations in a coded format, these
Chart 1 are called Station Circles. Using the information from the Station Circle, the
forecaster constructs the isobaric pattern.

23-2 Meteorology

The Synoptic Chart Chapter 23

Note: The remainder of this chapter is for information and is not currently examinable.

THE STATION CIRCLE DECODE

The observation reports submitted by Meteorological Observation Stations are transcribed, in
standard form, on to synoptic charts as the Station Circle which forms the basis for a
Meteorological Forecaster's forecasts. It is necessary for aviators to have a good general
knowledge of the symbols on the station circle. The format for the arrangement of the station
circle has been internationally agreed.

High Cloud Type

Medium Cloud Type

Amount/Base Height

Visibility Air Temperature TOTAL MSL Pressure
Present Weather CLOUD
Dewpoint Temperature COVER Barometric Tendency and
Characteristic

Past Weather

The wind symbol in a variable position Low Cloud Type
Amount/base height

PRESSURE (1 O'CLOCK)

QFF shown by three figures giving tens, units and tenths of a hectopascal (e.g. 721 = 72.1 hPa).
The expected QFF range is from 950 to 1050 hPa. The reader of the circle is required to prefix
the 3 figures by either a 9 or 10. In the above example, the full QFF is 972.1 hPa. Any figure
between 500 and 999 must be prefixed with a 9. Figures 000 to 500 with a 10 (e.g. 033 =
03.3 hPa which would be a QFF of 1003.3 hPa).

PRESSURE TENDENCY (3 O'CLOCK)

The pressure change in the past 3 hours is shown by two figures and a symbol. The symbols
show the type of change.

Overall Rise Overall Fall

A rise followed by a small A fall followed by a rise
fall

A rise and then steady A fall and then steady

A steady rise A steady fall

A small fall followed by a A small rise followed by a fall
rise

Meteorology 23-3

Chapter 23 The Synoptic Chart

The two figures show the amount of the pressure change in units and tenths of a hPa
(e.g. 36/ indicates in the past three hours there was a steady rise of 3.6 hPa).

PAST WEATHER (5 O'CLOCK)

All weather is shown by symbols only. Weather in this context refers to precipitation, mist, haze,
fog, thunderstorms, and snowstorms. Certain symbols are common to both "Past Weather" and
"Present Weather" (9 o'clock position).

Fog or ice fog Showers
Thunderstorms
, Drizzle

Rain Hail . The symbol is sometimes shown
black instead of open.
* Snow

The arrangements of the common symbols and their combinations have different meanings for
Past and Present Weather.

Past weather refers to the past 6 hours for the Major Synoptic hours of 0000, 0600, 1200, and
1800 UTC. It refers to the past 3 hours for the Minor Synoptic hours of 0300, 0900, 1500, and
2100 UTC. If hourly charts are produced then the past weather for the last hour.

ADDITIONAL PAST WEATHER SYMBOLS
A single symbol means that particular weather occurred for part of the time. Two identical
symbols means that it is continuous. Two different symbols means that each occurred, the first
being predominant.

Other combinations similarly apply. Meteorology
Note: Intensity of past precipitation is not shown.

23-4

The Synoptic Chart Chapter 23

LOW CLOUD OR VERTICAL VISIBILITY (6 O'CLOCK)

The cloud type is shown by a symbol. The amount of sky cover in eighths and the cloud base in
hundreds of feet are shown in figures, positioned below the symbol. The figures for cloud amount
and base are separated by a slash. Cloud height is given with reference to airfield level
(e.g. 3/12 indicates 3 oktas of stratus base 1200 ft above airfield level).

VERTICAL VISIBILITY

These figures are for use in indicating the sky is obscured. The 9 shows that the sky is obscured.
The 2 figures after the slash show the vertical visibility in hundreds of feet:

9/00 vertical visibility less than 100 ft
9/01 vertical visibility 100 ft

DEWPOINT (7 O'CLOCK)

This is shown by 2 figures for units and tens of degrees Celsius. Thermometers are read to the
nearest 0.1 of a degree and then rounded up or down to the nearest whole figure with 0.5 always
allocated to the nearest whole odd number. Values are positive unless prefixed by a minus sign.

VISIBILITY (9 O'CLOCK OUTER POSITION)

The two figures are for use in showing visibility in metres or kilometres using the following range
system:

00 to 50 metres (x 100)

56 to 80 subtract 50 giving an answer in kilometres

81 to 89 subtract 80, multiply the result by 5 then add 30. This
gives an answer in kilometres

e.g. 85 85 − 80 = 5
x 5 = 25

+ 30 = 55 km

Meteorology 23-5

Chapter 23 The Synoptic Chart

PRESENT WEATHER (9 O'CLOCK INNER POSITION)

Weather at the time of observation is shown by symbols only. Weather which has occurred during
the past hour, but not at the time of observation is also shown at this position.

