Jeppesen Meteorology

Climatology-The World Climate Chapter 25

CLIMATIC SUMMARY

Climate Weather Summary
Polar climate
Over the arctic anti-cyclonic regions, including NE Canada and
Cold temperate the most northern Russia.
climate
or Warmest month always below 10°C.
Moist mid-latitude
climates with cold Sporadic influences from travelling cyclones.
winters
Warm temperate Warm summer months, usually above 10° C, winter months
climate usually below – 3° C.
or
Moist mid-latitude Subdivided into two regions:
climates with mild
winters. Sub-arctic Canada, N. Sweden, Finland towards Siberia

Arid (dry) climates Humid Continental Sweden, Eastern Europe, SE. Russia,
N Japan and NE USA.

The coldest month is below 18°C but never lower than –3°C,
distinct summer and winter seasons are present.

Subdivided into three groups:

Mediterranean climate In the Mediterranean area but
also in California, SW Australia, and SW South Africa.

East coast or humid subtropical climate China, S
Japan, SE USA, Argentina, SE South Africa, E Australia.

West coast or marine Western Europe, NW-coast USA,
SE Chile, New Zealand.

Minimal precipitation most of the year.

Divisions include:

Arid desert North Africa, The Middle East towards
Himalayas, the interior of Australia, from N Mexico into
SW USA, the west coast of South America, and Africa
outside the equator area.

Steppe Great plains in USA, Interior of Asia north of the
Himalayas, around the deserts in South America, Africa
and Australia.

Tropical moist Temperatures above 18° C year round, significant rainfall usually
climates more than 1500 mm.

Subdivisions include:

Tropical rain forest The Amazon lowland, the Far East
islands from Sumatra to New Guinea and the Congo river
basin in Africa.

Tropical monsoon The coasts of Southeast Asia, India
and NE South America.

Savannah climate Central America, south central and
eastern Africa, in parts of India, Southeast Asia and in N
Australia.

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INTRODUCTION

This chapter looks at the prevailing winds of the world and the effect of ocean currents on the
climate.

EUROPE AND THE MEDITERRANEAN

MISTRAL

Meteorology

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Chapter 26 Climatology-Prevailing Winds and Ocean Currents

A cold wind in the winter and early spring. The wind blows down the Rhone Valley in the South of
France in to the Gulf of Lions. The wind is a combination of three factors:

¾ Katabatic effect
¾ Ravine effect
¾ The holding of a suitable pressure system over the Gulf of Genoa

The wind is Northerly, cold and of gale force. At certain times the winds can reach 70 to 80 kt.

As the wind blows over the sea it becomes unstable and CB may form.

BORA

A strong katabatic wind of up to 100 kt, that blows down the Balkan Plateau and Dalmation coast
in winter.

Bora

Over the Balkan Plateau the wind is dry and cloudless but is still strong and turbulent.
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The wind is north-easterly and of gale force. Like the Mistral, as this cold wind blows over the
warm sea instability occurs producing CB.
The wind is enhanced by some ravine effect and the possibility of depressions which are
apparent at this time of year in the Adriatic.

ETESIAN WIND

A summer wind which blows from the north over Greece and the Aegean. A similar wind blows
over Turkey and is known as the Meltemi.

High

Etesian Low
or

Meltemi

The wind is northerly and cool. The blowing of this wind can bring relief from the normal heat
wave conditions which are apparent in this region at this time of year. The wind regularly blows
between 10 to 30 kt, gusting to 40 kt at times.

The wind is caused by the meeting of the air from the Azores High and the Baluchistan Low.

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Chapter 26 Climatology-Prevailing Winds and Ocean Currents

GREGALE

A strong north easterly wind which blows over the Ionian Sea and the Mediterranean in the
second half of the year. The wind can reach gale force and last for 2 to 3 days. The prevailing
conditions are low cloud, rain, and poor visibility.

Gregale

The wind is not especially cold.

LEVANTER

A humid easterly wind which blows over Gibraltar when there is anticyclonic weather over Spain.

The air is generally moist after its sea track. It is not a strong wind but its passage is
characterised by the cap cloud that covers the Rock of Gibraltar.

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High
Levanter

The wind can blow at any time of year but is more prevalent during June to October

VENDEVALE

A south westerly wind that affects the Straits of Gibraltar at the beginning and end of winter. The
wind brings heavy rain.

Vendevale

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SIROCCO

A southerly wind that blows in the winter months. The wind is hot and dusty as it blows in
advance of travelling depressions moving from west to east in the Mediterranean.

Sirocco Ghibli Khamsin
Low Low Low

As the wind progresses over the sea it becomes hot and humid. The moistening of the air cools
and stabilises the wind. Eventually low stratus, drizzle, or advection fog are formed.

A similar wind blows over the Libyan desert and is called the Ghibli.

KHAMSIN

The Khamsin is similar to the Siroccco.

Originating over the desert the wind is supposed to blow for 50 days (the Arabic for 50 is Kham).
A southerly wind of late winter and Spring in Egypt occurring ahead of travelling depressions.

The wind is more persistent than the Sirocco because traveling depressions tend to slow down as
they reach the Eastern Mediterranean basin. It is hot and dry.

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AFRICA

HABOOB

Hab is Arabic for blow. This wind occurs in the Sudan in the afternoons and evenings between
May and September when the ITCZ is to the North.

Haboob

Moist air flows in from the Indian Ocean at both low and upper levels and convection produces
large CB. Ahead of the CB the squally winds raise dust storms to great heights.

As it approaches the Haboob is associated with an increase in wind speed and reduction in
visibility.

The dust storm is followed by torrential rain and conditions begin to improve.

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HARMATTAN

The Harmattan is a north or north easterly wind over West Africa and the Gulf of Guinea. The
wind is dependent on the position of the ITCZ.

In the Northern Hemisphere summer the ITCZ is well north of the Equator and the Gulf of Guinea
and West Africa are subject to the trade south westerly flow of the trade winds that cross the
Equator. The weather is typically equatorial.
In the Northen Hemisphere winter the ITCZ retreats over the Equator and West Africa is subject
to a hot, dry, dusty wind from the Sahara. It is known as the “Doctor” by Europeans because of its
dry characteristics rather than the humid tropical climate of the summer period.
Temperatures can reach 40°C with dew points as low as 7°C.
Dust carried by the wind can cause serious deteriorations in visibility up to 5000 ft.

SIMOON

The Simoon originates in the desert in the heat of the afternoon in Africa and the Middle East.
Simoon literally means poison, which sums up the characteristics of this wind.

