Chapter 8 — 2.4 Pressure Belts and Winds

Air is matter; it has mass and is pulled down by gravity, so it has weight. The weight of the column of air resting on a unit area of the earth's surface is the atmospheric pressure at that place. At sea level the column extends up through the whole atmosphere and presses on every square centimetre with a force of about one kilogram; on the palm of a hand the total is more than a hundred kilograms, but we do not feel it because the air presses equally from all sides and from inside our bodies.

Pressure is measured with a barometer. In the mercury barometer, invented by Torricelli in 1643, a glass tube closed at one end and filled with mercury is inverted in a bowl of mercury; the mercury falls until the weight of the column balances the pressure of the air on the bowl, and at sea level the column stands about 76 cm high. When the pressure rises the column rises, and when it falls the column falls. The aneroid barometer, which has no liquid, uses a sealed metal box that flexes with pressure and moves a needle; it is portable and, since pressure falls with height, is used as an altimeter in aircraft and by mountaineers. A barograph records pressure continuously on a chart.

Pressure is expressed in millibars (mb) or, in the newer unit, hectopascals (hPa); the two are equal. The average pressure at sea level is 1,013.25 mb, corresponding to 76 cm (29.92 inches) of mercury. Pressures above this are called high and below it low. The range at sea level is small in absolute terms, from about 1,050 mb in the strongest winter anticyclones of Siberia to about 870 mb in the eye of the most violent cyclones, but these small differences drive all the winds of the world.

On weather maps the distribution of pressure is shown by isobars, lines joining places of equal pressure reduced to sea level, usually drawn at intervals of 2 or 4 mb. Where isobars are close together the pressure changes rapidly over a short distance, and where they are far apart it changes slowly. The rate of change of pressure with distance, at right angles to the isobars, is the pressure gradient; it is the force that sets the air moving, and the steeper the gradient, that is, the closer the isobars, the stronger the wind. Closed isobars with the lowest pressure in the centre mark a low or cyclone; closed isobars with the highest pressure in the centre mark a high or anticyclone.

Pressure at a place changes with the time of day, showing two small maxima at about 10 a.m. and 10 p.m. and minima at about 4 a.m. and 4 p.m. (the atmospheric tide), with the season, and with the passage of weather systems; a falling barometer usually foretells rain and a rising one fair weather, which is why the barometer was for centuries the sailor's weather glass.

Atmospheric pressure varies from place to place and from time to time under the control of four main factors.

Altitude. Pressure decreases with height, because the higher one goes the less air remains above to press down, and because the air is compressed and densest at the bottom. The decrease is rapid near the surface, about 1 mb for every 10 m, and slower higher up; pressure falls by roughly half for every 5.5 km of ascent, from 1,013 mb at sea level to about 500 mb at 5.5 km and 250 mb at 11 km. On Everest the pressure is about a third of that at sea level. Because of this control, all pressures on a weather map are first reduced to sea level, otherwise every hill would appear as a low.

Temperature. When air is heated it expands, its molecules spread out, its density falls, and it becomes lighter; the lighter air rises and the pressure at the surface falls. When air is cooled it contracts, becomes denser and heavier, sinks, and the pressure rises. Hence, in general, hot regions have low pressure and cold regions high pressure: the equator, heated all the year, is a belt of low pressure; the poles, cold all the year, are regions of high pressure; and the interior of Asia, heated in summer, develops the great low that draws in the monsoon, while in winter, chilled, it develops the highest pressure on earth. Temperature and pressure are thus inversely related, and this thermal control is the basis of the pressure belts.

Water vapour. Water vapour is lighter than dry air (the molecular weight of water is 18 against 29 for air), so humid air is lighter than dry air at the same temperature and exerts less pressure. This is why the humid air over the oceans and over Bengal in the monsoon is associated with lower pressure, and why pressure falls as moist air moves in before rain.

Rotation of the earth and dynamic factors. The rotation of the earth deflects the moving air and piles it up in some latitudes and thins it in others. The air rising at the equator and flowing poleward at height is deflected and sinks at about 30° latitude, creating the subtropical highs, which are high not because they are cold but because air is descending; the rising of air where the warm westerlies meet the cold polar easterlies at about 60° creates the subpolar lows, which are low not because they are hot but because air is ascending. Highs and lows that owe their existence to the movement of air rather than to temperature are called dynamically induced, in contrast to the thermally induced equatorial low and polar highs.