23-6 Meteorology

The Synoptic Chart Chapter 23
The above descriptions also apply for drizzle and snow symbols.

WEATHER IN THE PAST HOUR BUT NOT AT THE TIME OF OBSERVATION
The situation is provided by a square bracket around a precipitation symbol.

Meteorology 23-7

Chapter 23 The Synoptic Chart

SURFACE AIR TEMPERATURE OR DRY BULB TEMPERATURE
(11 O'CLOCK)

This is shown by 2 figures for units and tens of degrees Celsius. Thermometers are read to the
nearest 0.1 of a degree and then rounded up or down to the nearest whole figure with 0.5 always
allocated to the nearest whole odd number. Values are positive unless prefixed by a minus sign.

MEDIUM LEVEL CLOUD (12 O'CLOCK LOWER POSITION)

The cloud type appears by symbol.

The cloud amount in eighths is sometimes given below the cloud symbol followed by a slash and
then two figures indicating the height of the cloud base. These two figures have a range of
56 − 80. The cloud base is then given in thousands of feet by subtracting 50.

In this example we have 4/8 of thin As with a base of 12 000 ft above airfield level.

23-8 Meteorology

The Synoptic Chart Chapter 23

HIGH LEVEL CLOUD (12 O'CLOCK UPPER POSITION)

The cloud type appears by symbol.

The cloud amount and cloud base are shown if there is no medium level cloud reported. Higher
cloud bases than the figure 80 (-50 = 30 000 ft) are catered for as follows.

Range of codes 81 − 89, subtract 80 and multiply the result by 5 and then add 30. This gives the
cloud base in thousands of feet.

In this example there is a 7/8 Cirrus, not increasing with a base of 40 000 ft above
airfield level.

Meteorology 23-9

Chapter 23 The Synoptic Chart

TOTAL CLOUD COVER (SHOWN IN THE CENTRE OF THE CIRCLE)

This is indicated in eighths of the total sky covered by cloud.

Sky clear 5/8ths

1/8th 6/8ths

2/8ths 7/8ths

3/8ths 8/8ths

4/8ths Indicates obscured
sky— usually by fog,
smoke or sand/dust

SURFACE WIND

Shown by a straight line from the periphery of the circle. This line indicates the direction from
which the wind is blowing (090° (T) in the examples below). The speed is shown by the feathers
at the end of the line.

Calm 20 kt, further additions up to 45 kt

1 to 2 kt 50 kt

5 kt 60 kt

10 kt 65 kt, further additions as
necessary

15 kt

Note: The feathers on the wind arrows conform with Buys Ballot's Law. The feathers indicate the
low pressure side. Left in the Northern Hemisphere, right in the Southern Hemisphere.

23-10 Meteorology

INTRODUCTION

Charts for the middle and upper levels must now be considered. These vary in coverage from FL
100 to FL 630 depending upon the area covered. The layout, and symbology used on these
charts is similar to ones already taught in previous sections. Other symbology includes:

SYMBOLS FOR SIGNIFICANT WEATHER

Notes

1. In flight documentation for flights operating up to FL 100 this symbol refers to a squall
line.

2. The following information referring to the symbol should be included in the side of the
chart.

¾ Volcanic eruption

¾ Name of volcano

¾ Latitude and longitude

¾ Date and time of the first eruption

¾ Check SIGMET for volcanic ash

3. This symbol does not refer to icing due to precipitation coming into contact with an
aircraft at a very low temperature.

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Chapter 24 Upper Air Charts

FRONTS AND CONVERGENCE ZONES AND OTHER SYMBOLS

Where the cold front, warm front, occlude front and quasi-stationary front symbols are not filled in
then the front is above the surface.

In the above diagram a cold front above the surface.