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Thermal Low

The thermal lows formed carry large amounts of sand and dust. A summer and autumn
phenomena which can last up to about 20 minutes.

ASIA

NORWESTER

Violent convective squalls which occur in Bengal/Assam. They are named after the direction from
which they come. Normally a summer phenomena.

The storms can occur as frequently as every 3 days.

Norwester

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SHAMAL

A north to north westerly wind that blows over Iraq during the summer months. The wind is
persistent during the day carrying a large amount of sand and dust in its wake. Visibility in this
wind is very poor.

Baluchistan
Low

At night the visibility may improve but in strong Shamal conditions the visibility may remain
throughout the 24 hour period.

SUMATRAS

The Sumatras are strong squalls with violent CB. The winds blow at night during the south west
monsoon in the Malacca Straits between Sumatra and Malaysia.
The high ground in Sumatra and Malaysia allows a katabatic flow to start at night. As the cold air
flows over the warm sea convection brings large CB.

The Sumatras are characterised by the formation of arches over the Malacca Straits when the
anvils of adjacent CB meet.

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THE INDIAN MONSOON

THE WINTER MONSOON

Starting from December to February, the monsoon is fully established over India between
January and February. High pressure is centred to the north west of the continent with an outflow
of air.

The Siberian High is the dominant
feature with North Westerly winds

flowing over China

The air is warm and dry so the weather is fine with little cloud and moderate to good visibility.
These conditions hold good to the lee of the landmasses over the sea. There is considerable
modification in other parts.

The winds flowing down the Ganges valley are turned to become the north east monsoon of the
Bay of Bengal. Due to the long sea passage over a warm sea a large amount of moisture is
picked up resulting in the south east coast of India and Sri Lanka experiencing considerable rain
with CU and CB giving TS. Over the low lying areas fog may form but this clears once the sun
rises.

Occasionally depressions originating in the Mediterranean penetrate across India and Pakistan.
The number and paths of these depressions vary considerably from year to year. They do seem
to depend on the intensity of the Siberian high.

In some parts of northwest India the winter rain is associated only with the passage of these
disturbances.

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THE HOT SEASON

The hot season is the inter-monsoon period between March and June. It includes mainly light and
variable winds with scattered TS. The TS is associated with depressions from the west. These
storms tend to become more frequent as the season advances. The feature of this season is the
thermal lows which form in the north west of India leading to:

¾ Very high temperatures
¾ Quite frequent sand and dust storms
¾ Severe wind squalls associated with the above as with TS

Between March and May in the vicinity of Calcutta there are violent storms known as the
Norwester.

THE SUMMER MONSOON

The summer monsoon is from June to September. The monsoon winds reach India after a long
sea passage where ocean temperatures are 27°C.

The South Easterly Trade Winds
cross the Equator and become

South Westerly with a long
maritime track

Summer

The air is moist and unstable.

The instability and the nature of the land mass, especially near the coast leads to considerable
orographic and convectional rain. The heaviest rain is in East Bengal and Bangladesh during this
monsoon.

Places to the lee of the mountain masses have a lighter rainfall. The southwest monsoon is
periodic where there are a few days of strong winds and bad weather interspersed with short
periods of fine weather. A feature of the onset of the southwest monsoon of India is the sudden
way in which it is established and the regularity of its onset.

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THE RETREATING MONSOON SEASON

The retreating monsoon season is from September to December. By the second half of
September the southwest monsoon is retreating south. The winds are generally light and variable
with TS at times. These are less severe than those of the southwest monsoon. In the north fine
weather is soon established.

The fine weather spreads gradually south until by December it covers the whole of the Indian
sub-continent.

SEASON OF MAXIMUM CYCLONE ACTIVITY

The Bay of Bengal is the most affected area where most storms move north towards the Ganges
valley. Associated with these storms is a wide band of cloud and rain which affects the coastal
areas of Madras. Tropical cyclones do occur in the Arabian Sea but are less frequent. They can
occur in the March to June period.

THE FAR EAST MONSOON

CHINA, JAPAN, SOUTH EAST ASIA, INDONESIA, AND MALAYSIA

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THE WINTER MONSOON

The winter monsoon is also known as the north easterly monsoon. This winter monsoon is
normally fully established by mid-October and lasts until late March/early April. The air flowing out
from central Asia is very cold and dry. As this air flows towards the equator and over the South
China Sea it is warmed and hence picks up moisture. It is during this season that we get the
Crachin.

¾ Drizzle, low ST, mist, and fog between January and April
¾ Forms in the South China Sea and in the coastal area between Cape Cambodia and

Shanghai
¾ The Crachin is caused by the interaction between the tropical maritime and polar

maritime air circulating round the eastern side of the Asiatic anti cyclone

Further inland the cool and dry northeast monsoon is experienced in southern China, Burma and
Thailand. To the north the mountains of Japan create orographic instability producing rain and
snow.

As the air moves south of 20°N the surface warming increases the degree of instability and the
humidity of the air. Over Malaysia and to the northeast this causes development of CU and CB
with the resultant heavy showers and TS.

FROM APRIL TO MID — JUNE

The north east monsoon degenerates as the Siberian high collapses. The winds become
variable, however, in May there is a tendency for south or south westerly winds. Frontal
depressions frequently affect the north of the area.

In the south, because of the moist tropical air the weather is warm and humid with CU type
clouds. Associated with the cloud are showers and TS.

THE SUMMER MONSOON

The southwest monsoon is fully established in the Far East in June and lasts until August. Over
China and Japan this monsoon is fully established in July and August. The weather is hot and
humid with heavy rain and TS near and over the land. Over the sea where there is shelter from
the land the conditions are better. Periods of broken Cu with quiet weather alternating with
showery periods. Morning mist and fog may affect Japan. Singapore is affected by thundery
weather. Many early morning storms are due to a build up in the Straits of Malacca, or the
Sumatras.

SUMATRAS

Violent, thundery squalls where the CB have taken on a characteristic arched shape. They form
at night due to the katabatic wind flowing down the mountains of Sumatra and the hills of
Malaysia with the winds meeting over the sea. Convergent lifting occurs. By dawn these storms
reach their maximum development but clear as the sun warms the land and the katabatic flow
ceases.

During this season, the seas and coasts north of 15°N are potentially affected by typhoons with
the main activity period being between July and September.

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FROM SEPTEMBER TO MID — OCTOBER

The southwest monsoon is receding and the north east monsoon develops as the Siberian high
develops. This period shows an increasing number of fair periods. These periods are interrupted
particularly in the north by the passage of active cold fronts which are usually narrow belts of
thundery rain and squalls from a northerly direction.