The distribution of land and sea modifies all these controls, because land heats and cools faster than water and so develops stronger seasonal highs and lows; the great continental centres of pressure over Asia and North America are the result, and they distort the simple belts described in the next topic.

If the earth were entirely covered by water and were not rotating, its surface pressure would be arranged in simple bands parallel to the equator, following the distribution of temperature. Even with the continents and the rotation, the pattern of pressure averaged over the year shows seven such pressure belts, four of high pressure and three of low, arranged symmetrically about the equator.

1. The equatorial low-pressure belt lies between about 5° N and 5° S. Here the sun is overhead throughout the year, the air is intensely heated, expands and rises in strong convection, and the pressure at the surface is low, about 1,008 to 1,010 mb. Because the air rises rather than blowing horizontally, this belt is a zone of calms, called the doldrums, in which sailing ships were becalmed for weeks; the light, variable winds and the daily thunderstorms of the rising air are its features. It is a thermal low.

2 and 3. The subtropical high-pressure belts lie between about 25° and 35° in each hemisphere, centred on 30°. The air that rose at the equator flows poleward at height, cools, is deflected eastward by the earth's rotation, piles up and descends at about 30°; the descending air is compressed, warmed and dried, and the pressure at the surface is high, about 1,020 mb. These are dynamic highs, and being zones of descending air they are calm and cloudless; most of the world's hot deserts, the Sahara, Arabia, the Thar, the Kalahari, the Atacama and the Australian desert, lie beneath them. Sailing ships crossing them were becalmed, and legend says the Spanish threw their horses overboard to save water, hence the name horse latitudes.

4 and 5. The subpolar low-pressure belts lie between about 60° and 65° in each hemisphere. Here the warm westerly winds from the subtropics meet the cold polar easterlies, and the lighter warm air is forced to rise over the cold; the rising air, helped by the outward fling of the earth's rotation at these latitudes, produces low pressure, about 990 to 1,000 mb. These are dynamic lows, and they are the stormiest belts on earth, the track of the temperate cyclones; the belt is well developed around Antarctica, where sailors named the latitudes the roaring forties, furious fifties and screaming sixties.

6 and 7. The polar high-pressure belts lie over the two poles beyond about 80°. The surface is permanently frozen, the air is intensely cold, dense and heavy, it sinks, and the pressure is high, about 1,020 mb or more. These are thermal highs. From them the cold air spreads outward as the polar easterlies.

The belts alternate: low at the equator, high at 30°, low at 60°, high at the poles, and the highs and lows are alternately thermal and dynamic in origin. Air flows at the surface from each high to the neighbouring lows, and this flow, deflected by the rotation of the earth, forms the planetary winds. The belts should be drawn on a circle representing the earth, with the latitudes marked and the letters H and L, and the causes written beside each.

The pressure belts are not fixed in position. Because the earth's axis is tilted at 23½°, the sun is overhead at the Tropic of Cancer on 21 June and at the Tropic of Capricorn on 22 December, and the belt of maximum heating, and with it the whole system of pressure belts, shifts north and south with the seasons. In the northern summer the equatorial low moves north to about 10° to 15° N, and the other belts follow; in the northern winter the whole system moves south. The shift is about 5° to 10° of latitude in each direction, and it is larger over the continents, which heat and cool strongly, than over the oceans.

The shifting has great consequences for climate, because a place near the boundary between two belts comes under one belt in one season and the other in the next. The regions between 30° and 40° on the western sides of the continents, the Mediterranean lands, California, central Chile, the Cape and south-western Australia, lie under the subtropical high in summer and are dry and sunny, but come under the rain-bearing westerlies in winter when the belts move equatorward; this gives them the Mediterranean climate of dry summers and wet winters. The savanna lands between 5° and 15° come under the equatorial low with its rains in summer and under the dry trade winds in winter.