CLOUD ABBREVIATIONS

CI Cirrus AS Altostratus ST Stratus
Nimbostratus CU Cumulus
CC Cirrocumulus NS Stratocumulus CB Cumulonimbus

CS Cirrostratus SC

AC Altocumulus

CLOUD AMOUNT

CLOUDS EXCEPT CB

SKC Sky clear 0/8 (0 oktas)
FEW Few 1/8 to 2/8 (1−2 oktas)
SCT Scattered 3/8 to 4/8 (3−4 oktas)
BKN Broken 5/8 to 7/8 (5−7 oktas)
OVC Overcast 8/8 (8 oktas)

24-2 Meteorology

Upper Air Charts Chapter 24

CUMULONIMBUS ONLY

ISOL Individual CBs (isolated)
OCNL
FREQ Well separated CBs (occasional)
EMBD
CBs with little or nor separation (frequent)

Thunderstorm clouds contained in layer of other clouds
(embedded)

WEATHER ABBREVIATIONS

DZ Drizzle GEN General
LYR Layer
LOC Locally BLW Below
SEV Severe
K Thunderstorm

COT At the coast

WDSPR Widespread

SH Showers

FZ Freezing

MAR Over the sea

LINES AND SYMBOLS ON THE CHART

15 SPEED OF A FRONT IN KNOTS
SLOW SPEED OF THE FRONT CAN BE DEPICTED IN WORDS

BOUNDARY OF AREA OF SIGNIFICANT WEATHER

BOUNDARY OF AREA OF CLEAR AIR TURBULENCE
THE CAT AREA MAY BE MARKED BY A NUMERAL INSIDE A SQUARE AND
A LEGEND DESCRIBING THE NUMBERED CAT AREA MAY BE ENTERED
IN A MARGIN

- - - 0° C: FL120 - - - ALTITUDE OF THE 0°C ISOTHERM IN FLIGHT LEVELS

Meteorology 24-3

Chapter 24 Upper Air Charts

SIGNIFICANT WEATHER CHART

The chart on the next page is an example of a high level chart issued by London. The chart
covers a considerable area of Europe, the Middle East, North Africa, and North America. These
charts are issued in advance of their valid times, which are 0000, 0600, 1200, and 1800 UTC.
The validity of this chart is 1200 UTC on 17 August. A Polar Stereographic or Mercator projection
is used for all middle and upper air significant weather charts.

Note: Take great care when measuring direction on all small scale meteorological charts. Use a
square navigation protractor.

a. The bottom right hand corner of this chart gives a box which:
i. Indicates the issuing station.
ii. The type of chart – Significant Weather.
iii. The depth of the atmosphere covered. In this case FL 250 - 630 this
is also given in hPa.
iv. The chart is a fixed time chart for 1200 UTC, 17 August.
v. The units used on the chart are Pressure Altitude (Hectofeet), knots,
and °C .

b. The bottom right box indicates that all heights are Flight Levels. Tropopause
heights are shown in boxes on the chart (Indicated by A on the chart). The
symbols and CB imply moderate or severe turbulence and icing.

c. The vertical distance at which phenomena are expected are indicated by flight
levels, top over base or top followed by base . 'XXX' means the phenomenon is
expected to continue above or below the vertical coverage of the chart (Indicated
by B on the chart).

d. The surface positions together with the direction and speed of movement of
pressure centres and fronts are denoted as shown on the chart. Where slow is
used this indicates movement of less than 5 kt (Indicated by C on the chart).

e. Dashed lines denote areas of CAT. These areas are numbered and are
associated with the decode box on the chart in the bottom right corner (Indicated
by D on the chart). (e.g. Area 4 Moderate turbulence FL 370 to FL 300).

f. On lower charts the 0°C Isotherm is also shown as a dotted line with the FL
indicated (e.g. - - - - - - - 0°C:FL130 - - - - - - - -).

24-4 Meteorology

Upper Air Charts Chapter 24

Meteorology 24-5

Chapter 24 Upper Air Charts

UPPER WIND AND TEMPERATURE CHARTS

Issued in conjunction with the significant weather chart, these charts give spot winds from 700
hPa (FL100) up to 200 hPa (FL390). Spot values of wind and temperature are shown at regular
intervals of latitude and longitude. The temperatures given are assumed negative unless prefixed
by PS. The wind arrow symbology is exactly the same as that for the synoptic chart.

The chart on the next page is again issued by London and is for Upper Wind and Temperature.
Remember that the maximum wind is contained on the significant weather chart. The chart is for
FL 340 and has the same validity as the upper wind chart. At the bottom is the time of issue 1200
UTC on the 16 August.

24-6 Meteorology

Upper Air Charts Chapter 24

Meteorology 24-7

Chapter 24 Upper Air Charts

AVERAGING WIND VELOCITIES

Apply common sense. For instance, if there is an east/west track with a wind velocity of
310°/20 kt to the north and 270°/20 kt to the south then the average wind is at 290°/20 kt.
Numerical averaging is the common sense way of approaching the problem.