Towards the end of October there is usually a fairly abrupt change to the northeast winds. This is
the definite onset of the northeasterly monsoon.

NORTH AMERICA

BLIZZARD

A blizzard is comprised of strong to gale force winds that are accompanied by falling or drifting
snow that is whipped up by the strong surface wind. It is prevalent in Northern USA and Canada.
Siberia has a similar wind called the Buran.

CHINOOK

A warm dry wind that is also known as the “Snow Eater”. This foehn wind produces a rapid rise in
temperature on the lee side of the rocky mountains. The wind blows in Alberta and Colorado.

SOUTH AMERICA

PAMPERO

The Pampero is a strong cold wind that develops behind cold fronts blowing at latitudes around
40°S. At this latitude the weather is influenced by the passage of depressions and anti-cyclones
moving to the east.

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Warm
Humid
Air

Cold
Dry Air

Warm humid air is drawn from the north ahead of a depression. The passage of the depression
then sees violent line squalls in association with the cold polar air flow from the south or west.
This wind is most frequent in summer but can flow any time of the year.

ZONDA

The South American equivalent of the Chinook. The wind blows off the lee slopes of the Andes.

AUSTRALIA

BRICKFIELDER

A summer wind which is hot dry and dusty and affects the areas of New South Wales and
Victoria.

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SOUTHERLY BUSTER

The Southerly Buster is similar to the Pampero occurring at latitudes around 40°S.

The wind blows most frequently in summer between travelling summer anticyclones when cold
unstable polar air moves behind a cold front which trails well to the south. The contrast between
the cold air and hot summer air is marked. Active line squalls form with strong winds. Low cloud
and poor visibility.

OCEAN CURRENTS

In the diagram below, the sub tropical high pressure systems give rise to warm water currents on
the west side of oceans and to cold water currents on the east side of oceans.

In an area of persistent offshore winds an upswell of cold water can be developed from the ocean
beds. This increases the effect of the cold water current and decreases the effect of the warm
water current.

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COLD WATER COAST

Due to the low rate of evaporation from the nearby cold ocean the air has a low vapour content.
Little cloud or precipiation forms. The cold water coasts bound the desert regions of the world.

During the night, cooling can produce advection fog or low stratus which disperses once the sun
is up.

WARM WATER COAST

Over both land and sea the air is humid due to the rapid evaporation from the warm ocean.

Over the land by day and the sea at any time the temperature is relatively high which results in
CU forming thunderstorms.

At night the diurnal variation of the surface temperature can cause CU clouds to disperse or form
SC. These clouds redevelop once insolation starts again.

Over the sea by night the CU persist because of the relatively constant temperature. It is these
areas that are suitable for tropical revolving storms to form.

SUMMARY OF THE LOCAL WINDS OF THE WORLD

Wind Location Season Brief Description
Bora Dalmatian Coast of Winter
North Easterly
Brickfielder Croatia Summer Cold
Australia
Chinook Anytime Strong gale force wind
Etesian North America Summer
Foehn Aegean Sea, Greece Anytime Northerly
Ghibli Late summer Hot and Dusty
Alps Blows from the interior
Gregale Libya Winter
Foehn wind blowing
Haboob Malta and environs Anytime over the Rockies
Harmattan November to March
Egypt and Sudan Northerly
Fine and clear
West Africa
Warm, dry stable wind
to the lee of mountains

Southerly
Hot and damp when

over the sea
Blows ahead of a

depression

North Easterly
Gales and squalls

Persistent

Sandstorm ahead of
advancing

thunderstorms

East North Easterly
Hot, dry and dusty

from the desert

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Wind Location Season Brief Description
Khamsin Egypt Late summer
Levanter From the Sahara
Straits of Gibraltar March to Summer Hot and dry
Meltemi
Turkey Summer Easterly
Mistral Hot and damp
Rhone Valley, Anytime, but more
Pampero France predominant in late Light winds

Argentina autumn to winter North Westerly to
Winter North Easterly
Fine and clear

Northerly
Cold, gale force wind
Often clear conditions

South Westerly
Gales with line squalls

Shamal Persian Gulf Summer North Westerly
Simoon Hot and dusty
Sumatra Palestine and Syria Summer and autumn Cloudless, calm nights
Southerly Buster Southerly to South
Vendevale Malacca Straits South West Monsoon
Zonda between Sumatra onset Easterly
Hot, dry and dusty
and Malaysia Anytime, more
Australia marked in January Squally
Thundery and wet
Gibraltar and Eastern and February
Spain Spring and autumn Squall line
Cool wind
Argentina Anytime
South Westerly
Strong squally wind

Thunderstorms
Foehn wind

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TROPICAL REVOLVING STORM (TRS)

The TRS is a very confined region of low pressure where the Isobars are tightly packed together.
TRSs mainly occur on the western side of an ocean during the summer and autumn period for
that hemisphere.

231/2° N Hurricanes Typhoon
Equator June-October July to
231/2° S October
June, October
and November Cyclone

January to January to
Tropical Cyclone March March

Note that there are no TRSs in the Southern Atlantic. This is probably because the ITCZ never
travels into the South Atlantic and one of the requisites of these storms is that they require
intense heating and low pressure. The water temperature is, therefore, too low. In the Atlantic
TRSs are called hurricanes, in the Indian Ocean they are called cyclones, and in the Western
Pacific they are known as typhoons.

The mechanism of the TRS is not fully understood, but they seem to breed in the vicinity of the
ITCZ. The ITCZ provides the convergence that provides high instability and high humidity.

The storms tend to follow an elliptical path, firstly moving westward. If the westward path is
maintained then the storm runs aground and peters out. If the storm moves in an elliptical path
then it turns toward the east and as it moves into higher latitudes loses its vigour.

It is rare for a TRS to form within 5° of the Equator where Coriolis is near to zero.

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CHARACTERISTICS

The TRS forms in defined locations between the ITCZ and the sub tropical high pressure belt.

The formation is in the trade wind belt where the weather is normally fine with fair weather CU.
Occasionally in this area a weak trough forms which moves slowly westward in the trade wind
drift. These are known as Easterly Waves.

The convection in the wave is normally checked by the Trade Wind inversion. If the heat and
humidity at low level are sufficiently high and the wind profile favourable, convection breaks
through the inversion. The sea temperature has to be above 26°C. During its motion eastward the
trough is amplified; its convection is intensified; Coriolis force starts a cyclonic airflow, and a
“comma” cloud can form.

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After this development the TRS can form.

The convergence in the trough encourages the development of bands of large CB and CU with
their associated precipitation.