The shift is most dramatic in South Asia. In June the equatorial low, drawn by the intense heating of the Indian and Tibetan land mass, moves far north to lie over the Punjab, Rajasthan and the Ganga plain at about 25° to 30° N. The south-east trade winds of the southern hemisphere are drawn across the equator towards this low, are deflected to the right on entering the northern hemisphere, and arrive over India as the south-west monsoon. In winter the low retreats south of the equator, high pressure forms over the cold interior of Asia, and the winds reverse. This seasonal reversal, the monsoon, is essentially the shifting of the pressure belts magnified by the great land mass of Asia, and it is treated in a later topic.

The belts are also broken and distorted by the continents. In January, high pressure develops over the cold interiors of Asia (the Siberian high, over 1,040 mb) and North America, and the subpolar lows are concentrated as the Icelandic low and the Aleutian low over the warmer oceans. In July, the continents develop deep lows (the Asian low centred on Baluchistan and the Punjab, below 995 mb) and the subtropical highs strengthen over the oceans as the Azores high and the Hawaiian high. In the southern hemisphere, which is mostly ocean, the belts remain nearly continuous and regular throughout the year.

For the examination the student should be able to state that the belts shift with the apparent movement of the sun, north in the northern summer and south in the northern winter, by 5° to 10°, and to give the Mediterranean climate and the Indian monsoon as its results.

Wind is the horizontal movement of air over the earth's surface, from a region of high pressure to a region of low pressure. Vertical movements of air are called currents, not winds. A wind is always named after the direction from which it blows: a westerly blows from the west towards the east, a north-easterly from the north-east. Wind speed is measured with an anemometer in kilometres per hour or knots, and wind direction with a wind vane.

The immediate cause of wind is the pressure gradient force. Wherever pressure differs between two places, the air is pushed from the higher to the lower pressure, at right angles to the isobars, and the steeper the gradient the stronger the push. If the earth did not rotate, every wind would blow straight down the gradient, from high to low.

But the earth rotates, and every body moving freely over its surface is deflected from its straight path. This apparent deflecting force, due to the rotation of the earth, is the Coriolis force, named after the French mathematician Gaspard-Gustave de Coriolis who described it in 1835. Its effect is expressed in Ferrel's law, stated by the American meteorologist William Ferrel in 1856: in the northern hemisphere a moving body is deflected to the right of its path, and in the southern hemisphere to the left. The deflection is zero at the equator and greatest at the poles, and it increases with the speed of the moving body. Rivers, ocean currents, artillery shells and aircraft are all affected, but its greatest effect is on the winds.

The Coriolis force arises because the earth turns from west to east and the speed of its surface varies with latitude: about 1,670 km per hour at the equator, 835 km per hour at 60°, and zero at the poles. Air moving poleward from the equator carries the equator's high eastward speed with it, arrives over ground that is moving eastward more slowly, and so runs ahead of the ground towards the east, that is, it is deflected to the right in the northern hemisphere; air moving equatorward lags behind the faster-moving ground and is deflected to the west, again to the right. In the southern hemisphere the same reasoning gives deflection to the left.

The result is that winds do not blow straight from high to low pressure but at an angle to the isobars, and, in the case of the planetary winds, in a slanting direction across the pressure belts: the winds that would blow from north to south in the northern hemisphere blow from the north-east, and those that would blow from south to north blow from the south-west. High in the atmosphere, where friction is absent, the deflection is complete and the wind blows parallel to the isobars; this is the geostrophic wind. Near the ground, friction with the surface reduces the deflection so that surface winds cross the isobars at about 10° to 30°.

The Coriolis force also governs the circulation of air round centres of high and low pressure: air spirals into a low anticlockwise in the northern hemisphere and clockwise in the southern, and out of a high clockwise in the northern and anticlockwise in the southern. This is the rule of Buys Ballot: stand with your back to the wind in the northern hemisphere, and low pressure lies on your left.

The winds that blow throughout the year from one pressure belt to the next over large parts of the earth, in nearly the same direction, are called planetary winds, permanent winds or prevailing winds. There are three systems in each hemisphere: the trade winds, the westerlies and the polar easterlies. Together they form the general circulation of the atmosphere, which carries the surplus heat of the tropics towards the poles.