Example: As well as taking spot winds and temperatures from specific points, winds, and
temperature might require averaging over a route.

Because of the time limitations of the Flight Planning examination the rule of KISS (keep
it simple stupid) applies.

Temperature
STEP 1 Along the route add up the temperatures and numerically average the

sum total.
Temperature 48°C

The above system is quite a simple way of arriving at the mean temperature. To
average the wind velocity over a route is not as simple.

STEP 1 Look at the wind directions involved at approximately 10° spacing.

80W 320/20
70W 250/35
60W 240/50 (average between the two velocities spanning the track)
50W 270/15
40W 020/65
30W 020/20
20W 210/70
10W 290/30 (average between the two velocities spanning the track)
0E/W 270/50
The winds are predominantly westerly. Ignore the two north-easterly
winds as they distort the figures.
Direction 265°

STEP 2 For the speed use the same principle as the direction. Give westerly
winds a + configuration and easterly winds a – configuration.
Speed 25 kt

Time may mean that you are not able to make these calculations. If not, try to come to a sensible
wind by inspection.

24-8 Meteorology

INTRODUCTION Hadley Cell

Climatology is the long term study of the behaviour of the weather carried out by looking at the
average weather of the various areas of the Earth. Much of the information is open to discussion.
Remember, these are the ideals not the actual.
Climatology needs a basic knowledge of the location of countries, major cities, the Tropic of
Cancer (Northern Hemisphere), and the Tropic of Capricorn (Southern Hemisphere).

IDEAL GLOBAL CIRCULATION

Initially, assume that the Earth has a uniform surface, that it is not rotating, and it is not tilted.

H

Surface Flow

Equator

H

The circulation of the air resembles a large scale sea breeze.

Meteorology

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Chapter 25 Climatology-The World Climate

The Equator receives more insolation than the poles. This insolation causes the Equator to have
a higher temperature than at the poles.

The air at the surface is warmed, expands, and rises. This rising air creates a high pressure at
altitude over the Equator. This flow starts an outflow of air from the high pressure at height. A low
pressure system is formed at the surface which draws air in.

At the poles the low temperature causes a high pressure system at the surface and subsidence
occurs. The subsidence allows a low pressure system to form at height drawing air from the
Equator.

ROTATION OF THE EARTH

North Pole

30°
Equator

30°

Because of the Earth’s rotation, take into account the geostrophic force or Coriolis.

In the upper levels as the air travels towards the poles from the equator it comes under the
influence of Coriolis. In the:

Northern Hemisphere The air is deflected to the right
Southern Hemisphere The air is deflected to the left

This movement takes place at approximately 30° from the Equator.

The deflection means that the flow is eastwards in both hemispheres. The air is cooled as it
moves parallel to the Equator and eventually subsides to the surface. The falling of the air causes
a high pressure to form at the surface. Known as the Sub-Tropical High these are recognisable
on the average pressure charts.

At height, strong westerly winds form the sub-tropical jet stream.

25-2 Meteorology

Climatology-The World Climate Chapter 25

IDEALISED PRESSURE ZONES

The Coriolis effect forms the first of three cells in the idealised circulation. This first cell is known
as the Hadley Cell.

Polar High Polar
Polar Front Cell

H Ferrel Polar Easterlies 60°
L Cell Subpolar Low

Horse 60° N L
Latitudes
L
H Hadley Westerlies

30° N H Cell 30°
Doldrums
H Subtropical High

0° L L NE Trade Winds
H
Equatorial 0°
Lows
30° S H Intertropical Convergence Zone (ITCZ)

SE Trade Winds

30°

60° S Subtropical High

Westerlies 60°

Subpolar Low

Polar Easterlies

The sub-tropical high pressure system has an outflow of air to both the Equator and towards the
poles.

The flow of air from the poles and the flow from the sub-tropical high meet in the temperate
latitudes. Convergence occurs and air rises. A surface low pressure forms with a high pressure
area at height.

The three distinctive cells are:

¾ The Hadley Cell
¾ The Mid-Latitude Cell (Ferrel Cell)
¾ The Polar Cell

THE EARTH’S TILT

The Earth is tilted by 23° 27’. The sun travels to the Tropic of Cancer in the Northern hemisphere
summer and the Tropic of capricorn in the winter. As the sun moves across the Earth’s surface so
does the Equatorial Low pressure belt.

This general picture ignores certain features such as the irregular surface of the land and the
different characteristics of the land and sea surfaces.

These features do have a major influence on the climate and the weather.

