Atmosphere, Pressure Belts and Winds

The atmosphere is the blanket of gases held around the Earth by gravity. It gives us air to breathe, protects us from harmful ultraviolet rays and meteors, keeps the planet warm enough for life, and is the stage on which all weather happens. About 99 per cent of the air lies within the lowest 32 km, so the atmosphere is very shallow compared with the size of the Earth. It has no sharp upper edge; it thins out gradually into space. For convenience, the Kármán line at about 100 km is taken as the boundary between the atmosphere and space.

Why is the troposphere thicker at the equator? The hot equator drives strong convection currents that carry heat and moisture to a greater height, so the layer is about 18 km deep there, while at the cold poles the air barely rises and the layer is only about 8 km. Why is it the densest layer? Because it is squeezed by the weight of all the air above it. That is also why it is harder to breathe on a mountain: the air is less dense, not because oxygen has a smaller share.

The ionosphere is not a separate layer but an electrically charged region (about 80 to 400 km) lying mostly inside the thermosphere. Ultraviolet and X-rays from the Sun knock electrons off gas molecules there, making ions and free electrons. These charged particles reflect radio waves back to Earth, which makes long-distance radio communication possible. The ozone layer (roughly 15 to 35 km, in the stratosphere) absorbs most of the Sun's harmful ultraviolet radiation. It is thinned by chlorofluorocarbons (CFCs) used in refrigerators and air conditioners (also called Freon), which the Montreal Protocol (1987) phased out. The chapter on environment takes this further.

The Sun is the engine of the weather. Its energy reaches Earth as short-wave radiation, called insolation (INcoming SOLar radiATION). Only a tiny fraction of the Sun's output reaches Earth, but it is enough to heat the planet, run the water cycle and drive the winds. The amount of insolation is not the same everywhere. It is highest at the equator and lowest at the poles, and this unequal heating is the root cause of pressure belts, winds, ocean currents and climates.

The Earth is a sphere tilted on its axis, so the angle at which the Sun's rays strike the surface changes with latitude and season. At the equator the rays are almost vertical, so the same energy falls on a small area and heats strongly. Near the poles the rays are slanting, so the same energy is spread over a large area and also has to pass through a thicker slice of atmosphere. That is the reason why regions on the same Earth, under the same Sun, have different climates.

Where does all the incoming energy go? Take the incoming insolation as 100 units. About 35 units are reflected straight back to space (by clouds, by snow and ice and by scattering in the air). This reflected share is the Earth's albedo. About 14 units are absorbed by the atmosphere and about 51 units are absorbed by the Earth's surface. The warmed surface then sends out heat as long-wave terrestrial radiation. Of this, about 17 units escape directly to space, while 34 units are absorbed by the atmosphere (through radiation, convection and the latent heat of condensation). The atmosphere finally radiates 48 units (14 + 34) to space. Add up: 35 + 17 + 48 = 100. Income equals expenditure, so the Earth's average temperature stays steady. This balance is the heat budget.

The atmosphere is heated from below, not directly from above. Most of the sunlight passes through clear air without heating it. The ground absorbs it, warms up and then heats the air touching it by long-wave radiation. That is why air temperature falls as we go up, and why it is cold on a high mountain even though it is nearer to the Sun. The greenhouse gases (water vapour, carbon dioxide, methane) trap part of this outgoing long-wave heat and keep Earth warm; this is the natural greenhouse effect.

Temperature distribution. Temperature is shown on maps by isotherms, lines joining places of equal temperature (reduced to sea level). Isotherms run roughly east-west, because temperature falls from the equator to the poles. They bend where land meets sea: land heats and cools faster than water, so in January isotherms bend poleward over the oceans and equatorward over the continents. Temperature also falls with height at the normal lapse rate of about 6.5 degrees Celsius per 1,000 m (the textbooks also state about 1 degree per 165 m).

Temperature inversion means the normal fall of temperature with height is reversed: a layer of warm air lies above cold air near the ground. The ideal conditions are a long winter night, clear skies, calm air, and dry air or snow cover. On such a night the ground loses heat quickly by radiation and chills the air touching it, while the air above stays warmer. Such an inversion is called a surface (radiation) inversion. In mountain valleys, cold heavy air drains down the slopes at night and settles on the valley floor, so the valley is colder than the slopes above; this is why orchards and villages in the Himalaya are often on the slopes.