The trade winds blow from the subtropical high-pressure belts at about 30° towards the equatorial low. If the earth did not rotate they would blow due south in the northern hemisphere and due north in the southern; deflected by the Coriolis force, they blow from the north-east in the northern hemisphere and from the south-east in the southern, so that they are called the north-east trades and the south-east trades. They meet near the equator in the doldrums, along a line called the Inter-Tropical Convergence Zone (ITCZ), where the air rises. The name trade comes from an old English word meaning track or course, because these winds blow steadily along the same path; they carried the sailing ships of the trade routes from Europe to the Americas and from India to Africa, and Columbus rode the north-east trades to the Caribbean.

The trade winds are the most regular and constant winds on earth. They blow at a steady 15 to 25 km per hour, day and night, throughout the year, over the tropical oceans, and their direction seldom varies. Because they blow from cooler latitudes towards the hot equator, they are warming as they travel and so their capacity to hold moisture increases; they therefore bring little rain over the continents and are drying winds in the interior, which is why the great deserts lie in the trade-wind belt on the western sides of the continents. But as they cross the oceans they pick up moisture, and where they strike the eastern coasts of the continents and are forced to rise over mountains they bring heavy rain: the east coasts of Brazil, Central America, Madagascar, Queensland and the Caribbean islands are wet trade-wind coasts.

The trade winds are best developed over the oceans, especially the Atlantic and Pacific. Over the Indian Ocean north of the equator they are replaced in summer by the south-west monsoon, and only in winter does the north-east trade appear over India as the dry north-east monsoon that brings winter rain to Tamil Nadu.

The trade winds are the surface branch of a great vertical circulation called the Hadley cell, named after George Hadley who explained it in 1735: air rises at the equator, flows poleward at height, sinks at 30° and returns to the equator as the trades. The Hadley cell is the engine of the tropical climate.

The westerlies blow from the subtropical high-pressure belts at about 30° towards the subpolar lows at about 60°. Deflected by the Coriolis force, they blow from the south-west in the northern hemisphere and from the north-west in the southern; the name westerlies records that they come from a westerly direction. They are also called the anti-trades, since they blow in the opposite sense to the trade winds from the same highs.

The westerlies are far less regular than the trades. Their belt, the middle latitudes between 35° and 65°, is crossed by a constant procession of travelling cyclones and anticyclones, which swing the wind round the compass from day to day, so that the westerly direction is only the average; the weather of the belt is changeable, with alternating rain and fair spells, and it is the belt of the temperate cyclones and of the greatest day-to-day variation of temperature. The westerlies blow from warmer towards cooler latitudes, so they are cooling as they travel and readily give up their moisture as rain; they bring rain to the western coasts of the continents in all seasons, giving the mild, wet climate of Britain, western Europe, Oregon and British Columbia, New Zealand, Tasmania and southern Chile. The west coasts are wet and the east coasts of the middle latitudes comparatively dry.

In the northern hemisphere the westerlies are broken up by the large land masses and are strongest in winter. In the southern hemisphere between 40° and 60° S there is almost no land to obstruct them, and they blow around the whole earth with tremendous force and constancy over the Southern Ocean; sailors called them the roaring forties, the furious fifties and the screaming sixties, and the clipper ships used them to race from the Cape to Australia. Above the surface westerlies, near the tropopause, blow the jet streams, described later.

The polar easterlies blow from the polar high-pressure areas towards the subpolar lows at about 60°. Deflected by the Coriolis force, which is strongest at the poles, they blow from the north-east in the northern hemisphere and from the south-east in the southern. They are extremely cold and dry winds, since they come from the frozen polar surface, and they are irregular and weak in the northern hemisphere, where the Arctic Ocean and the continents break them up, but strong and persistent around Antarctica, where the cold dense air pours off the ice cap. Where the polar easterlies meet the warm westerlies at about 60°, along the polar front, the warm air is forced to rise over the cold, and the temperate cyclones of the subpolar low are born; this front is the birthplace of the storms that sweep across Europe and North America in winter.

The three wind systems in each hemisphere, the trades, the westerlies and the polar easterlies, together with the calms of the doldrums and the horse latitudes, form the complete pattern of planetary winds, which the student must be able to draw on the diagram of the pressure belts, with the deflection shown correctly in both hemispheres.

Winds that reverse their direction with the season or with the time of day are called periodic winds. The greatest of them is the monsoon, from the Arabic mausim, a season, the name given by Arab sailors to the winds of the Indian Ocean that blew from the south-west for half the year and from the north-east for the other half, carrying their dhows to India and back.

