Wind Pressure Sys. Structure of a Depression Anticyclone Weather Services TRS




Wind and Pressure Systems


The diagram below shows the distribution of pressure and the winds, which would result over a featureless earth.

The Trade Winds blow on either side of the Equatorial Trough, NEly in the N hemisphere and SEly in the S hemisphere.

The Trades blow with great persistence and each embraces a zone of some 1,200 miles of latitude.

Trade winds, however, do not blow throughout this zone.

The South West Monsoon winds blow instead off part of the West coast of Africa, in the North Indian Ocean and in the W part of the North Pacific Ocean.

The trades move slightly north and south with the sun; their approximate limits are as follows:

February August

Atlantic Doldrums 000 - 002N 005N - 010N

N.E. Trade 002N - 025N 010N - 030N

S.E. Trade 000 - 030S 005N - 025S


Pacific Doldrums 004N - 008N 008N - 012N

N.E. Trade 008N - 025N 012N - 030N

S. E. Trade 004N - 030S 008N - 025S


Indian S.E. Trade 015S - 030S 000 - 025S


The ANTI-TRADES are winds, which blow above about 2500 metres in the opposite direction to the trades on the surface.

Climatic Chart for July

The average strength of the Trades is about Force 4, though variations occur between different oceans and at different seasons.

The weather in Trade Wind zones is generally fair with small-detached cumulus clouds.

On the E sides of the oceans cloud amounts and rainfall are small, while on the W sides cloud amounts are larger and rainfall is frequent, being a maximum in the summer months.

Cloud amounts and the frequency and intensity of rain all increase towards the Equatorial Trough.

Poor visibility often occurs at the E end of the Trade Wind zones, due partly to mist or fog forming over the cold currents and partly to sand and dust being carried out to sea by prevailing offshore winds.

At the W end of the zones visibility is good, except when reduced in rain. Fog is rare.

In certain seasons and in certain localities the generally fair weather of the Trades is liable to be interrupted by tropical storms. These are described in detail later.

The Variables

Over the areas covered by the oceanic anticyclones, between the Trade winds and the Westerlies further toward the poles, there exist zones of light and variable winds which are known as The Variables, and the N area is sometimes known as the Horse Latitudes (30- 40). The weather in these zones is generally fair with small amounts of cloud and rain.

The Westerlies

On the polar sides of the oceanic anticyclones lie zones where the wind direction becomes predominantly Wly. Unlike the Trades, these winds known as The Westerlies are far from permanent. The continual passage of depressions from W to E across these zones causes the wind to vary greatly in both direction and strength. Gales are frequent, especially in w inter. The weather changes rapidly and fine weather is seldom prolonged. Gales are so frequent in the S hemisphere that the zone, S of 40S, has been named the Roaring Forties.

In the N hemisphere fog is common in the W parts of the oceans in this zone in summer.


Climatic Chart for January

Areas where fog is likely and those where ice may be encountered are shown on the below in the climatic charts:

The Polar Regions which lie on the polar side of the Westerlies are mainly un-navigable on account of ice. The prevailing wind is generally from an Ely direction and gales are common in winter, though less so than in the zones of the Westerlies. The weather is usually cloudy and fog is frequent in summer.


Over certain parts of the oceans the general distribution of pressure and wind in the zones described above is greatly modified by the seasonal heating and cooling of adjacent large, landmasses.

The annual range of sea temperature in the open ocean is comparatively small, whereas large landmasses become hot in summer and cold in winter.

This alternate heating and cooling of the land results in the formation of areas of low and high-pressure respectively.

This redistribution of pressure results in a seasonal reversal of the prevailing wind over the adjacent oceans.

The most important oceanic areas subject to these seasonal winds are the Indian Ocean, West Pacific Ocean and those adjacent to the coast of West Africa.


The regular alternation of land and sea breezes is a well-known feature of most tropical and sub-tropical coasts and large islands.

These breezes also occur at times in temperate latitudes in fine weather in the summer, though they are here much weaker and less well marked than is the case in lower latitudes.

The cause of these breezes is the unequal heating and cooling of the land and sea. By day the surface of the land rapidly acquires heat from the sun whereas the sea temperature remains virtually unaffected.

The heat of the land is communicated to the air in contact with it, which expands and rises. Air from over the sea flows in to take its place, producing an onshore wind known as a sea breeze.