Why does inversion matter? It acts like a lid. Cold, dense air trapped near the ground cannot rise, so fog, smog and pollutants are trapped in it. North Indian winter fog and Delhi's winter smog are made worse by inversion. It also causes frost, which is dangerous for standing crops.

Air has weight. The column of air standing above any spot presses down on it, and this pressure is called atmospheric pressure. At sea level it is about 1013.25 millibars (mb), which is the same as 1013.25 hectopascals (hPa) or 1 atmosphere. It is measured with a barometer: the mercury barometer was invented by the Italian scientist Torricelli in 1643, and the portable aneroid barometer has no liquid in it. Why should you care? Because wind is nothing but air moving from a place of higher pressure to a place of lower pressure, so every wind question begins with pressure.

Pressure falls with height, and it falls fastest near the ground. Air is squeezed by the weight above it, so the lowest layers are the densest. Roughly half of all the air in the atmosphere lies below about 5.5 km. Near the surface, pressure drops by about 1 mb for every 8 to 10 metres you climb. This is why mountaineers feel breathless: the air is not poorer in oxygen, there is simply less of it in every breath.

On weather maps, places with equal pressure are joined by lines called isobars. Where isobars are crowded together, pressure changes quickly over a short distance (a steep pressure gradient) and the wind is strong; where they are wide apart, the wind is gentle. Four things decide the pressure of a place. Temperature: warm air expands and rises, so heating lowers pressure. Altitude: higher places have lower pressure. Water vapour: moist air is actually lighter than dry air (a water molecule weighs 18 units, nitrogen 28 and oxygen 32), so humid air has lower pressure. Earth's rotation and the movement of air: sinking and sideways-moving air can raise or lower pressure without any heating.

The ITCZ (Intertropical Convergence Zone) is the line where the trade winds of the two hemispheres meet, and it is the same region as the Doldrums. Air there rises, so the sky is cloudy and rainy, yet the surface winds are weak. Old sailing ships could get stuck for days in this calm, hence the name 'Doldrums'. The ITCZ follows the overhead Sun, moving north in June and south in December. In July it reaches the Gangetic plain, where it is called the monsoon trough, and this northward pull is a key reason why the south-west monsoon enters India.

Seasonal shift of the belts. Because the Sun's vertical rays move between the Tropic of Cancer and the Tropic of Capricorn, all the belts shift north in the northern summer (June) and south in the northern winter (December), by roughly 5 to 10 degrees of latitude. This small shift has big results. The Mediterranean lands lie at the edge of the subtropical high, so in summer they are dry under the high and in winter, when the belts move equatorward, they get rain from the westerlies. That is exactly the Mediterranean climate: dry summer, wet winter.

Wind is air moving horizontally across the Earth's surface. Three forces decide where it goes. The pressure gradient force pushes air from high to low pressure, at right angles to the isobars. The Coriolis force bends the moving air sideways. Friction with the ground slows the air, mainly in the lowest kilometre or two. A wind is named after the direction it blows from: a westerly comes from the west, a north-easterly from the north-east.

The Coriolis effect. The Earth spins from west to east. A wind that travels over this spinning globe does not move in a straight line as seen from the ground; it is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The force is zero at the equator, grows with latitude and is maximum at the poles; it also grows with wind speed. It changes only the direction of the wind, never its speed, and it is named after the French scientist Gaspard-Gustave de Coriolis. This is why there is no cyclone right on the equator: the swirl needs Coriolis, and cyclones need a minimum of about 5 degrees of latitude.

Geostrophic wind. High above the ground, where friction is absent, the pressure gradient force and the Coriolis force balance each other. The wind then stops crossing the isobars and blows parallel to them. This is why upper-air winds and jet streams run along the isobars, while surface winds, slowed by friction, cross the isobars at an angle and drift towards low pressure.

Now put the pressure belts and the Coriolis effect together and you get the planetary (prevailing) winds, which blow all year in the same general direction. In each hemisphere the air circulates in three huge loops called cells. The Hadley cell (equator to about 30 degrees) is driven by heating: air rises at the equator, sinks at the subtropical highs and returns along the surface as the trade winds. The polar cell (60 to 90 degrees) is driven by cold: air sinks at the pole and flows out as the polar easterlies. Between them sits the Ferrel cell (30 to 60 degrees), which is not driven by heat directly but is dragged along by its two neighbours, and its surface wind is the westerlies.