The monsoon is best understood as a land and sea breeze on a continental scale, combined with the seasonal shifting of the pressure belts. Land heats and cools faster than water. In summer the vast land mass of Asia, and especially the Indian subcontinent and the Tibetan plateau, is intensely heated; the air over it expands and rises, and a deep low-pressure centre, below 995 mb, forms over the Punjab, Rajasthan and Baluchistan by June, while the Indian Ocean, cooler, remains under relatively high pressure (the Mascarene high near Madagascar, about 1,020 mb). Air flows from the ocean towards the land. At the same time the equatorial low and the ITCZ have shifted north over India, and the south-east trade winds of the southern hemisphere are drawn across the equator; on crossing it they are deflected to the right by the Coriolis force and become south-west winds. This is the south-west monsoon or summer monsoon, which blows from June to September. Having crossed thousands of kilometres of warm ocean, it is laden with moisture, and it gives India about three-quarters of its annual rain. It arrives in two branches: the Arabian Sea branch, which strikes the Western Ghats and drenches the west coast, and the Bay of Bengal branch, which sweeps up the Bay, strikes the hills of Meghalaya and the Himalaya, gives Mawsynram the heaviest rain on earth, and turns west up the Ganga plain, reaching Kolkata in early June and the Punjab by July. West Bengal receives most of its 1,500 to 3,000 mm of rain from this branch.

In winter the process reverses. The land cools rapidly, a high-pressure centre forms over the cold interior of Asia and north-west India, and the ocean, warmer, has lower pressure; the equatorial low has retreated south of the equator. Air flows from the land to the sea as the north-east monsoon or winter monsoon, from December to February. Coming from the interior of the continent, it is cold and dry, and most of India has clear, fine, rainless weather; only where it crosses the Bay of Bengal and strikes the Coromandel coast does it bring rain, giving Tamil Nadu its main rainy season in October to December.

Between the two, in October and November, the summer monsoon retreats and the north-east monsoon sets in; this transition, the retreating monsoon, is the season of the cyclones of the Bay of Bengal, which strike Odisha, Bengal and Bangladesh with great violence. The hot months of March to May before the monsoon bring the nor'westers or kalbaishakhi to Bengal, violent afternoon thunderstorms born of the convection over the heated plateau to the west.

Monsoons also occur, less strongly, in East Asia, northern Australia and West Africa, but the Indian monsoon is the greatest, and the whole agriculture, economy and calendar of India and Bengal depend on its arrival, its strength and its retreat. Its year-to-year variations are influenced by El Niño and the jet streams, described in the last topics.

Two smaller periodic winds reverse their direction not with the season but with the time of day. They are local in extent but are felt by millions of people every day, and they are regular examination items with diagrams.

Sea breeze and land breeze. On a coast, the land and the sea are heated unequally by the sun. By day the land heats quickly, the air over it expands and rises, and a local low pressure forms over the land, while the sea, heating slowly, remains cooler and under higher pressure. Air flows from the sea to the land as the sea breeze, a cool, moist wind that sets in during the late morning, is strongest in the afternoon, reaches 20 to 30 km inland, and dies away at sunset. It moderates the heat of coastal towns, which is why Mumbai, Chennai and Digha are more bearable in summer than places a hundred kilometres inland, and why Kolkata, 130 km from the sea, feels its relief only weakly. At night the land cools quickly by radiation and becomes cooler than the sea, which retains its heat; the pressure over the land is now higher and over the sea lower, and the air flows from the land to the sea as the land breeze, a gentle, dry wind that blows from late night to early morning. Fishermen on the Bengal and Odisha coasts have always used the land breeze to sail out before dawn and the sea breeze to return in the afternoon.

Valley breeze and mountain breeze. In hilly country a similar reversal occurs between the slopes and the valley. By day the sun heats the mountain slopes strongly, the air in contact with them warms, expands and rises up the slopes, and cooler air from the valley floor moves up to replace it. This upslope wind is the valley breeze or anabatic wind. As the rising air cools, its vapour condenses into the cumulus clouds that gather over the ridges of Darjeeling every afternoon and often give afternoon showers on the hilltops. At night the slopes cool rapidly by radiation, the air on them becomes cold and heavy and slides downhill under gravity into the valley, where it collects as a pool of cold air; this downslope wind is the mountain breeze or katabatic wind. It produces the night-time temperature inversion of the valleys, with frost and fog on the valley floor while the slopes are milder, which is why the hill settlements, orchards and tea gardens are placed on the slopes and the valley bottoms are avoided. In the Himalaya the cold air draining down the great valleys at night is felt as a chilly wind at their mouths in the plains, as at Siliguri at the foot of the Teesta valley.