When the pressure patterns are discussed later in the chapter it is apparent that the general
circulation discussed so far does in reality exist.

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Chapter 25 Climatology-The World Climate

PRESSURE ZONES

EQUATORIAL LOW (TROUGH)

The Equatorial Low is an area of convergence at the surface created by the outflow of air in the
upper atmosphere. The surface winds travel in towards the Low pressure area. In the Northern
Hemisphere these winds are deflected to the right, and to the left in the Southern Hemisphere.
These are the Trade Winds:

¾ In the Northern Hemisphere they are North Easterly
¾ In the Southern hemisphere they are South Easterly

In Sea areas, occassionally there is an area known as the Doldrums, where the trade winds are
light and variable. Over land the winds can get up to 25 kt.

SUB-TROPICAL HIGHS

Created by the upper level air from the Equator being deflected by the geostrophic force.
Eventually the flow is near parallel to the parallels of latitude at 30°N/S. The sub-tropical highs
are seen in both hemispheres in summer and winter.

The source area for the Trade Winds.

TEMPERATE LOW

The area where the sub-tropical and polar airmasses meet in each hemisphere. An area of
convergence, the temperate low is defined by travelling depressions forming on the polar front.
These travelling depressions appear on the pressure chart with sufficient frequency for them to
appear permanent.

The circulation described is bounded by low temperatures at the poles and high temperatures at
the Equator. To maintain a temperature balance the air must move between the poles and the
Equator. At irregular periods North-South surges do occur which distort the climatological pattern.
The best example of this is “El Nino.”

POLAR HIGH

The polar high is apparent in both hemispheres. The Antarctic polar high is similar to the ideal
circulation and is a near constant feature. The Arctic polar high is not so permanent. The area is
surrounded by land and suffers from regular travelling depressions which remove the high
pressure system.

PREVAILING SURFACE WINDS

WESTERLY WINDS

The outflow of air on the polar side of each sub-tropical high is deflected by the geostrophic
forces. These forces create mainly westerly winds in both hemispheres. These winds are often
strong, especially in the Southern Hemisphere between 40°S and 60°S where they are known as
the “Roaring Forties.”

25-4 Meteorology

Climatology-The World Climate Chapter 25

EASTERLY WINDS

The outflow of air from the polar high pressure regions results in the formation of easterly winds
at the surface in high latitudes. In reality the polar highs are less well defined than the idealised
circulation. The polar easterlies are therefore variable.

CLIMATIC ZONES

Because the zones are essentially the product of solar heating, the pressure zones change
latitude with the seasonal movement of the sun. These pressure zones in turn produce climatic
zones. The pressure zones are complex and depend upon the nature of the surface of the Earth
and the movement of the sun.

Because rising air cools, in the temperate and equatorial low pressure areas large amounts of
cloud and precipitation are found. At the poles and sub-tropical high belts subsiding air disperses
any cloud and dry areas occur.

EQUATORIAL CLIMATE (0° TO 10° LATITUDE)

This area is also known as humid tropical and occurs up to 10° either side of the Equator. Over
the sea areas, light winds, high temperature, and high humidity are apparent all year. Convective
activity prevails giving heavy showers and TS. The slack pressure gradient causes strong sea
breezes on coasts. The zone has two rainy seasons which occur at the equinoxes in March and
September when the sun crosses the Equator.

Neither rainy season is a distinct feature, the days are wetter than normal.

TROPICAL TRANSITION CLIMATE (10° TO 20° LATITUDE)

Also known as the Savannah. Zones occur in both hemispheres. The area has a marked wet and
dry season associated with the passage of the sun. In the area nearest the Equator there is a
possibility of two wet seasons. The edge of the area nearest the pole only has one wet season.
The amount of rain decreases as the latitude increases. In winter the area is one of dry trade
winds. In summer there are belts of equatorial trade winds. Temperatures are fairly high
throughout the year. Annual and diurnal temperature ranges increase with latitude.

ARID SUB-TROPICAL (20° TO 35° LATITUDE)

Also known as the Steppe. The edge of the equatorial low pressure area is not well defined. This
area is always under the influence of the sub-tropical high pressure belt. The subsiding air is
cloudless and hot. In the summer there are large diurnal and annual temperature ranges. This
area contains most of the Earth’s deserts:

¾ In the Northern Hemisphere Sahara, Arabia, Arizona
¾ In the Southern Hemisphere Kalahari, Australia

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Chapter 25 Climatology-The World Climate

Trade winds flow and are consistent in direction. In the desert interior there is little or no rain.
Bordering the desert is the Steppe. An area of treeless plains with short rainy seasons. In the
Northern hemisphere this area is the region north of the deserts in winter, south of the deserts in
summer. Steppe regions include:

¾ Algeria
¾ The Veldt of South Africa
¾ Central and southern Russia

The upper winds are westerly sub-tropical jet streams.