As the trough grows, the convergence and convection become organised and the pressure at the
surface begins to fall rapidly.

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A deep depression is formed which is characterised by a central pressure between 900 to 960
hPa (870 hPa is the lowest recorded value). High winds develop between 50 to 100 kt with CB
and torrential rain.
Much of the energy obtained from the latent heat of condensation is released in the atmosphere
as the high humidity is lifted.
The structure of the cloud is still under investigation. However, it is known that the isobars are
roughly circular with the depression having a diameter of ⊄ 350 nm. A mid latitude depression
has a diameter of approximately 1000 nm and so the smaller diameter reflects a steep pressure
gradient. The winds are strong below 10 000 ft and tend to spiral inwards giving the highest
speeds 10 to 20 nm from the centre of the storm.
Above 25 000 ft the winds spiral outwards carrying with them an extensive cloud table. Other
outward spirals of lesser extent are found at medium and low levels. These can also form cloud
tables.
The storm has great vertical extent with the CB in excess of 40 000 ft. The centre of the storm is
always marked by “the eye” which is a roughly circular area with a radius between 10 to 20 nm.
The area is one of subsidence which gives light winds and broken clouds.

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A wall of CU and CB surrounds the eye; these are formed in patterns which mark the spiralling
nature of the wind.

Stratiform tables appear out of the sides of the storm forming cloud tables. At low level they
consist of SC, at medium level they consist of AC or AS, and at high level they consist of CI or
CS.

Heavy showers accompanied by TS and severe squalls accompany the main wall of cloud.

The most severe weather is just outside the ring of the strongest surface winds which exist just
outside the eye. Satellite imagery can clearly depict the eye and the extent of the cloud.

The storm moves at approximately 15 kt. The speed changes frequently, slowing down as the
path or movement curves. The TRS can then accelerate as it passes 30° latitude where speed of
movement is up to 50 kt.

At lower or higher latitudes cold air is pulled into the system. The TRS develops into a very active
tropical depression. Over Western Europe these depressions can bring Hurricane force winds
with the associated weather of a depression.

If the moisture content of the storm is cut off then the storm dies out. This normally happens when
the storm travels over land.

The warning of the approach of the TRS is now done by satellite. These predictions are not totally
accurate, as the storm tends to move in an erratic manner.

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VISUAL INDICATIONS OF THE ADVANCE OF THE TRS

Surface Pressure
The diurnal variation in pressure in the tropics is suppressed. The pressure has a
tendency to fall.

Ocean Swell
At coastal sites an abnormally heavy swell can be seen. This is a result of the strong
winds that spread out from the centre of the disturbance.

Cloud
Extensive tables of CI can be detected up to 600 nm from the storm.

TORNADO

The term is applied to disturbances that are also known as whirlwinds. They are common in the
USA and Australia. Even though the Australian continent can have up to 150 disturbances a year
they are rarely reported, as they are much less severe than the storms in the USA.

The storm consists of a violent circular whirlpool of air shaped like a funnel between
100 to 1000 m in diameter. It is when the funnel reaches the surface that the storm becomes
destructive.
The extremely low central pressure makes the Tornado the most destructive storm known.

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The storm has limited dimensions and is difficult to assess accurately. However, the following is
typical.

¾ Wind speeds can range between 100 to 300 kt.
¾ The Tornado has a twisting appearance due to the strong winds.
¾ The pressure can change by 100 hPa in as little as 50 m.
¾ The advance is between 15 to 20 kt.
¾ Like a TS the Tornado lasts approximately 2 hours.

The Tornado forms in association with a marked trough of low pressure along which there is
marked instability. These troughs are:

¾ Linked to a frontal depression
¾ A single cold frontal trough
¾ A non-frontal trough

These troughs are generated when cold dry air from the western plateau overrides the tropical
maritime air. Instability is generated and this allows the trough to form overland.

If the Tornado forms over the sea it is known as a waterspout. This storm is much less violent and
lasts in the region of 20 minutes.

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TROPICAL REVOLVING STORM AREAS

Area Formation
West Indies
5 to 10 hurricanes per year
West and Central Pacific The storms originate in or east of the Caribbean. Movement is
then westerly or northwesterly. Some affect the USA. Most
Western North Pacific curve across the islands in the Caribbean or Florida passing
and the China Sea into the North Atlantic.
Bay of Bengal June to October
Arabian Sea
Tropical Cyclones which form in the area of New and Old
South Indian Ocean Caledonia
North West Australian January to March
Coast
Typhoons which affect the Philippines, Hong Kong, and Taiwan
West African Tornado July to October

Cyclones occurring in advance of the SW Monsoon in June and
during the retreat of the monsoon in October and November.

Cyclones form over the sea to the east of Oman to Bombay.
Associated with the ITCZ and the SW monsoon.
Times are the same as those for the Bay of Bengal.

Tropical Cyclones in the Madagascar area
December to April

Tropical Cyclones, or “Willy Willys”.
Be careful with this second name, as the Willy Willys are really
an inland dust storm.
These storms occur NW of Darwin but originate in the Timor
Sea.
These storms can flow down the Coral Sea to Brisbane.
Occur between January to March

Occur in the Gulf of Guinea and are severe TS.
These are not technically TRS but an extensive line squall
which affects the area twice a year.
March to May
October to November

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EUROPE

The area lies in the same climatic zone as the North Atlantic and is considered a disturbed
temperate.

In Europe, the flow of weather is determined by the travelling depressions from the Atlantic. In
winter a dominant Siberian high can make the flow change. Norway is the exception where the
coastal mountains, which run north/south, cause a block to east-west flow.

The changes in temperature and weather conditions from summer to winter are less extreme than
the larger continents of Asia and North America.

NORTH WEST EUROPE

The climate is affected by the prevailing south westerly winds which transport warm air from the
North Atlantic drift to the land. The absence of any major topographical barriers allows the
maritime influence to extend deep into Europe. As depressions move into the land mass they
tend to dry out so the rainfall in the east of Europe is only about half that in the west.

The position of the polar front over the North Atlantic has a strong influence over European
weather. Depressions travelling east along it progress well into the continent; especially because
there are few mountains to oppose their progress. The only major topographical barrier is the
Alps which impedes the progress of cold fronts. These fronts slow down and cause widespread
cloud and rain. The final movement of easterly moving depressions is often dictated by the
position of the Siberian High in winter.

The Siberian High can become a dominant feature on the European weather map during the
winter months causing depressions to track around it.

In summer, the low pressure over Siberia and Asia is less dominant and the weaker and less
frequent depressions continue without deflection and follow the line of the polar front.