All four breezes are simple convection systems, driven by the unequal heating and cooling of two neighbouring surfaces, and each diagram should show the warmer surface with rising air and low pressure, the cooler surface with sinking air and high pressure, and the surface wind blowing from the high to the low.

Local winds blow over a small area, for a short period, under local conditions of temperature, pressure and relief, and each has a local name. They are of two kinds, hot and cold, and the examination asks for their names, places and characteristics; a table is the best way to learn them.

Hot local winds

Cold local winds

Two Indian winds beyond the loo should be known. The kalbaishakhi or nor'wester of Bengal, Assam and Odisha is a violent thunderstorm squall from the north-west on afternoons of April and May, born of convection over the heated Chotanagpur plateau; it brings hail and 100 km/h gusts, damages crops and houses, but relieves the heat and helps the jute and tea crops, and its name means the calamity of the month of Baisakh. The mango showers of Kerala and Karnataka and the cherry blossom showers of the Karnataka coffee belt are pre-monsoon thundershowers of April and May. The Andhi of Rajasthan and the Punjab is the dust storm of the loo season.

The foehn and chinook illustrate an important principle: air forced to rise over a mountain cools at the slower saturated rate while it drops its rain on the windward side, then descends the lee side warming at the faster dry rate, so it arrives at the foot warmer and drier than it started. The same effect gives the rain shadow of the Deccan behind the Western Ghats and of Ladakh behind the Himalaya.

High in the troposphere, just below the tropopause at heights of 9 to 14 km, blow narrow bands of extremely fast westerly wind called jet streams. They were discovered during the Second World War, when American bombers flying to Japan at 10 km found themselves facing headwinds so strong that they made no progress over the ground. A jet stream is typically a few hundred kilometres wide and a few kilometres thick, thousands of kilometres long, and its core wind reaches 150 to 300 km per hour, occasionally over 400. It flows from west to east in both hemispheres in a meandering, wave-like path, the waves being called Rossby waves.

Jet streams arise where the temperature difference between neighbouring air masses is greatest, because a sharp horizontal temperature difference produces a sharp pressure difference at height, and the Coriolis force turns the resulting flow into a strong westerly. There are two main jets in each hemisphere. The polar front jet or polar jet blows at about 9 to 12 km above the polar front, where the cold polar air meets the warm westerlies at 40° to 60°; it is strongest in winter, when the temperature contrast is greatest, and its meanders steer the temperate cyclones beneath it, so that it governs the weather of Europe, North America and East Asia. The subtropical jet blows at about 12 to 14 km above the subtropical highs at 25° to 30°, at the poleward edge of the Hadley cell, and is more constant in position. Both shift towards the equator in winter and towards the poles in summer with the pressure belts.

The jet streams matter to India through the monsoon. In winter the subtropical westerly jet lies across northern India at about 25° N, south of the Himalaya, at 12 km; the descending air beneath it keeps the winter dry, and the disturbances it brings from the Mediterranean, the western disturbances, give the winter rain and snow of the Punjab and Kashmir. In late May and June the intense heating of the Tibetan plateau forces this jet to shift suddenly north of the Himalaya, and only when it has done so can the low over north-west India deepen fully and the south-west monsoon burst over India. At the same time a tropical easterly jet develops at about 15 km over peninsular India and the Indian Ocean, blowing from east to west, and its strength is linked to the strength of the monsoon rains. When the westerly jet is slow to move north, the monsoon is delayed; when it returns south in September or October, the monsoon withdraws. The jet streams are thus the switch that turns the Indian monsoon on and off.

Jet streams are of great practical use. Aircraft flying eastward, from India to Japan or from America to Europe, ride the jet and save an hour or more and much fuel; westbound flights avoid it. Weather forecasters track the jet to predict the movement of cyclones and anticyclones several days ahead. Clear-air turbulence at the edges of jets is a hazard to aircraft. The jets also spread volcanic ash and pollutants round the world within days.