MEDITERRANEAN CLIMATE (35° TO 40° LATITUDE)

A warm temperate transition zone, which exhibits the following:

In Winter

¾ Disturbed temperate climate
¾ Travelling frontal depressions
¾ Prevailing westerly winds
¾ Cloud and precipitation
¾ Cool and unsettled

In Summer

¾ Dry sub-tropical climate
¾ Anticyclonic in nature
¾ Hot fine sunny weather
¾ Land and sea breezes on the coasts
¾ Westerly upper winds

The areas of the world include California, the Mediterranean, Central Chile, and the cape area of
South Africa.

DISTURBED TEMPERATE (40° TO 65° LATITUDE)

The weather is controlled by travelling frontal depressions. Less frequently high pressure systems
may affect the area. The winds are normally westerly. There is no dry season. In winter the polar
front lows are more frequent. Winters are cold and in Western Europe wet. Gale force winds are
experienced at any time.

The areas of the world include Western Europe and New Zealand

25-6 Meteorology

Climatology-The World Climate Chapter 25

POLAR CLIMATE (65° TO 90° LATITUDE)

Around the edge of the polar zone is the Tundra. The mean temperature of this area rises above
0°C for only a few months of the year. Subsoil temperatures remain below 0°C permanently,
giving the term perma frost. No trees are found in this area. The vegetation consists of grass,
lichens, and moss.

The area is subject to 24 hours darkness for 3 months in winter and 24 hours daylight for 3
months in summer. The area is generally anticyclonic which is occasionally replaced by travelling
depressions. The travelling depressions are more common in the Northern Hemisphere.

MODIFICATIONS TO THE IDEALISED CIRCULATION

The seasonal movement of the sun distorts the pressure zones. To allow for this movement
climatology takes two extremes, the months of January and July.

The Earth is assumed to have a uniform surface. In the standard pressure and temperature
variations for January and July the complications of large land and sea masses become
apparent.

GLOBAL TEMPERATURE DISTRIBUTION

All climate and weather, energy, and movement are caused by solar energy. The weather is
related to the temperature distribution over the Earth. In the ideal case the temperature
decreases from the Equator to the poles evenly. In the Southern Hemisphere this is nearly the
case and in the temperature distribution charts for January and July the isotherms nearly follow
the lines of latitude. In the Northen Hemisphere the distribution is distorted by the large land
masses.

MEAN SEA LEVEL TEMPERATURES — JANUARY

This is the Southern hemisphere summer.

The highest temperatures occur between 10°S to 20° over the land areas. At these latitudes the
sea temperatures are lower than land temperatures at the same latitude. For example, at 20°S
the temperature over the land is 25°C to 30°C, while over the sea it is 20°C to 25°C.

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Chapter 25 Climatology-The World Climate

The coldest temperatures are found over the Northern Hemisphere land masses. Note the
extremes are found in Canada and Siberia. The isotherms are distorted by the land masses in
both hemispheres. In the Northern Hemisphere the relatively warm temperatures of the North
Atlantic and Pacific contrasts to the colder land temperatures.

MEAN SEA LEVEL TEMPERATURE — JULY

The Northern Hemisphere summer.

The highest temperatures are over the land between 20°N and 40°N. Sea temperatures are
slightly lower than the land temperatures. Note that the extremes occur over the land masses of
South America, Africa, and South East Asia.

Over the Southern hemisphere oceans there are no land masses to distort the isotherms and
they parallel the lines of latitude.

On both charts the highest isotherm value is emboldened. This marks the position of the heat
equator, or the equatorial low pressure area. This belt is known as the Inter Tropical
Convergence Zone (ITCZ). This is discussed in the next chapter.

SEASONAL VARIATIONS IN TEMPERATURE

In tropical regions the mean temperature only varies by about 5°C throughout the year. This is
more apparent over large ocean areas. The largest temperature variations are found over the
large land masses such as Northern USA and Siberia.

In the Southern Hemisphere the lack of large land masses mean little temperature variation
throughout the year.

