Atmosphere and Winds: Layers, Pressure Belts and Jet Streams

The atmosphere is the envelope of gases held to the earth by gravity. It supplies the oxygen we breathe and the carbon dioxide plants use, filters ultraviolet radiation, keeps the surface warm through the greenhouse effect and carries the moisture and heat that make weather possible. Almost every question on winds, pressure belts and jet streams in a competitive paper comes from the facts of its composition and its layered structure, so the two are best learnt together.

Composition of the atmosphere

By volume, dry air near the surface is about 78 per cent nitrogen and about 21 per cent oxygen, and these two make up almost the whole of it. Argon accounts for a little under one per cent, and carbon dioxide for about four hundredths of one per cent, a share that has been rising through the burning of fossil fuels. Water vapour is not counted in dry air but is the most variable gas, from almost nothing over a desert to about four per cent over a warm ocean; it is the source of all precipitation and it also absorbs heat radiated by the earth. Ozone is present in very small quantities but is vital because it absorbs ultraviolet radiation. Dust and salt particles act as nuclei on which water vapour condenses, which is why the air over a city is hazier than the air over the open sea.

Layers of the atmosphere

The atmosphere is divided into layers by the way temperature changes with height. The troposphere, the lowest layer, reaches about 18 kilometres over the equator and about 8 kilometres over the poles, because heated air at the equator expands upward. Temperature falls in it at the normal lapse rate of about 6.5 degrees Celsius per kilometre, and all clouds, rain and storms occur here; its upper limit is the tropopause. Above it the stratosphere holds the ozone layer, so temperature rises with height instead of falling; the air is stable and cloudless, which is why jet aircraft prefer to fly in its lower part. In the mesosphere temperature falls again to the lowest values found anywhere in the atmosphere at the mesopause, and meteors entering the earth burn up here. Above that the thermosphere, which contains the ionised layers that reflect radio waves back to the earth, shows a steep rise in temperature, and beyond it the exosphere thins away into space.

Pressure belts and planetary winds

Unequal heating creates four belts of pressure in each hemisphere. The equatorial low, also called the doldrums, is a belt of rising air and calms; the sub-tropical high near thirty degrees, the horse latitudes, is a belt of sinking air where the great deserts lie; the sub-polar low near sixty degrees is a belt of convergence and storms; and the polar high is a belt of cold, heavy air. Winds blow from high pressure to low pressure but never in a straight line, because the rotation of the earth deflects them to the right in the northern hemisphere and to the left in the southern hemisphere. This is the Coriolis effect, stated as Ferrel's law. Buys Ballot's law puts the same idea another way: stand with your back to the wind in the northern hemisphere and low pressure lies to your left.

The permanent or planetary winds follow from these belts. Trade winds blow from the sub-tropical high to the equatorial low, as north-easterlies in the northern hemisphere and south-easterlies in the southern. The westerlies blow from the sub-tropical high towards the sub-polar low and become very strong over the southern ocean, where they are called the roaring forties, the furious fifties and the shrieking sixties. The polar easterlies blow out of the polar high towards the sub-polar low.

Seasonal and local winds

Seasonal or periodic winds reverse with the season or with the day. The monsoon is the largest example, reversing between summer and winter as the land heats and cools faster than the sea. The same difference on a smaller scale gives the sea breeze by day and the land breeze by night, and in mountains the valley breeze blows upslope by day and the mountain breeze downslope by night. Local winds are named in every syllabus and are best learnt as a table.

Jet streams

Jet streams are narrow bands of very fast winds in the upper troposphere. The subtropical westerly jet and the polar front jet blow from west to east and steer the weather systems of the middle latitudes; over India the westerly jet lies south of the Himalaya in winter and brings in the western disturbances. In summer it shifts north of the plateau of Tibet and an easterly jet appears over peninsular India, a change that goes with the burst of the south-west monsoon.