Besides the belts and the planetary winds, the pressure map shows travelling centres of low and high pressure, each with its own circulation of wind. A centre of low pressure surrounded by closed isobars, into which winds blow spirally, is a cyclone; a centre of high pressure from which winds blow spirally outward is an anticyclone. Under Ferrel's law the wind spirals into a cyclone anticlockwise in the northern hemisphere and clockwise in the southern, and out of an anticyclone clockwise in the northern hemisphere and anticlockwise in the southern.

In a cyclone the air converges towards the centre and rises; the rising air cools, its vapour condenses, and thick cloud and heavy rain result. Cyclones are therefore associated with bad weather, and because the pressure gradient is steep the winds are strong. There are two families. Tropical cyclones, called hurricanes in the Atlantic, typhoons in the western Pacific and simply cyclones in the Indian Ocean, form over warm seas of at least 27 °C between 5° and 20° latitude, chiefly in late summer and autumn. They are compact, a few hundred kilometres across, with a calm, clear eye at the centre surrounded by a wall of towering cloud, extremely low central pressure, winds of 120 to 250 km per hour and torrential rain, and they raise a storm surge of sea water that drowns low coasts. The Bay of Bengal breeds the deadliest cyclones on earth, because its shallow funnel shape magnifies the surge and its coasts are low and crowded: the Bhola cyclone of 1970 killed some 300,000 people in Bangladesh, the Odisha super-cyclone of 1999 about 10,000, and Aila (2009), Amphan (2020) and Yaas (2021) devastated the Sundarbans and southern Bengal. They occur mainly in April–May and October–December, before and after the monsoon. Temperate cyclones or depressions form along the polar front in the middle latitudes, are much larger, 1,000 to 3,000 km across, but milder, with fronts of warm and cold air, and bring days of cloud, rain and changing wind to Europe and North America; the western disturbances of the Indian winter are weak temperate cyclones from the Mediterranean.

In an anticyclone the air descends in the centre and spreads outward at the surface; descending air is warmed by compression and dried, so the sky is clear and the weather calm and fine, with light winds because the pressure gradient is gentle. Anticyclones bring hot dry sunny spells in summer, and in winter clear cold nights with frost, fog and, over cities, smog under the inversion that the sinking air creates. The great winter anticyclone over Siberia and north-west India brings India its fine, dry, cool winter weather, and the subtropical highs are permanent anticyclones. Anticyclones move slowly and may sit over a region for days or weeks, giving droughts and heat waves.

The contrast is simple to remember: cyclone, low, converging, rising, cloudy, rainy, windy; anticyclone, high, diverging, sinking, clear, dry, calm.

The pressure and wind systems of the earth are linked across oceans, and a change in one part of the world can alter the weather of another thousands of kilometres away. The most important example for India is the El Niño of the Pacific Ocean.

In a normal year the south-east trade winds blow steadily across the tropical Pacific from the coast of South America towards Indonesia and Australia. They push the warm surface water westward, so that the sea is warm, the pressure low and the rain heavy in the western Pacific, while off Peru cold, nutrient-rich water wells up from the depths, the sea is cool, the pressure high and the coast dry; the Peruvian anchovy fishery is the richest in the world. This normal pattern is part of the Walker circulation.

Every two to seven years, for reasons not fully understood, the trade winds weaken or reverse. The warm water that had piled up in the west flows back eastward, the upwelling off Peru stops, and the sea off South America becomes several degrees warmer than usual. Peruvian fishermen, who noticed the warm current arriving around Christmas, named it El Niño, the Christ child. The warming shifts the zone of rising air and heavy rain from the western to the central and eastern Pacific: Peru and Ecuador suffer floods, while Indonesia, Australia and often India suffer drought. The reversal of pressure between the eastern and western Pacific that accompanies El Niño is called the Southern Oscillation, and the whole phenomenon is ENSO (El Niño–Southern Oscillation). An El Niño event lasts about a year and warms the whole globe slightly; 1997–98, 2015–16 and 2023–24 were strong events.

La Niña, the girl, is the opposite phase, when the trade winds are unusually strong, the eastern Pacific unusually cold and the western Pacific unusually warm and wet. La Niña years tend to bring good monsoons to India but floods in eastern Australia and Indonesia.