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TEMPERATURE DISTRIBUTION

JANUARY

The isotherms run north/south indicating the temperature contrast between the mild waters of the
North Atlantic drift and the colder continent. The temperature gradient is much shallower than that
found on the east coast of the USA where the contrast is not as significant except that in
Scandinavia where the Norwegian mountains separate the mild ocean from the severe winter
temperatures of Siberia.

JULY

The isotherms conform to the lines of latitude and there is little contrast between the land and
water temperatures.

PRESSURE DISTRIBUTION

JANUARY
The general pressure distribution is as follows:

¾ Icelandic Low to the north approximately 1000 hPa.
¾ Azores high (Sub-tropical high) to the south approximately 1020 hPa at 30°N.
¾ The Siberian high to the east normally around 1035 hPa.

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The ridge of the Azores high extends eastward over the cold lands of southern Europe and the
Icelandic low deepens. The Siberian high is intensified by the snow covered terrain of
Scandinavia and Eurasia and is the other major influence

JULY

The Icelandic low intensifies in pressure to 1010 hPa and the Azores high which moves north to
35°N is 1025 hPa.

The Siberian high is now replaced by the Baluchistan or monsoon low of India.

The Azores high has deepened and moved north and the Icelandic low has weakened and
moved north.

UPPER WINDS

JANUARY
The upper winds are westerly, normally 40 to 60 kt with frequent jet streams reaching 150 kt
associated with fronts. The jet stream direction is variable because of their positioning in relation
to the travelling depressions.

JULY
The upper winds weaken but are still westerly at 20 to 40 kt. Jet stream speeds are decreased
due to the weaker temperature gradients found in the summer period. Speeds of 100 to 150 kt
are seen.

SURFACE WINDS

JANUARY
The prevailing winds are from the south west. Winds from the east can persist for several days or
even longer when the Siberian high becomes well established over Scandinavia.

JULY
The prevailing winds are still from the west but are weaker.

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HEIGHT OF TROPOPAUSE AND 0°C ISOTHERM

JANUARY

Tropopause 35 000 ft
0°C Isotherm 2000 ft

JULY

Tropopause 40 000 ft
0°C Isotherm 12 000 ft

CLOUD

In winter, like the North Atlantic region, the average cloud cover is 6/8. Cloud types are those
associated with frontal depressions and their respective warm and cold air masses.

In summer the cloud cover reduces slightly to an average of 5/8. Frontal depressions travel the
area less frequently and this small reduction in cloud cover is due to the high incidence of thermal
lows over the continent.

ICING

WINTER
The 0°C is low, often at the surface especially in central and eastern Europe. Conditions are
therefore favourable for icing in the extensive cloud of the travelling depressions. High ground in
the region can cause the icing to become severe in warm fronts or the convective clouds which
form in the unstable polar air.

SUMMER
The 0°C isotherm rises and the incidence of icing is reduced. With the travelling depressions that
travel across Europe icing can still be a problem at times during the summer months.

PRECIPITATION

The annual rainfall in the west is about double that in the east because of the drying out of the air
as it travels east. Normal rainfall in the west is 1000 mm against 500 mm in the east.

The western coastal parts of the region have the heaviest rainfall in winter. Elsewhere, the
wettest period is late summer and the driest period late winter or early spring.

Precipitation is liable to be snow in winter particularly in the east and south east where the ground
is occasionally snow covered for long periods.

VISIBILITY

WINTER
Greatest problem in Europe is poor visibility due to the high frequency of fog and very low cloud.
Both occur very readily in the maritime air masses and little cooling being required to produce
condensation.

In anticyclonic conditions fog may become widespread and dense, aggravated further by
industrial smoke in the Eastern European states.

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SIGNIFICANT WEATHER

JANUARY
Frontal weather associated with the depressions travelling in from the west. When the pressure is
high then the visibility is often severely reduced by radiation fog.

Frost and severe wintry weather are frequently associated with an easterly flow from the
dominating Siberian high.

Advection fog is occasionally expected during periods of thaw and in coastal areas in spring and
early summer when the sea temperatures are at their lowest.

JULY
Some frontal depressions can still be expected though they are fewer in number. Less vigorous
than in the winter period the depressions still bring typical frontal weather but on a reduced scale.

SPECIAL FEATURES OF EUROPE

WINTER
If a trailing cold front is held up by the Alps in the south of France it can produce a belt of rain and
cloud on the northern slopes. However, more important is the possibility of waves forming on the
front which develop into vigorous secondary depressions which can move rapidly north east with
their associated weather.

Lee side orographic depressions can be formed in Northern Italy.

Occasionally, lows form in the Danube basin, in the south east of the region, due to the incursion
of warm air from the Mediterranean. These low pressure areas give rise to extensive low cloud
which can extend as far as eastern England. Associated precipitation, which may fall as snow, is
frequently heavy.

A low pressure over Scandinavia can bring Arctic air to the west of the region which again brings
snow.

SUMMER
Occasionally, large-scale thermal depressions form over the continent and these lows give rain
and thunder with extensive masses of cloud. The thermal low is most evident over the continental
areas of France and Spain.

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MEDITERRANEAN

An area with a transitional climatic zone:

¾ To the north is the disturbed temperate climate of Europe.
¾ To the south are the arid sub-tropical regions of North Africa.

The weather in the Mediterranean is noted for its marked seasonal variations. The Mediterranean
is a sea surrounded by land. Remember that water warms up and cools down much slower than
land surfaces.

TEMPERATURE DISTRIBUTION

JANUARY
The sea is relatively warm but is surrounded by cold land.

JULY
The sea is relatively cool surrounded by warm land.

PRESSURE DISTRIBUTION

JANUARY
The sub-tropical high moves south and the disturbed temperate weather of northwest Europe
penetrates to the Mediterranean.

The water is warm compared to the land which leads to low pressure over the sea.

Incursions of cold air over the warm sea, in the western basin of the Mediterranean, help to
create, or enhance, the depressions that reach that area from the Atlantic.

Depressions can enter the Mediterranean via:

¾ The Carcassonne Gap which is between the Pyrenees and the Masif Centrale.
¾ The Straits of Gibraltar.
¾ From the orographic or lee depressions that form over the Gulf of Lyons, Gulf of

Genoa, and the northern Adriatic as a cold front advances from the north or with a
broad northerly airstream over Europe.

Saharan depressions in the lee of the Atlas Mountains travel from the western end of the
Mediterranean to the eastern basin where they:

¾ Slow down
¾ Are sometimes regenerated by polar continental air from Russia and the Balkans
¾ Sometimes continue to the Arabian Gulf

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In winter, a depression passes through the Mediterranean in approximately 10 days. The warm
fronts associated with these depressions are not very active but the cold fronts can be quite
vigorous.

