Atmospheric Circulation and Weather Systems

Air is always on the move. The reason is simple: it flows from where pressure is high to where pressure is low. From this one rule grows the whole machinery of the world's winds: the global pressure belts, the steady planetary winds, the high-altitude jet streams, and the spinning storms we call cyclones. This is one of the most heavily tested areas of physical geography. It ties together pressure, wind, the Earth's rotation and weather.

Atmospheric pressure is the weight of the column of air pressing down on a unit area of the surface. It is measured with a barometer in units called millibars. Because air is densest near the ground, pressure is highest at sea level and falls rapidly with height.

Differences in pressure from place to place set the winds in motion. Where air is warm, it expands, becomes lighter, and rises. This leaves low pressure at the surface. Where air is cold, it contracts, becomes heavier, and sinks. This creates high pressure. Wind is simply air flowing from the high-pressure area to the low-pressure area.

On a global scale the pressure is organised into seven belts running around the Earth.

At the Equator the intense heat makes air rise, creating a belt of equatorial low pressure (the doldrums). Around 30 degrees north and south the descending air forms the subtropical high-pressure belts (the horse latitudes). Around 60 degrees north and south lie the subpolar low-pressure belts, and over the cold poles sit the polar high-pressure areas. These belts shift a little north and south with the seasons as the Sun moves.

The permanent winds that blow steadily between the pressure belts are the planetary or prevailing winds. They blow from the high-pressure belts towards the low-pressure belts. But they do not move in a straight north–south line. The rotation of the Earth deflects them. This deflection is the Coriolis force. It turns winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Two further properties of the Coriolis force are tested again and again. First, its strength grows with wind velocity: the faster the wind blows, the more it is deflected. Second, it varies with latitude: it is maximum at the poles and absent at the equator, where winds feel no deflection at all. At any given latitude its magnitude is the same in both hemispheres; only the direction of the turn differs.

There are three great wind systems in each hemisphere:

• The trade winds blow from the subtropical highs towards the equatorial low.
• The westerlies blow from the subtropical highs towards the subpolar lows.
• The polar easterlies blow from the polar highs towards the subpolar lows.

The trade winds blow towards the equator from the north-east in the Northern Hemisphere and the south-east in the Southern. Moving into ever warmer latitudes, they pick up moisture rather than release it, so they arrive dry. On the western margins of continents within the trade-wind belt, the trades blow offshore, away from the land, and cold ocean currents chill the coast and suppress rain. This is why the great tropical deserts, such as the Sahara, the Kalahari and the Atacama, lie along the western margins of continents in the trade-wind latitudes. The same logic applies to India in winter: the north-east winds then blow from land to sea and are dry, so even the East Himalayan region receives no heavy rainfall from them.

The westerlies blow from the south-west in the Northern Hemisphere and from the north-west in the Southern. For India, their most important cargo is the western disturbances: moist air masses originating over the Mediterranean region that travel eastwards embedded in the mid-latitude westerlies. They bring the winter rainfall of north-western India, the light rains valuable for the wheat crop.

The westerlies of the Southern Hemisphere are stronger and more persistent than their northern counterparts. The reason is the map, not the physics. The southern mid-latitudes are almost entirely ocean, so there is far less landmass and therefore little surface friction to slow or interrupt the winds. It is not a stronger Coriolis force: at the same latitude the Coriolis force is equal in magnitude in both hemispheres. Between 40 and 50 degrees south, these fierce, constant westerlies are called the Roaring Forties. They blow only in the Southern Hemisphere, generally from the north-west towards the east, and they bring overcast skies, rain and raw, stormy weather. Sailors named the still wilder belts further south the Furious Fifties and the Shrieking Sixties.

Far above these surface winds, near the top of the troposphere, flow the fast jet streams that steer the weather systems below.

Besides the great planetary systems, many regions have their own local winds. These are produced by local differences in heating, or by air descending the lee side of a mountain range. A descending wind is compressed as it sinks, so it arrives warm and dry. Examiners love to match these winds with their home regions, so the names and places must be learnt as fixed pairs.

• Fohn: a warm, dry wind descending the leeward slopes of the Alps in Europe. It melts snow and ripens grapes early.
• Chinook: the North American counterpart of the Fohn, blowing down the eastern slopes of the Rockies. It is called the snow-eater.
• Santa Ana: a hot, dry wind of California. It dries vegetation and fans wildfires.
• Zonda: a warm, dry wind descending the Andes into Argentina.
• Samun: a hot, dry wind of Kurdistan, in the Iran and Iraq region.
• Harmattan: a dry, dusty land wind blowing from the Sahara across the West African coast, not East Africa. It brings relief from humid heat, so locals call it the doctor.
• Loo: the scorching hot summer wind of the plains of northern India.