By night, land rapidly loses heat by radiation and becomes much colder than the adjacent sea. The air over the land is chilled, becomes denser and heavier and flows out to sea under the influence of gravity, producing an offshore wind known as a land breeze.

Sea breezes usually set in late in the forenoon and reach maximum strength, about force 4 (occasionally they reach force 5 or even 6), around 1400.

They die away around sunset.

Land breezes are usually less well-marked and weaker than sea breezes. The effect of these breezes may be to deviate the prevailing wind, reinforce it, neutralize it or even reverse it.

The following factors favour the formation of well-marked land and sea breezes:

(a) A dry desert coast as opposed to forests or swamps.

(b) High ground near the coast.

(c) A weak prevailing wind.

(d) A clear or partly cloudy sky.

A cold current along the coast also has the effect of favouring the establishment of a well-marked sea breeze.

Small islands less than 5 to 10 miles in diameter will not usually produce land and sea breezes.

As the land is heated during the daytime the air over it will be heated by conduction.

This heating causes a decrease in the density of the air, and the pressure falls.

The sea temperature remains more or less the same and the pressure over it is high compared with that over the land. The pressure gradient is sufficient for air to flow from over the sea to the land; this is the sea breeze.

The sea breeze (about force 3 4) sets in during the morning, reaches its maximum strength about 1400 hours and then dies away towards sunset.

After sunset the land cools rapidly and the air above it also cools and its density increases giving rise to an increase in pressure.

The pressure over the sea is now low compared with that over the land. The pressure gradient causes air to flow from the land to the sea; this is the LAND BREEZE.

The land breeze (generally very light compared to sea breeze) sets in shortly after sunset and continues until dawn.


During the northern winter the Asian continent is cooled and an intense high pressure area forms over Eastern Siberia.

The winds circulating round this form the N.E. Monsoon.

In the northern part of the China Sea the pressure gradient is large and winds are likely to be North Westerly force 6 - 7, further south where the pressure gradient is smaller the winds will be northerly force 5 - 6. In the Bay of Bengal and Arabian Sea winds are force 3 - 4.

Rainfall of the showery type is likely on windward coasts especially in the China Sea. As the air is of Polar origin the relative humidity will be low.

The N.E. monsoon becomes established in early December and continues until April.


During the northern summer the Asian continent is warmed and the high pressure over Siberia declines and is replaced by a low pressure over N.W. India.

There is a pressure gradient between the high pressure over the South Indian Ocean and the lows over Africa and India. This causes, the S.E. Trade to blow up to the equator, and on crossing it, the wind near the African coast circulates round the low over Africa and brings rainfall to East Africa.

Away from the coast the wind is deflected to the right and blows from a southwesterly direction.

In the Arabian Sea the wind is S.W. force 7 - 8,

In the Bay of Bengal S.W. winds of force 6 - 7 can be expected, whilst in the southern part of the China Sea, the wind is Sly force 4 - 5.

The northern part of China Sea has light winds.

The air forming the S.W. Monsoon his traveled over thousands of miles of ocean and is saturated.

This results in heavy rain, especially near the coasts, and poor visibility.

The S.W. monsoon sets in during the beginning of June and lasts until the beginning of October.


Katabatic winds are offshore winds caused by the drainage of cold air from high ground under the influence of gravity.

In temperate and high latitudes, where snow-covered mountains back the coast, intense radiation occurs and this causes cold air to accumulate over the high ground. A light offshore wind suffices to start this mass of cold air moving down the seaward facing slopes with gathering momentum reaching the coast without warning, as a strong Or even gale force wind, endangering small craft or ships at anchor. The following are some of the areas where Katabatic winds are common: Greenland, Norway, N Adriatic, E Black Sea and Antarctica.


When the earths surface cools by night, the air next to the surface is also cooled and thus its density is increased. If this cold air is on high ground there is a tendency for it to sink down- to lower ground. If the high ground is a cliff top or a high coastal plateau, the down flowing cold-air will move horizontally when it reaches sea level.

The ANABATIC WIND is less noticeable as it blows up the sides of valleys with considerably less force than the KATABATIC wind. The air at the bottom of the valley is warmed by conduction from the heated land during the day, and this air, being less dense than the air above it, takes the easiest path to the top of the valley by following the warm sides.