Depressions following a path close to the northern shores of the Mediterranean cause Italy and
the Balkans to have similar weather to that experienced over the UK. Where depressions follow
the southern coast, there is less cloud and precipitation as the air in the warm sector is from the
Sahara and thus very dry.

Ahead of the warm front the surface wind is southerly or south easterly and is often strong
enough to lift sand and dust off the desert. The obvious result is sand and dust storms causing
hot dusty winds over the Mediterranean (e.g. Sirocco, Ghibli and Khamsin).

JULY
The sub-tropical high-pressure belt moves north and now the sea is colder than the surrounding
land.

The Azores high extends over the area giving fine or fair weather. Occasionally, the north west
experiences a depression.

UPPER WINDS

JANUARY
The marked contrast in temperature between cold air from Siberia and warm air from North Africa
leads to a steep temperature gradient over the North African coast at the eastern end of the
Mediterranean.

This sub-tropical jet stream reaches speeds of 100 kt over Cairo and is about 80 kt over Cyprus.
At the western end of the Mediterranean the winds are westerly with a mean speed of 40 kt.

JULY
The upper winds are westerly at approximately 30 kt, due to the sub-tropical jet stream moving
north and reducing in speed.

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SURFACE WINDS

JANUARY
At the eastern end of the Mediterranean, the surface wind is generally westerly to north westerly
but it can be variable. The wind speeds are moderate but can be increased to gale force when
depressions are reinvigorated by cold air from the Siberian high.

In the western basin the winds are moderate westerly to north westerly, but can be gale force
when associated with depressions. Both Mistral and the Bora winds occur.

JULY
The surface winds are predominantly from the north, such as the Etesian. Local sea breezes are
also evident during the day.

SIGNIFICANT WEATHER

JANUARY
The most noticeable features are the winter depressions with their attendant unstable squally
weather. Vigorous cold fronts on the depressions have attendant CU and CB with strong winds
and heavy rainfall. The visibility can deteriorate significantly especially when a Sirocco or
Khamsin is blowing from the south in advance of the depression.

JULY
The pressure is generally high which means warm cloudless conditions. Occasional TS can
generate near high ground.

NORTH ATLANTIC AND NORTH AMERICA

TEMPERATURE DISTRIBUTION OVER THE NORTH ATLANTIC

The temperature is regulated by both warm and cold water currents.

GULF STREAM
The warm water Gulf Stream from the Caribbean flows up the eastern seaboard of the USA. It
then turns east around the sub tropical high pressure zone and then divides into two distinct
currents. One element, the North Atlantic drift, fetches up against north west Europe and
Scandinavia. It is this current that keeps the coast of Norway, north of the Arctic Circle, ice free
throughout the year.

The second element flows eastward and eventually turns south around the east side of the sub-
tropical high pressure zone (The Azores High).

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CANARIES CURRENT
The cold water current emanates from the more northerly latitudes towards the northwest coast of
Africa. Hence the typical cold water coast of Morocco with a tendency for fog formation over the
Canary Islands and North West coast of Africa.

THE LABRADOR CURRENT
The Labrador Current is the cold water current emanating from high latitudes and flowing south
between Labrador and Greenland. When air, warmed by passing over the warm waters of the
Gulf Stream moves north over this current, advection fog forms. This is a well known feature of
the area known as the Grand Banks off the east coast of Newfoundland. The fog forms frequently
between May and August and can persist for several days at a time.

TEMPERATURE DISTRIBUTION OVER NORTH AMERICA

Land masses heat up and cool down comparatively rapidly. This is certainly noticeable with the
large North American landmass.

JANUARY
The land cools down rapidly, and when looking at a chart of isotherms at surface level you can
see they are tightly packed over the Eastern seaboard. This distribution occurs because of the
considerable temperature difference between the cold land mass and the warm waters of the Gulf
Stream. The steep temperature gradient is typical of the western sides of oceans in winter. The
temperature gradients form because in low latitudes the ocean currents circulate around the sub-
tropical high pressure areas and the flow of warm water from equatorial regions is on the west
side of oceans.

This steep temperature gradient over the eastern seaboard produces a strong thermal wind
component from the south west. Steep temperature gradients and the accompanying strong
thermal wind component form in winter whenever a cold land mass is adjacent to a warm ocean
current.

JULY
The North American landmass is warm. The steeper temperature gradient is now to be found on
the west coast. The clockwise circulation of ocean currents around the North Pacific sub-tropical
high creates a cold water current flowing south from the Aleutian Islands. This results in a cold
water current off the coast of California and the formation of advection fog when the warm moist
air from the Pacific drifts over the cold Californian current. This is very prevalent in the region of
San Francisco.

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PRESSURE DISTRIBUTION OVER THE NORTH ATLANTIC

JANUARY
The general pressure distribution is shown below:

¾ Icelandic Low to the north 1000 hPa.
¾ Azores high (Sub-tropical high) to the south 1020 hPa at 30°N.
¾ High pressure to the west over the USA.
¾ The Siberian high well to the east, 1035 hPa.

A large number of depressions pass over Iceland in winter creating the mean Icelandic low which
dominates the temperate latitudes. Families of travelling depressions move eastward. The large
landmass of North America allows cold polar air to move well south before meeting warm tropical
air from the Azores high around Florida and Bermuda.

Depressions form and run along a line roughly from Florida to south west England. These
travelling depressions are interspersed with ridges of high pressure.

Polar air depressions (Polar Lows) can form in the polar air as it moves into an area of the North
Atlantic to the North West of the UK. The general movement of these depressions is west to east.

JULY

¾ The Azores high intensifies and moves to 35°N 1030 hPa. Meteorology
¾ The Icelandic low is reduced in size with a pressure rise to 1010 hPa.

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The Icelandic Low is less of a dominant feature. The two continental high-pressure zones from
winter are replaced by low-pressure with the monsoon low of India being the predominant feature.

The Polar Front over the North Atlantic, along which we get the travelling depressions changes
position to run from Newfoundland to north of Scotland. Ridges of high pressure and anticyclones
last longer as the contrast in temperature between polar air and tropical air masses is reduced.
Fronts have less marked features and higher sea temperatures reduce the incidence of polar air
depressions.

GENERAL
Depressions that form on the polar front are more frequent in winter with 12 to 14 depressions
travelling per month. In summer, there are fewer depressions and they are much less vigorous:

¾ To the north of the area 6 to 8 per month.
¾ To the south of the area 1 to 2 per month.

