Atmospheric Pressure, Winds & Pressure Belts

9.1 Why Air Moves — Pressure as the Engine of Wind

Take a deep breath. Hold it. Now exhale. You have just ridden the same engine that drives every cloud, every cyclone and every monsoon on the planet — a difference in pressure. Earlier in Chapter 8 you learned that the Sun heats the earth's surface unevenly. Air expands when it is heated and is compressed when it is cooled. Out of this heating and cooling come variations in atmospheric pressure, and out of those variations come horizontal movements of air which we call wind. The wind is no accident — it is nature's response to a pressure imbalance, redistributing heat and moisture across the planet so that the earth as a whole maintains a constant temperature.

Pressure is measured with a mercury barometer^? — a vertical glass tube of mercury invented by Torricelli in 1643 — or with a portable aneroid barometer^?, which uses a sealed metal capsule that flexes as outside pressure changes. Although our body is constantly subjected to about a tonne of air pressure on every square metre, we feel it only when we move up rapidly — for instance, on take-off in an aircraft or while climbing a high pass — when the air becomes rarefied and we may feel breathless.

9.2 Vertical Variation of Pressure

In the lower atmosphere the pressure decreases rapidly with height. The decrease amounts to roughly 1 mb for every 10 m of elevation gain near the surface, though this rate is not constant — at greater heights the air is thinner and pressure falls more slowly. Table 9.1 sets out the standard pressure and temperature for a calm, dry "standard atmosphere" used by aviation and weather services worldwide.

9.3 Horizontal Distribution of Pressure — Isobars and Pressure Systems

Small horizontal differences in pressure are highly significant for the direction and velocity of wind. To map them, meteorologists draw isobars^? — lines connecting places that have equal atmospheric pressure. Because pressure naturally varies with altitude, all readings used to plot isobars are first reduced to sea level using a standard correction. Without this step a high mountain station would always look like a "low" simply because it is high up.

9.4 World Distribution of Sea-Level Pressure

Plotted on a January or July map, the world's sea-level pressure organises itself into a remarkably regular set of latitudinal belts. Near the equator the pressure is low and the area is known as the equatorial low. Along 30° N and 30° S sit two great rings of high pressure called the subtropical highs. Further polewards, along 60° N and 60° S, lie the low-pressure belts called the sub-polar lows. And near the poles, both north and south, the pressure is high — the polar high. These belts are not permanent; they oscillate with the apparent movement of the Sun, shifting southwards in the northern winter and northwards in the northern summer.

9.5 Forces that Move the Wind

Wind is air in horizontal motion. At any moment a parcel of surface air responds to the combined effect of three forces — and a fourth, gravity, which acts vertically:

Pressure Gradient Force

The differences in atmospheric pressure produce a force. The rate of change of pressure with respect to distance is the pressure gradient^?. The pressure gradient is strong where the isobars on a weather map are crowded close together, and weak where the isobars are spaced far apart. Wherever the gradient is strong, the wind will be fast.

Frictional Force

Friction affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1 to 3 km. Over the relatively smooth sea surface friction is minimal; over rugged land surfaces and forests it is much larger.

Coriolis Force

The rotation of the earth on its axis affects the direction of the wind. This force is named the Coriolis force^? after the French physicist Gaspard-Gustave Coriolis, who described it mathematically in 1844. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection grows with wind speed and with latitude — the Coriolis force is directly proportional to the angle of latitude, being maximum at the poles and absolutely zero at the equator.

Pressure and Wind — The Geostrophic Balance

The velocity and direction of the wind are the net result of these forces. Above 2–3 km — out of reach of surface friction — winds in the upper atmosphere are controlled mainly by the pressure gradient force and the Coriolis force. When the isobars are straight and friction is absent, the pressure gradient force is exactly balanced by the Coriolis force, and the resulting wind blows parallel to the isobars. This balanced upper-air wind is called the geostrophic wind^?.

The wind circulation around a low is called cyclonic circulation; around a high it is called anticyclonic circulation. Generally, over a low-pressure area surface air converges and rises; over a high-pressure area air sinks from above and spreads out at the surface. Apart from convergence, eddies, convection currents, orographic uplift, and uplift along fronts also lift air — and rising air is a precondition for cloud formation and precipitation.

