Atmosphere: Pressure, Winds and Climate
Atmosphere: Pressure, Winds and Climate
The atmosphere gives 3 questions and is the part of physical geography with the most direct bearing on Indian life, since it explains the monsoon. Pressure belts, wind types and cyclones are the three areas the Part-III paper examines.
Composition and Layers
Air is about 78 per cent nitrogen and 21 per cent oxygen, with argon, carbon dioxide and other gases making up the rest.
| Layer | Feature |
|---|---|
| Troposphere | All weather occurs here; temperature falls with height |
| Stratosphere | Ozone layer; temperature rises with height; used by aircraft |
| Mesosphere | Coldest layer; meteors burn up here |
| Thermosphere | Temperature rises sharply; radio waves reflect from the ionosphere |
| Exosphere | Outermost, merging into space |
Temperature falls with height in the troposphere and rises with height in the stratosphere, because the ozone there absorbs ultraviolet radiation. That alternation is asked directly.
Pressure Belts
| Belt | Latitude | Character |
|---|---|---|
| Equatorial low | 0 degrees | Air rises; the doldrums, calm and rainy |
| Subtropical high | About 30 degrees N and S | Air sinks; the horse latitudes; world's deserts |
| Subpolar low | About 60 degrees N and S | Rising air; stormy |
| Polar high | 90 degrees N and S | Cold sinking air |
Wind blows from high pressure to low, and the world's great deserts lie under the subtropical high because sinking air does not produce rain. That is the reasoning question, and it explains the Sahara and the Thar in one sentence.
✗ Deserts occur at 30 degrees because it is hottest there | ✓ They occur because sinking air in the subtropical high belt prevents rainfall
Types of Wind
- Permanent or planetary winds blow all year — trade winds, westerlies, polar easterlies.
- Seasonal winds reverse with the season — the monsoon is the great example.
- Local winds affect a small area — the loo of north India, the chinook, the mistral.
- