Atmospheric Circulation and Weather Systems

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 9.

If air flows from high to low, why do storms spiral?

Air always wants to move from high pressure to low pressure, which sounds like a straight line. But look at a satellite picture of a cyclone: the clouds swirl in a great spiral. Something is bending the wind. The chapter explains what: the spinning earth, friction with the ground, and the pressure differences created by uneven heating. Once you know the three forces, circulation cells, local breezes, fronts and cyclones all follow from them.

The link with the last chapter is direct. Air expands when heated and is compressed when cooled, so the uneven temperature of the earth creates uneven pressure, and pressure differences set air moving. Wind then redistributes heat and moisture over the planet, which helps keep the planet's overall temperature steady. And when moist air rises, it cools and forms clouds and rain.

Atmospheric pressure

Atmospheric pressure is the weight of a column of air on a unit area, from sea level to the top of the atmosphere. It is measured in millibars (mb) with a mercury barometer or an aneroid barometer. At sea level the average pressure is about 1,013.2 mb. Gravity pulls air towards the surface, so surface air is denser and pressure is highest there. Pressure decreases with height, which is why people feel breathless on mountains.

In the lower atmosphere the pressure falls rapidly with height, by about 1 mb for every 10 m of elevation, though the rate is not always the same. The pressure difference between the bottom and top of the air is large, but gravity balances it almost exactly, so we do not feel strong upward winds.

Worked example: pressure at a height. If the surface pressure is 1,000 mb, what is the pressure 1 km above it? A rise of 1 km is 1,000 m, and the fall is about 1 mb per 10 m, which is 100 mb. The pressure is about 900 mb. The common wrong step is to think the pressure rises with height, or to use the whole 1,013 figure.

Horizontal differences matter most. The small variation in pressure from place to place is what drives winds. To compare stations, pressure is reduced to sea level, which removes the effect of altitude. Weather maps show isobars, lines joining places of equal pressure. A low pressure system has one or more isobars closed around the lowest pressure at the centre, and a high pressure system has the highest pressure in the centre.

World pattern of sea level pressure. Four pressure belts appear, as shown here.

These belts are not fixed. They oscillate with the apparent movement of the sun, moving south in the northern hemisphere's winter and north in summer.

Three forces and one rule

The wind blows from high pressure to low pressure. Near the surface, horizontal winds respond to the combined effect of three forces, while gravity acts downward.

The Coriolis force acts at right angles to the pressure gradient force, and the pressure gradient force is at right angles to the isobar.

Geostrophic wind. Above about 2 to 3 km from the surface, friction no longer matters. Where isobars are straight, the pressure gradient force and the Coriolis force balance, and the wind blows parallel to the isobars. This is the geostrophic wind.

Why cyclones do not form at the equator. At the equator the Coriolis force is zero, so the wind blows straight across the isobars, perpendicular to them, and the low pressure simply fills up rather than intensifying. Away from the equator, the two forces at right angles make wind circle around a low. So tropical cyclones do not form near the equator.

Cyclonic and anticyclonic circulation. The wind pattern around a low is called cyclonic, and around a high it is anticyclonic. The direction depends on the hemisphere.

Over a low, air converges and rises. Over a high, air sinks from above and spreads out (diverges) at the surface. Rising air is essential for cloud and rain. Besides convergence, rising happens through eddies, convection currents, orographic uplift (air forced up a mountain) and uplift along fronts.

Pause and predict: a wind blows in the northern hemisphere from the subtropical high towards the equator, which is from north to south. In which direction will the Coriolis force turn it, and what wind do you get?

Check your answer: the Coriolis force turns it to the right of its path, so a wind heading south bends towards the west. The result is a wind from the north-east, the north-easterly (trade wind) of the tropics.

General circulation: the world's wind machine

The pattern of planetary winds depends on five things: the uneven heating of latitudes, the pressure belts, their shift with the sun, the distribution of land and ocean, and the earth's rotation. The movement pattern is the general circulation of the atmosphere.

At the Inter Tropical Convergence Zone (ITCZ), near the equator, strong insolation causes convection and low pressure. Winds from the tropics converge there, and the air rises up to about 14 km at the top of the troposphere and moves towards the poles. By about 30° N and S, the air accumulates, cools and sinks to form the subtropical high. Near the ground it flows back to the equator as easterlies, which complete the loop.

These three cells set the pattern of general circulation, and the transfer of heat from low latitudes to high latitudes maintains it. The big winds also start the slow, large ocean currents, and the oceans in turn supply energy and water vapour to the air.

Beyond the textbook. For the exam: the atmosphere is organised into three cells, Hadley, Ferrel and polar. In reality: the three cell picture is a simplified average. The tropical Hadley cell is a fairly direct response to heating, but the middle latitude cell is largely an average of the swirling storms and waves of the westerlies rather than a neat, steady loop. That is why a diagram of three tidy loops describes the idea well but cannot predict a day's weather.

El Nino, the southern oscillation and ENSO

Warming and cooling of the Pacific Ocean are very important for global circulation. Warm water from the central Pacific slowly drifts towards the South American coast and replaces the cool Peruvian current. The appearance of warm water off the coast of Peru is called El Nino. It is linked to changes in pressure over the central Pacific and Australia, known as the southern oscillation. The two together are ENSO.