The polar fronts are more apparent in the summer than the winter. In the North Atlantic the polar
front lies:

In Summer Newfoundland to the North of Scotland
In Winter Florida to South West England

25-8 Meteorology

Climatology-The World Climate Chapter 25

UPPER AIR TEMPERATURE DISTRIBUTION

The upper air temperature is controlled by the surface temperature. Because the height of the
tropopause is higher over the Equator than the poles the 2°C lapse rate applies over a greater
atmospheric depth. The diagram below gives an illustration of the temperature deviation in the
upper atmosphere. The temperature at the tropical tropopause is likely between -75° to -80°C. At
the poles -55°C.

WORLD PRESSURE DISTRIBUTION

Temperature variations found across the world can link to the January and July Pressure charts.

MEAN SEA LEVEL PRESSURE — JANUARY

The high and low pressure areas are very apparent on the chart.

Meteorology 25-9

Chapter 25 Climatology-The World Climate

The simplified diagram on the previous page shows low and high pressures that relate to the
idealised circulation. In January the sun is overhead the Tropic of Capricorn in the Southern
Hemnisphere. The warm air over Australia, Africa, and South America creates surface low
pressure areas. These low pressure areas break up the sub-tropical high pressure belts between
20°S to 40°S. In the Northern Hemisphere the sub-tropical high is apparent over the oceans at
30°N. The land masses distorting the picture because of the well established cold anticyclones.
The Siberian high is the dominant feature of the Eurasian land mass.

There are two mean low pressure areas:

North Atlantic The Icelandic low
North Pacific The Aleutian low

Neither of these pressure areas is permanent. Travelling depressions are so common that they
show up as a permanent low pressure area on the chart.

MEAN SEA LEVEL PRESSURE — JULY

The sun is now overhead the Tropic of Cancer.

In the Southern Hemisphere the sub-tropical high is well established. The pressure system
moved to approximately 30°S. This picture is near the ideal pattern discussed earlier.

The sub-tropical high pressure areas in the Northen Hemisphere moved north to 35°N. These
areas are now more dominant than in January. The Siberian High is replaced by a low pressure
area which extends over the land masses of India and the Gulf States. The Monsoon Low, or
Baluchistan Low, dominates the area.

The low pressure areas in the North Atlantic and the North Pacific are now weaker and retreat
northwards as the high pressure systems move north.

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Climatology-The World Climate Chapter 25

UPPER WINDS

The temperature decreases in the troposphere from the tropics to the poles. The thermal wind
component in both hemispheres is therefore westerly. The mean circulation is also westerly.

The wind circulates around the upper air depressions at each pole.

Southern Hemisphere

Outside the tropics the winds follow the ideal circulation. The prevailing westerlies at
temperate latitudes increase with height.

Northern Hemisphere
The same applies, with westerly winds increasing with height.

The higher wind speeds associated with jet streams are transient in the temperate latitudes. The
sub-tropical jet streams are a normal feature of the meteorological chart.

MEAN UPPER WIND — JANUARY Westerly Winds
(Up to 300 Knots)
80°

Westerly
Winds

55°
Polar Front Jet Stream (70 to 200 Knots)

40°
Sub-Tropical Jet Stream (70 to 200 Knots)

25°
Westerly Wind

10°
0°

Easterly Wind (Maximum 40 Knots)

20°

Westerly Wind

40° Sub-Tropical Jet Stream (70 to 200 Knots)
45°

Polar Front Jet Stream Westerly Wind
55° 180°

Westerly Wind

180° 0°

The normal flow is westerly. South of the Equator is an easterly flow. This flow is never greater
than 40 kt, normally 15 to 25 kt.

The mean position of the sub-tropical jet stream is:

Northern Hemisphere

North Africa to Japan passing over the Persian Gulf. The highest wind speeds in the
world are found along the Chinese/Japanese Coast.

Southern Hemisphere
Approximately 40°S

The polar front jet streams do not appear on mean wind charts normally as they are a transient
feature. Strong wind speeds occur along the east coast of North America.

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Chapter 25 Climatology-The World Climate

The strong winds over the coastal areas of the USA and China are caused by the strong
temperature gradient found between the cold polar air over the land and the warm tropical
maritime air over the sea.

MEAN UPPER WIND — JULY

The sub-tropical jet stream moves north in the Northern Hemisphere to 40°N to 45°N.
Temperature gradients are now weaker in this hemisphere and the mean speeds reduce.

In the Southern Hemisphere the sub-tropical jet moves north to 30°S. The highest speeds being
towards Australia and the South Pacific

Away from the Equator the winds are generally westerly. Easterly winds do affect the Equatorial
region with the possibility of an easterly jet stream over India up to 70 kt at 50 000 ft. The polar
front jet stream in the Northern Hemisphere is less evident although strong winds are
experienced over the eastern seaborad of the USA.