The link with the Indian monsoon is statistical but strong. In El Niño years the shifting of rising air to the central Pacific weakens the pressure gradient that drives the south-west monsoon, the monsoon tends to arrive late, break early and yield below-normal rain, and India has experienced many of its worst droughts, in 1972, 1987, 2002, 2009 and 2015, in El Niño years. La Niña years, by contrast, tend to be years of good or excess rain. The Indian Meteorological Department watches the Pacific closely each spring for the state of ENSO when it issues its monsoon forecast, together with the Indian Ocean Dipole, a similar see-saw of sea temperature between the western and eastern Indian Ocean whose positive phase helps the monsoon and can offset El Niño. The forecast matters to West Bengal's rice, jute and potato crops and to the water levels of the Damodar and Teesta reservoirs.

El Niño illustrates the central lesson of this section: the pressure belts, winds and ocean currents of the earth form one connected system, and the weather of Bengal is decided as much by the trade winds of the Pacific and the jet streams over Tibet as by the local heating of the Ganga plain.

Pressure belts and winds are the most examined part of the atmosphere unit, and the diagram of the pressure belts with the planetary winds is asked almost every year.

One-mark questions ask: the instrument for pressure (barometer); the unit (millibar or hectopascal); normal sea-level pressure (1,013 mb); the line of equal pressure (isobar); the belt of calms at the equator (doldrums); the calms at 30° (horse latitudes); the number of pressure belts (seven); the wind blowing from the subtropical high to the equatorial low (trade wind); from the subtropical high to the subpolar low (westerlies); the law of deflection (Ferrel's law); the direction of deflection in the northern hemisphere (right); the hot wind of northern India (loo); the snow-eater of the Rockies (chinook) and Alps (foehn); the cold wind of the Rhone valley (mistral); the wind that reverses seasonally (monsoon); the wind that reverses daily on the coast (land and sea breeze); the very fast upper-air wind (jet stream); the warm current off Peru (El Niño); the direction of wind round a northern-hemisphere cyclone (anticlockwise); the Bengal thunderstorm of spring (kalbaishakhi). Match-the-column items pair local winds with countries and pressure belts with their latitudes.

Two-mark questions ask for definitions: atmospheric pressure, isobar, pressure gradient, wind, doldrums, horse latitudes, Ferrel's law, monsoon, jet stream, El Niño, cyclone, anticyclone, sea breeze, foehn.

Three-mark questions ask for causes and differences: Why is pressure low at the equator and high at 30°? Why do the trade winds blow from the north-east? Why do the pressure belts shift? How does a sea breeze form? Distinguish between cyclone and anticyclone, between trade winds and westerlies, between land and sea breeze, between monsoon and trade wind. What is the effect of El Niño on the Indian monsoon? Explain the role of the jet stream in the monsoon.

Five-mark questions ask: Describe with a diagram the pressure belts of the earth. Describe with a diagram the planetary winds. Explain the origin of the Indian monsoon. Describe the local winds of the world. Explain the formation and weather of a tropical cyclone.

Guidance for the diagram: draw a large circle, mark the equator, the tropics, 30°, 60° and the poles on both sides; write L at 0°, H at 30°, L at 60° and H at 90° in each hemisphere with the name of each belt; then draw the wind arrows, always slanting: north-east trades and south-west westerlies and north-east polar easterlies in the northern hemisphere, south-east trades, north-west westerlies and south-east polar easterlies in the southern; label the doldrums and the horse latitudes. Check that every arrow in the northern hemisphere bends to the right of the straight north–south line and every arrow in the southern hemisphere to the left. For the monsoon, draw the two seasonal maps and explain the reversal in terms of land–sea heating, the shift of the belts and the Coriolis deflection of the trades on crossing the equator. For local winds, give the name, place, season and one effect in a single line each. Use figures wherever known: 1,013 mb, 15–25 km/h for the trades, 150–300 km/h for jets, 27 °C for cyclone formation, 1 June for the monsoon onset over Kerala. Avoid the common errors of drawing straight north–south winds, deflecting winds the wrong way, naming winds by the direction they blow towards, placing the horse latitudes at the equator, and calling the monsoon a planetary wind.