In a strong ENSO year, large scale weather variations occur around the world. The normally arid west coast of South America receives heavy rainfall, drought occurs in Australia and sometimes in India, and floods occur in China. ENSO is closely monitored and used for long range forecasting in many parts of the world.

Beyond the textbook. For the exam: a strong ENSO year brings drought to Australia and sometimes to India. In reality: the word "sometimes" matters. India's monsoon rainfall is affected by many factors, and not every El Nino year brings a poor monsoon. ENSO is one strong signal among several, which is why forecasters treat it as a warning rather than a guarantee.

Seasonal and local winds

The world pattern shifts through the year as the zones of maximum heating, pressure and wind belts move. The most pronounced result is the monsoon, especially over south-east Asia. Beyond that, small differences in heating and cooling create local winds.

Pause and predict: why does a sea breeze blow in the afternoon and not at night?

Check your answer: during the day the land warms faster than the sea. Air rises over the land, which creates low pressure there, while the sea stays cooler with higher pressure, so air flows from sea to land. At night the land cools faster and the pattern reverses into a land breeze.

Air masses

When air stays over a uniform area for long enough, it takes on the temperature and humidity of that area. A large body of air with little horizontal variation in temperature and moisture is an air mass. The uniform area where it forms is the source region: a vast ocean or a vast plain. There are five major source regions, and each gives a named type.

Tropical air masses are warm, and polar air masses are cold. A mountain or plateau such as the Himalayas or the Deccan is not a source region, because the surface is not a large uniform plain or ocean. A source region such as the Siberian plain is.

Fronts and extra tropical cyclones

When two different air masses meet, the boundary zone is called a front, and its formation is frontogenesis. Fronts occur in the middle latitudes and are marked by a steep gradient in temperature and pressure. They cause sudden temperature changes and force air to rise, making clouds and rain.

How an extra tropical cyclone lives and dies. These systems form in the middle and high latitudes beyond the tropics, along the polar front.

Tropical cyclones

Tropical cyclones are violent storms that originate over warm tropical oceans and then move on to coastal areas. They bring violent winds, very heavy rain and storm surges, and are among the most devastating natural calamities. Their name depends on where they form.

What a cyclone needs. Five conditions favour formation and intensification: a large sea surface with temperature above 27°C, the presence of the Coriolis force, small variations in vertical wind speed, a pre-existing weak low pressure area or low level cyclonic circulation, and upper divergence above the sea level system.

Why cyclones die over land. The storm's energy comes from the condensation of water vapour in the towering cumulonimbus clouds around the centre. A continuing supply of moisture from the sea strengthens it. On reaching land the moisture supply is cut off, and the storm dies out. The point where a tropical cyclone crosses the coast is its landfall. Cyclones that cross 20° N latitude generally recurve and are more destructive.

Anatomy of a mature cyclone. The strong wind spirals around the centre, called the eye. The eye is a region of calm with sinking air. Around it is the eye wall, where air spirals upward to the tropopause, the wind reaches its maximum (as high as 250 km per hour) and torrential rain falls. Rain bands radiate outwards with trains of cumulus and cumulonimbus clouds. The circulating system has a diameter of 150 to 250 km, and the storm over the Bay of Bengal, the Arabian Sea and the Indian Ocean is 600 to 1,200 km across. It moves slowly, about 300 to 500 km per day, and the storm surge floods coastal lowlands.

Beyond the textbook. For the exam: a tropical cyclone's damage comes from violent winds, very heavy rain and storm surges. In reality: in many cyclone disasters the storm surge, the sea driven inland, has done the greatest harm to low lying coasts, more than the wind itself. This is why cyclone warnings stress evacuation from coastal lowlands, and why coastal mangroves and dunes are valued as natural buffers.

Thunderstorms and tornadoes

Thunderstorms and tornadoes are short-lived, small in area and violent. A thunderstorm is a well grown cumulonimbus cloud producing thunder and lightning, caused by intense convection on moist hot days. If the cloud reaches heights where the temperature is below freezing, hail forms. With insufficient moisture, a thunderstorm can generate dust storms. An intense updraft of warm air grows the cloud, and later the downdraft brings cool air and rain to the ground.

From a severe thunderstorm, a spiralling wind can descend like an elephant's trunk, with very low pressure at the centre and great destructive force. This is a tornado, and tornadoes generally occur in the middle latitudes. A tornado over the sea is a water spout. These storms show the atmosphere adjusting to uneven energy distribution, as potential and heat energy turn into kinetic energy and the atmosphere returns to a stable state.

Exam traps

60-second revision

Where this chapter connects

• Previous: Solar Radiation, Heat Balance and Temperature explains the uneven heating that creates the pressure differences here.
• Next: Water in the Atmosphere follows the rising, cooling air into condensation, clouds and rainfall.
• Related: Movements of Ocean Water shows how these winds drive ocean currents, Natural Hazards and Disasters covers cyclone impacts, and Climate carries the monsoon story forward for India.