9.6 General Circulation of the Atmosphere — Three Cells, Three Wind Belts

The pattern of planetary winds depends on five key factors: (i) the latitudinal variation of atmospheric heating; (ii) the emergence of pressure belts; (iii) the migration of those belts following the apparent path of the Sun; (iv) the distribution of continents and oceans; and (v) the rotation of the earth. Together, these produce the general circulation of the atmosphere — the long-term average of the wind pattern, organised into three vertical cells and three horizontal wind belts in each hemisphere.

The mechanism works like this. Air at the Inter-Tropical Convergence Zone (ITCZ) rises because of intense convection driven by high insolation, creating a low pressure that pulls in winds from both tropics. The rising air cools and reaches the top of the troposphere at about 14 km, then drifts polewards. Air piles up at around 30° N and 30° S and part of it sinks back down — partly because of cooling at the higher latitude, partly because of accumulation. The descending dry air forms the subtropical high, and at the surface it splits: some flows back equatorwards as the easterlies, completing a tropical loop called the Hadley Cell.

In the middle latitudes the circulation reverses: cold polar air sinking near the poles flows equatorwards while warm subtropical air rises along the polar front, giving the Ferrel Cell. The surface winds of this cell are the westerlies — pushed eastwards by the planet's rotation. At very high latitudes, cold dense air subsides over the poles and blows toward middle latitudes as the polar easterlies, creating the polar cell. Together these three cells transfer heat energy from low to high latitudes — the prime task of the entire global circulation.

9.7 Pressure and Wind Belts — Trades, Westerlies, Easterlies

At the surface, the planetary winds organise themselves into three named belts in each hemisphere. They are summarised below, and each comes with its own famous calm zones at the boundaries.

9.8 Seasonal Winds — The Monsoons

The pattern of planetary winds is significantly modified in different seasons because the regions of maximum heating, the pressure belts and the wind belts all migrate with the apparent movement of the Sun. The most pronounced effect of this seasonal shift is seen in the monsoons^?, particularly over South-East Asia and the Indian sub-continent.

The word monsoon derives from the Arabic mausim, meaning season. In summer the vast Asian land mass heats intensely, generating a deep continental low that draws moisture-laden winds inland from the Indian Ocean — the South-West Monsoon, source of nearly 80% of India's annual rainfall. In winter the cold land mass becomes a high-pressure source and the wind reverses, blowing dry from the land out to the sea — the North-East Monsoon. The seasonal reversal is essentially a giant continental-scale land-and-sea-breeze cycle. (You will study the Indian monsoon in detail in the companion textbook India: Physical Environment.)

9.9 Local Winds — Famous Names from Around the World

Differences in the heating and cooling of land surfaces, valleys and slopes create local winds on daily and annual cycles. Each region has its own — many of them famous enough to have nicknames.

Land and Sea Breezes — A Daily Cycle

Land and sea absorb and lose heat at very different rates. During the day, the land heats faster than the sea, the air over the land rises, a low pressure forms on land, and a cooler sea breeze blows from sea to land. At night the situation reverses: land cools faster, becomes the relative high, and the land breeze blows from land to sea. Coastal residents in places like Mumbai or Chennai feel this rhythm every day.

Mountain and Valley Winds

In mountain country a similar daily cycle plays out vertically. During the day the slopes heat up faster than the air at the same altitude over the valley; warm air glides upslope and air flows up out of the valley to fill the gap — the valley breeze (also called an anabatic wind). At night the slopes lose heat by radiation, the cool dense air drains downslope, and a mountain wind flows into the valley — a katabatic wind. The cold air of high plateaux and ice fields draining off Greenland and Antarctica are extreme examples of katabatic winds, sometimes reaching 200 km/h.

Foehn-Type Warm Dry Winds — Leeward of Mountain Ranges

A second important kind of mountain wind appears on the leeward side of major ranges. As moist air is forced over the range it cools, condenses and precipitates on the windward slope, losing most of its moisture. As the now-dry air descends the leeward slope, it warms by adiabatic compression. The resulting wind is unusually warm and dry, and it can melt snow within hours.