PRESSURE DISTRIBUTION OVER NORTH AMERICA

JANUARY
Because of the cold land mass, the area becomes a centre for high pressure.

JULY
The heated land mass now becomes a centre of low pressure Upper Winds.

UPPER WINDS

JANUARY
The predominant feature is the south west sub-tropical jet stream formed by the large
temperature difference between the cold land and the warm sea.

Strong upper westerly winds prevail in the mid-latitudes normally in conjunction with travelling
depressions on the polar front. The wind direction varies from south west to north west and is
often of jet stream proportions, the speed increasing with height to an average 50 to 60 kt. Jet
stream speeds are between 100 to 200 kt.

JULY
Upper winds are still westerly but the speed decreases to between 40 and 50 kt. Jet streams
become less frequent, however the speeds are still between 100 to 200 kt.

LOW LEVEL WINDS

Over the ocean, the winds are westerly of a moderate speed, stronger in winter than summer.

The winds circulate anti-clockwise around the depressions so that to the north of a depression
exists an easterly flow, whilst to the south of a depression exists a westerly flow.

The north east trade winds blow in the southern part of the area.

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OVER NORTH AMERICA

In winter the winds blow from the north. In summer, the winds blow from the south, except in
Canada where they blow from west to north.

TROPOPAUSE AND 0°C ISOTHERM

JANUARY

Tropopause 56 000 ft in the south
0°C Isotherm 30 000 ft in the north
10 000 ft in the south
Close to the surface in the north

ICING

The 0°C isotherm is low and on the surface on the eastern seaboard of the USA swinging
north to lie to the north of the UK. Conditions are favourable for icing in frontal clouds and
the CU and CB found in the polar air.

PRECIPITATION

Widespread and continuous ahead of warm fronts, showery at, and behind the cold
fronts.

The stable conditions found in the warm sector usually give drizzle. Snow can reach the surface
in the north and north west of the area when the surface temperatures become less than 4°C.

JULY

Tropopause 55 000 ft in the south
0°C Isotherm 35 000 ft in the north
15 000 ft in the south
10 000 ft in the north

ICING

The 0°C isotherm is higher so the incidence of icing is less. It still may present a major
problem.

CLOUD

In winter, an average of 6/8 cover with the cloud types varying dependent on the air mass and
frontal system.

Frontal

Frontal clouds are extensive both horizontally and vertically. They can sometimes extend
from the surface to the tropopause.

Polar Air

The convective cloud behind cold fronts is usually scattered, but often extensive in active
polar air depressions.

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Tropical Air
In the warm sector of frontal depressions, widespread SC exists with tops not above
4000 ft. In anticyclones there is low SC over both sea and land.

In summer, the basic cover remains 6/8 due to the extensive SC in the tropical air.

SIGNIFICANT WEATHER

NORTH AMERICA
Winter
In winter, depressions moving from west to east produce most of the weather. Gale force winds
can produce blizzard conditions with minimal visibility. Precipitation, often in the form of snow,
accompanies the depressions. As spring arrives then the Chinook is a feature to the lee side of
the Rocky Mountains.

Summer
TS can build up over the mountains and also form when moist air from the Gulf of Mexico is over
ridden by cold dry air from the high mountain plateaus. The instability that results encourages the
formation of tornadoes in the mid-west especially in spring and early summer.

NORTH ATLANTIC
Over the ocean, the season for tropical revolving storms in the low latitudes is June to October.
About 3 to 5 hurricanes occur per year. These storms form in the low latitudes near the ITCZ
initially moving west then turning north and finally curving to the north east. If the TRSs reach the
higher latitudes they take on the characteristics of a severe temperate latitude depression. These
often reach northwest Europe bringing the wet windy conditions normally associated with a
travelling low in winter.

Advection fog occurs between May and August on the east coast of Canada and in the south
west approaches to the English Channel during spring and early summer

AFRICA

With most of the continent lying within the tropics there is no defined winter or summer period.
The most important aspect of the weather is the ITCZ and its seasonal movement. Because of
this movement there are clearly defined wet and dry seasons over the continent.

The northern area borders the Mediterranean and experiences the weather and temperature
changes of that zone.

The extreme south is outside the Tropic of Capricorn so can also be said to experience
Mediterranean style weather.

In January the land mass to the south of the equator receives the greatest amount of heat and
therefore has the higher temperatures. In July the thermal equator lies to the north.

PRESSURE DISTRIBUTION

The most distinctive feature is the ITF/FIT, which is directly influenced by the sun, which, in turn,
creates a low pressure convergence zone by heating up the land mass.

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Over the adjacent oceans the ITCZ is less marked as the water does not respond as readily to
the sun’s heating.

JANUARY

ITF 5°N in West Africa
20°S in South Africa

Travelling depressions affect the Mediterranean.

JULY

ITF 20°N

Travelling depressions affect the Cape of Good Hope.

UPPER WINDS

JANUARY
To the north of the area, the winds are westerly with speeds up to 50 kt. These decrease to 10 to
20 kt in the lower latitudes. In the equatorial regions these winds become easterly at 10 to 20 kt.

Once in the southern hemisphere the winds increase from the west.

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JULY

The upper winds are westerly in the higher latitudes both north and south of the equator. In the
lower latitudes the wind is easterly with speeds up to 60+ kt over the Guinea coast at the 200 hPa
level.

HEIGHT OF THE TROPOPAUSE AND 0°C ISOTHERM

JANUARY

Tropopause Equator 56 000 ft
0°C Isotherm Higher Latitudes 50 000 ft
Equator 18 000 ft
Higher Latitudes 12 000 ft

JULY

Tropopause Equator 55 000 ft
0°C Isotherm Higher Latitudes 50 000 ft
Equator 18 000 ft
Higher Latitudes 12 000 ft

SURFACE WINDS

JANUARY

Over the adjacent oceans, the trade winds blow from the north east and south east. In the Gulf of
Guinea the trade winds are deflected by Coriolis and so blow from the south west.

West Africa

The Harmattan blows from the north east as a hot dusty dry wind from the Sahara. The
resultant visibility is poor because of dust haze.

North African Coast
The winds are generally from the west.

East Africa
This region is affected by the trade winds.

South Africa

The winds are from the south west having circulated around the southern hemisphere
sub-tropical high pressure area in the south Atlantic.

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JULY
North Africa
Northerly winds from the Mediterranean.

West Africa
The ITF has moved north and the West African monsoon from the south west brings
warm humid air in from the Gulf of Guinea.

Sudan
The warm humid air is drawn in from the Indian Ocean. This is the season for frequent
Haboobs.