In the Southern Hemisphere the polar front jet is approximately 50°S.

80° (70 to 200 Knots) Westerly Winds
(70 to 200 Knots)
Westerly Westerly Wind
65° Winds 0° 180°

Polar Front Jet Stream (70 to 200 Knots)
45°
40° Sub-Tropical Jet Stream (70 to 200 Knots)

Westerly Wind
20°

Easterly Wind (30 to 50 Knots)
10°

Westerly Wind
25°

Sub-Tropical Jet Stream
40°

Polar Front Jet Stream
55°

180°

Note the position of the easterly winds:

In January Between 10°N and 20°S
In July Between 20°N and 10°S

INTER TROPICAL CONVERGENCE ZONE (ITCZ)

The Equatorial low moves across the Earth’s surface with the movement of the sun. The
Equatorial low is fed with trade winds from the two sub-tropical high pressure belts and because
of these convergent winds is termed the ITCZ.

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Climatology-The World Climate Chapter 25

ITCZ — JANUARY

Maximum heating of the land mass is in the Southern Hemisphere.

The effect of heating the land mass moves the ITCZ well south of the Equator over the land
areas. Over the sea areas the ITCZ is just north or follows the line of the Equator.

ITCZ — JULY

The ITCZ is moved north of the Equator by the heating of the land masses.

The general position of the ITCZ is well north of the Equator. The most northerly position is over

China at 45°N. There is little travel over the sea areas, and over the Atlantic and the Pacific the

ITCZ lies between 10°N and 15°N.

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Chapter 25 Climatology-The World Climate

STABILITY AND MOISTURE CONTENT OF THE ITCZ

The trade winds that flow into the ITCZ are from a relatively dry and stable area, originating from
the sub-tropical high pressure belt. The passage of the air over warmer seas towards the ITCZ
produces instability due to heating from below; this coupled with a rapid increase in moisture
content due to evaporation in to the air at lower levels means convective cloud formation.

ITCZ WEATHER

There are wide variations in the weather along the ITCZ. Over the land the ITCZ is often very
narrow and resemble a temperate latitude cold front. Over the sea the ITCZ varies between 30 to
300 nm wide. The cloud varies between fair weather CU to CB.

INTER TROPICAL FRONT (ITF/FIT)

Most of the Equatorial region is water. The converging airstreams in these areas are very similar
in both moisture content and temperature. This gives rise to lines of CU and CB. The approach is
the same whether from the north or the south.

On approach into the ITCZ the weather is:

¾ Fair weather cumulus
¾ Due to heating CU with great depth form. There is usually an inversion from between

3000 ft to 8000 ft.
¾ CB form with the cloud tops possibly over 50 000 ft.
¾ If there are stable layers at mid-levels then CU build up ceases and extensive

Stratiform layers can form.

The main aspect of the ITCZ is the potential for the warm moist air to produce heavy cloud and
heavy precipitation.

If the air stream has a continental track then the change in moisture content and temperature is
often quite marked. Normally, when the ITCZ travels over the land the term ITF/FIT is used.

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Climatology-The World Climate Chapter 25

LOW LEVEL WINDS

LOW LEVEL WINDS — JANUARY

Outside 40°S.40°N the winds are generally westerly. The greatest deviation is over the Northern
Hemisphere.

The Southern Hemisphere flow is similar to the ideal flow. At approximately 40°S the “Roaring
Forties” blow. Because the ITCZ is south of the Equator in places, the north east trade winds
cross the Equator. As they cross the Equator they are influenced by the geostrophic force in the
Southern Hemisphere and become the north west monsoon winds of the southern hemisphere.

Monsoon Winds
Monsoon is derived from the Arabic for season. The monsoon winds blow quite steadily
for long periods near the ITCZ. The monsoon winds are often the trade winds. The trade
winds are considered to exist up to 10 000 ft.

Outflow of air from the Siberian High moves over China and Japan. The winds become North
easterly and follow the chinese coast to the coast of Malaysia.

High pressure over the north west indian plain results in air flowing down the ganges valley. This
air meets with the North easterly monsoon from the Siberian High.

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Chapter 25 Climatology-The World Climate

LOW LEVEL WINDS — JULY

The ITCZ moves over the Asian continent, as far as 45°N over China. The south east trade winds
become the south west monsoon winds as they cross the Equator.

Outside 40°S/40°N the winds are predominantly westerly.

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