Why are the Southern-Hemisphere westerlies so fierce? In the south, between 40 and 60 degrees, there is almost only open ocean and no land to slow the wind with friction. Sailors gave these winds famous names by latitude: the Roaring Forties (about 40 to 50 degrees S), the Furious Fifties (50 to 60 degrees S) and the Screaming Sixties (beyond 60 degrees S). The trade winds got their name from an old word for 'track' or 'course', because they blow steadily along the same path, and they carried merchant ships across the oceans.

Why are hot deserts on the west side of continents? In the tropics the trade winds blow from the east. They pick up moisture over the ocean and drop it on the eastern coasts (eastern Brazil, Madagascar, Queensland, the Philippines). By the time they reach the western edge of the land they are dry, and the sinking air of the subtropical high adds to the dryness. That is why the great hot deserts sit on western sides: the Sahara, the Atacama, the Namib, the Kalahari and the Australian deserts. In the temperate belt it is the opposite: the westerlies bring heavy rain to the western coasts (British Isles, Norway, British Columbia, southern Chile, New Zealand).

Planetary winds and India. India lies in the trade-wind belt, so in the cold season the north-east trade winds blow over the country from land to sea. Having crossed dry land they carry little moisture, which is why most of India has a dry winter. The winds of the summer monsoon are different: they are the south-east trades of the southern hemisphere, pulled across the equator and bent to the right by the Coriolis effect, so they arrive as the south-west monsoon. On the Tamil Nadu coast, the north-easterlies that pick up moisture over the Bay of Bengal in October and November give the 'north-east monsoon' rainfall.

Planetary winds are constant, but many winds are shaped by local heating and cooling. Periodic winds change their direction with the time of day or the season. The big ones are the monsoons (seasonal), the land and sea breezes (daily) and the mountain and valley breezes (daily). Local winds are winds of a particular small region, and each one has a name, a place and a character (hot or cold, dry or wet). Exams love these names, so we will learn them as a table, with one idea behind every row.

The monsoon (from the Arabic word mausim, meaning 'season') is a wind that reverses its direction with the season. In summer the land is hotter than the sea, so a low forms over the land and moist winds blow in from the sea; in winter the land is colder, a high forms over it and dry winds blow out to the sea. The monsoon is best developed in South and South-East Asia, and weaker versions occur in northern Australia, East Africa and West Africa. The full mechanism for India is taught in the chapter on India's climate.

The Foehn effect (why Chinook, Foehn and Zonda are warm and dry). Moist air is forced up the windward slope of a mountain. As it rises it cools, its vapour condenses and it drops rain or snow, and the condensation gives back some heat. After the crest, the now-dry air sinks down the leeward slope and warms at the faster dry rate (about 10 degrees Celsius per km). So at the same height the leeward side is warmer and drier than the windward side. That is how a wind from the cold mountains can melt snow in hours, and it also explains the rain-shadow deserts.

A jet stream is a narrow, fast, meandering river of air high in the atmosphere, usually near the tropopause at about 9 to 16 km. Its winds blow at well over 100 km/h and in winter can exceed 300 km/h in places. Most jet streams blow from the west to the east (westerlies), but one important exception, the Tropical Easterly Jet of the Indian summer, blows from the east. Jet streams were first noticed clearly by high-flying bomber pilots during the Second World War, who found that flying eastwards was suddenly faster and westwards slower.

Why do jet streams exist? Wherever a warm air mass sits next to a cold one, the pressure difference between them grows rapidly with height, so the wind high above that boundary becomes very strong. The Coriolis effect then turns this wind so that it blows along the boundary, from west to east. That is why the polar front jet lies where polar and tropical air meet (near 60 degrees) and the subtropical jet lies at the edge of the Hadley cell (near 30 degrees). The jets sit near the tropopause, where the difference in temperature is sharpest.

Why jet streams matter. First, they steer weather: the polar front jet guides temperate cyclones eastwards, and the subtropical jet carries western disturbances to India. Second, their wavy path (the Rossby waves) can dip far south, pulling a cold wave from the pole into the mid-latitudes, or bulge north, bringing a heatwave. Third, aviation: aircraft flying with the jet save time and fuel, and flying against it costs both; the sharp edge of a jet also causes clear-air turbulence. This is why a flight from India to Europe or the USA is usually quicker in one direction than the return.