South of the Equator
The south east trades blow in towards the equator.

South Africa (Cape Province)
The winds are often from the west with travelling depressions providing stormy
conditions.

SIGNIFICANT WEATHER

Over the continent in both January and July, the weather is occasionally severe in the vicinity of
the ITF which lies across some part of Africa throughout the year. Typical ITF weather is TS with
CB cloud extending to the tropical tropopause with the attendant rain and squally winds.

Long and short rains occur annually where there is a double passage of the ITF, usually at
locations close to the equator. One example is Nairobi/Seychelles where the long rains occur
when the sun moves north and the short rains occur when the sun moves south.

JANUARY
The Harmattan blows over West Africa where visibility is reduced to 4000 m and at times can be
as low as 1000 m. The dust carried by this wind can extend to considerable altitudes. Tropical
cyclones occur in the Mozambique Channel from January to March.

JULY
Haboobs form in the East African desert regions. In West Africa the south west monsoon moves
in behind the ITF. On the front there are often severe TS and heavy rain. The Guti affects
Zimbabwe and sometimes the Transvaal. The Guti is formed when moderate to strong south
easterly winds bring moist air from the Mozambique Channel. Conditions associated with this
wind are very low St and Sc. The wind occurs in spells of 1 to 5 days especially in the dry season
from April onward.

West African tornadoes, a line of TS moving westward, are a feature of spring and autumn.

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ASIA

TEMPERATURE DISTRIBUTION

JANUARY
The vast Asian land mass is cold north of the Himalayas. This means that a steep temperature
gradient forms at this mountain barrier and the eastern seaboard of the continent.

The warm Kuro Siwo current running up the Chinese coast to Japan creates a strong
temperature gradient similar to that on the eastern seaboard of the USA

JULY
In the summer the sun migrating north of the equator heats the land mass. The isotherms now
conform to the lines of latitude except on the eastern seaboard where the land has warmed up
more than the ocean and the isotherms parallel the coast.

PRESSURE DISTRIBUTION

JANUARY
The Siberian high is the dominant pressure system affecting the continent. The cold is intense
where temperatures of –40°C can be reached.

The pressure reaches values which can be in the order of 1070 hPa. Air flows out from this high
pressure which gives rise to the winter monsoon. Winds in northern China are westerly, but
further south the winds become northerly. Finally, they become north easterly to become the
north east monsoon of south east Asia and Indonesia.

India is cut off from the Siberian high by the Himalayas and it develops its own high pressure
system centred in north west India and Pakistan. The resultant wind from this high pressure
system flows out along the Ganges valley and eventually joins the north east monsoon over the
Bay of Bengal.

Over China, Japan, and East Asia the air is cold, warming up as it flows toward the equator.

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JULY

The high land temperatures create the monsoon low. Low pressure is over the continent, while
high pressure is over the oceans. The resultant airflow is from sea to land.

The air may come from the southern hemisphere as the south east trade winds may be turned to
from the south west monsoon winds by Coriolis. The south west monsoon has its direction
changed as it reaches the land masses. For example, it is diverted to flow from the south east up
the Ganges valley toward the low centred in north west India.

UPPER WINDS

JANUARY
The sub-tropical jet stream blows parallel to the steep temperature gradient created by the
Himalayan barrier and the Kuro Siwo current in the east. It blows from the west over northern
India to the south of the Himalayas. From the south west over Japan, the jetstream reaches
speeds of over 100 kt. At low latitudes, an easterly jetstream at 10 to 15 kt prevails.

JULY
Westerly winds prevail at higher latitudes. Between 20°N and the equator the winds are easterly
above 20 000 ft increasing in speed with height until at 30 000 ft they are 40 to 50 kt. At the 200
hPa level they become the equatorial jet stream at 80 kt.

HEIGHT OF THE TROPOPAUSE AND 0°C ISOTHERM

JANUARY
Over the Persian Gulf and India:

Tropopause 56 000 ft
0°C Isotherm 12 000 ft

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Further north over China and Japan:

Tropopause 49 000 ft
0°C Isotherm 8000 ft
JULY
Over the Persian Gulf and India:

Tropopause 55 000 ft
0°C Isotherm 18 000 ft

Further north over China and Japan:

Tropopause 52 000 ft
0°C Isotherm 10 000 ft

SURFACE WINDS

JANUARY
The north east monsoon dominates much of the area. Over central Asia the winds circulate
around the Siberian high. This circulation produces northerly winds over eastern Siberia, Japan,
and Korea. Towards the north and west of Asia the flow is south westerly.

JULY
The situation is reversed where much of the area is under the influence of the south west
monsoon. The flow is modified over China and Japan where the monsoon is from the south and
south east. The northern and eastern areas of the continent experience a northerly flow.

AUSTRALIA AND THE PACIFIC

TEMPERATURE DISTRIBUTION

JANUARY
With the absence of any significant land mass in the South Pacific, apart from Australia, the
isotherms conform to the lines of latitude. Over Australia the land heats up to a greater extent
than the surrounding sea. Over parts of Australia temperatures can exceed 30°C.

JULY
The isotherms conform to the lines of latitude. There is some distortion over Australia where the
southern half of the continent is slightly cooler than the ocean.

PRESSURE DISTRIBUTION

JANUARY
A zone of low pressure forms in the centre of Australia due to the high temperatures. This
contrasts to the sub-tropical zones of high pressure which occur in both hemispheres.

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1005

L

The ITCZ extends from the north of Australia north east to the equator and then across the north
Pacific to Columbia. Low pressure zones occur in both hemispheres in the temperate latitudes. In
the northern hemisphere this is known as the Aleutian low, the equivalent to the Icelandic low.

JULY
The ITCZ moves as far north as Hong Kong in the west Pacific basin.

1010

1020 H 1020

H

Over the ocean, the ITCZ follows a similar line to its January alignment across to South America.
The sub-tropical high and temperate low pressure zones are still apparent in their respective
hemispheres.

In addition, there is now a high pressure zone over Australia similar to the sub-tropical high
pressure zone over the oceans.

UPPER WINDS

JANUARY
Temperate latitude westerlies with jet streams in the vicinity of travelling depressions occur in the
north Pacific. Over Australia and the South Pacific westerlies at speeds of 60 to 70 kt blow. In the
equatorial regions the upper wind is easterly at 20 to 30 kt.

JULY
Temperate latitude westerlies still blow in the north Pacific and South Pacific. These winds reach
jet stream proportions in association with mid-latitude travelling depressions. Upper easterlies still
prevail in the equatorial regions.

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