Pariksha SaathiLesson 6 of 10 · Physical and World Geography
Pressure Belts, Planetary and Local Winds, Jet Streams, Cyclones and Rainfall
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Air pressure
Air pressure is the weight of the column of air above a place. It is measured with a mercury barometer, invented by Evangelista Torricelli in 1643, or an aneroid barometer; average sea-level pressure is 1013.25 millibars. Near the surface pressure falls about 1 millibar for every 10 m of height. Lines of equal pressure are isobars, and closely spaced isobars mean strong winds, since air always moves from high pressure to low.
Pressure belts and planetary winds
The Equatorial Low (about 5 degrees N to 5 degrees S) is a calm zone of rising hot air, the doldrums, where the trade winds of both hemispheres meet in the Inter-Tropical Convergence Zone (ITCZ). The Subtropical Highs (about 30 degrees N and S), the horse latitudes, have sinking dry air, and most hot deserts lie under them. The Subpolar Lows lie at about 60 degrees and the Polar Highs over the poles. The trade winds blow from the subtropical highs to the Equator, from the north-east in the north and the south-east in the south. The westerlies blow from the subtropical highs to the subpolar lows, from the south-west in the north and the north-west in the south; over the open southern oceans they are so strong that sailors named the Roaring Forties, Furious Fifties and Shrieking Sixties. The polar easterlies blow from the poles. The belts shift north in the northern summer and south in winter with the overhead Sun, which helps explain the monsoons and the winter rain of Mediterranean lands.
The Coriolis effect
Because the Earth rotates, moving air is deflected to the right in the Northern Hemisphere and to the left in the Southern. This Coriolis force, described by Gaspard-Gustave de Coriolis in 1835, is zero at the Equator and greatest at the poles; the rule of deflection is Ferrel's law. By Buys Ballot's law, if you stand with your back to the wind in the Northern Hemisphere, low pressure is on your left.
Local winds of the world
Warm, dry winds descending the lee side of mountains include the Chinook of the Rockies (the snow eater), the Foehn of the Alps, the Zonda of the Argentine Andes and the Santa Ana of southern California. Hot desert winds include the Sirocco, blowing from the Sahara to Italy, the Khamsin of Egypt, the Simoom of Arabia and the Sahara, the Harmattan of West Africa, called the Doctor because its dryness relieves humid heat, the Brickfielder of south-east Australia and the Loo of north India. Cold winds include the Mistral down the Rhone valley of France, the Bora of the Adriatic coast, the Pampero of the Argentine pampas, the Southerly Buster of New South Wales, the Purga of Siberia and the snowy Blizzard of North America and the polar lands.
Jet streams
Jet streams are narrow bands of very fast winds, mostly westerly, near the top of the troposphere, roughly 9 to 16 km up. They blow fastest in winter: NCERT gives their speed as about 110 km per hour in summer and about 184 km per hour in winter, and their cores can exceed 300 km per hour. In winter the subtropical westerly jet blows south of the Himalayas and steers the western disturbances that bring rain and snow to north-west India; in early summer it shifts north of the Tibetan plateau, which helps the monsoon burst, and a tropical easterly jet then blows over peninsular India.
Cyclones and anticyclones
A cyclone is a low-pressure system with winds spiralling inward, anticlockwise in the Northern Hemisphere and clockwise in the Southern; an anticyclone is a high with sinking air, outward winds and fair weather. Tropical cyclones form over seas warmer than about 27 degrees Celsius, between about 5 and 30 degrees of latitude, since the Coriolis force needed to start the spin is missing near the Equator. They feed on latent heat released by condensation, so they weaken after landfall. The calm eye is ringed by the eyewall, where wind and rain are fiercest, and the storm surge causes most deaths. In the north Indian Ocean they are most frequent in the Bay of Bengal, before and after the monsoon. Temperate cyclones form in the middle latitudes along fronts where warm and cold air masses meet; they are larger, move west to east with the westerlies and bring days of rain, and the western disturbances are of this type.
Humidity, clouds and rainfall
Relative humidity is the water vapour in the air as a percentage of what it could hold at that temperature; air is saturated at 100 per cent, and the temperature at which this happens is the dew point. Cirrus clouds are high (8,000 to 12,000 m), feathery and made of ice crystals; cumulus are cotton-wool heaps of fair weather; and towering cumulonimbus bring thunder, lightning and hail. Convectional rain comes from strongly heated air rising, as in the afternoon thunderstorms of the equatorial belt. Orographic rain falls when moist winds are forced up mountains: the windward side is wet and the leeward side lies in a rain shadow, as with the Western Ghats and the dry Deccan interior. Cyclonic or frontal rain falls when warm air is lifted over cold air along fronts.
Key facts
- Pressure belts: Equatorial Low (doldrums, ITCZ), Subtropical Highs at about 30 degrees (horse latitudes, hot deserts), Subpolar Lows at about 60 degrees, Polar Highs.
- Coriolis force: zero at the Equator, greatest at the poles; deflects to the right in the north and left in the south (Ferrel's law). Buys Ballot: back to the wind, low pressure on your left (north).
- Trade winds: north-east (north) and south-east (south), towards the Equator. Westerlies: south-west (north) and north-west (south). Roaring Forties, Furious Fifties, Shrieking Sixties.
- Warm descending winds: Chinook (Rockies, snow eater), Foehn (Alps), Zonda (Argentina), Santa Ana (California).
- Hot winds: Sirocco (Sahara to Italy), Khamsin (Egypt), Simoom (Arabia and Sahara), Harmattan (West Africa, the Doctor), Brickfielder (Australia), Loo (north India).
- Cold winds: Mistral (Rhone valley, France), Bora (Adriatic), Pampero (Argentina), Southerly Buster (New South Wales), Purga (Siberia), Blizzard (North America and polar lands).
- Subtropical westerly jet: south of the Himalayas in winter, steers western disturbances; moves north of Tibet as the monsoon bursts; tropical easterly jet over peninsular India in summer.
- Tropical cyclone: sea above 27 degrees C, latitude 5 to 30 degrees, calm eye, fiercest eyewall; anticlockwise in the north, clockwise in the south; weakens after landfall.
- Clouds: cirrus high and feathery (ice), cumulus fair-weather heaps, stratus grey layers, cumulonimbus thunder, lightning and hail.
- Rain types: convectional (equatorial afternoons), orographic (wet windward side, rain-shadow leeward side), cyclonic or frontal (along fronts).
Quick revision
Sea-level pressure 1013.25 mb; air moves from high to low; doldrums near the Equator, horse latitudes at 30 degrees, subpolar lows at 60 degrees; trades NE and SE, westerlies SW and NW, polar easterlies; Coriolis zero at Equator; Roaring Forties; Chinook, Foehn, Zonda, Santa Ana warm; Mistral, Bora, Pampero cold; Sirocco, Khamsin, Harmattan hot; jet streams steer western disturbances; tropical cyclones need sea above 27 C; temperate cyclones form on fronts; cirrus high, cumulonimbus hail; convectional, orographic, cyclonic rain.