Multi-statement questions often pair a local wind with unrelated geography claims, and elimination then depends on those claims too. Two recur with the Harmattan. A nautical mile measures 1.852 km, so it is longer than the terrestrial or statute mile of 1.609 km, not shorter. And Greece and Albania lie on the Balkan Peninsula of Europe, not the Iberian Peninsula, which holds Spain and Portugal.

A large body of air that has roughly the same temperature and humidity throughout is called an air mass. It forms when air rests for a long time over a homogeneous surface called a source region, such as a vast ocean or plain. The air slowly takes on the character of that surface: warm and moist over tropical seas, cold and dry over polar lands. As an air mass moves away from its source, it carries that weather with it.

Air masses are classified by their source region on two axes. Moisture gives maritime (m, over oceans) or continental (c, over land). Temperature gives tropical (T), polar (P) or arctic (A). The main types are:

• Maritime tropical (mT): warm and moist.
• Continental tropical (cT): warm and dry.
• Maritime polar (mP): cold and moist.
• Continental polar (cP): cold and dry.
• Continental arctic (cA): very cold.

When two different air masses meet, they do not mix easily. The sloping boundary between them is called a front. Fronts are zones of active, changeable weather, and they are the birthplace of mid-latitude weather. The concept of fronts was introduced by V. and J. Bjerknes, Norwegian meteorologists, in 1918. There are four types:

• Warm front: warm air rises gently over retreating cold air, bringing a broad belt of steady rain and nimbus clouds.
• Cold front: advancing cold air undercuts warm air steeply, bringing a sharp temperature drop, towering cumulonimbus clouds and short, heavy showers, often with thunder.
• Occluded front: a faster cold front catches up with a warm front and lifts it off the ground, merging the two. Its weather mixes warm-front and cold-front features.
• Stationary front: two air masses meet but neither advances, so the boundary stays put.

A thunderstorm is a violent local storm built around a towering cumulonimbus cloud, the thundercloud. Building such a cloud needs several ingredients working together. High temperature and humidity supply the energy and the moisture. A trigger such as intense surface heating or orography, the forced lifting of air over hills and mountains, starts the air rising. Strong vertical winds, the updrafts, carry the moist air to great heights. As the air rises, condensation releases latent heat, which warms the air further and drives it still higher. All four together, heat and humidity, orography, vertical wind, and condensation, build the storm cell.

What actually makes the sound of thunder is a common trap. It is not clouds colliding, and lightning does not separate any clouds. A lightning discharge superheats the narrow channel of air it passes through. That air expands explosively, and the shock of this sudden expansion is the sound we hear as thunder. Light travels faster than sound, so the flash always arrives before the rumble.

A cyclone is a system of low pressure with winds spiralling inwards (anticlockwise in the Northern Hemisphere and clockwise in the Southern).

Tropical cyclones form over warm tropical seas. They draw their energy from the heat and moisture of the ocean. They are violent storms with torrential rain and destructive winds. They are known as hurricanes in the Atlantic and typhoons in the Pacific. The calm centre is the eye of the cyclone.

Temperate cyclones form in the mid-latitudes along the fronts where warm and cold air masses meet. They are larger but gentler than tropical cyclones. They bring widespread cloud, rain, and changeable weather over several days. In contrast, an anticyclone is a system of high pressure with outward-spiralling winds. It brings calm, clear weather.

Northern India offers the classic seasonal example. In winter, cold, dense air settles over the land and builds high pressure, creating anticyclonic conditions across the northern plains. The winter rainfall that northern India receives in this season comes from the western disturbances riding the westerlies, and it is associated with this anticyclonic cold-weather pattern.

The Indian monsoon is shaped not only by winds over India but also by ocean temperatures far away in the Pacific. El Nino is an unusual warming of the sea surface in the eastern equatorial Pacific, off the coast of Peru. It comes paired with the Southern Oscillation, a see-saw of air pressure between the eastern and western Pacific. Together they are called ENSO (El Nino and Southern Oscillation). An El Nino year is the event most strongly associated with weak south-west monsoon rains over the Indian subcontinent, so it is the key signal for short-term climate prediction.

La Nina is the opposite phase: unusually cold sea-surface temperatures in the equatorial Pacific Ocean, not the Indian Ocean. Far from having no effect on India, La Nina generally strengthens the south-west monsoon. It also brings heavy rain and floods to Australia. Two refinements matter for the exam:

• El Nino Modoki: a variant in which the warming occurs in the central Pacific, while a normal El Nino warms the eastern Pacific. A normal El Nino suppresses Atlantic hurricanes, but El Nino Modoki produces a greater number of Atlantic hurricanes with greater frequency.
• Indian Ocean Dipole (IOD): a difference in sea-surface temperature between the tropical western Indian Ocean and the tropical eastern Indian Ocean, near Indonesia. Both poles lie inside the Indian Ocean, not the Pacific. A positive IOD, with a warmer western pole, favours good monsoon rain and can offset the damaging effect of an El Nino on the monsoon.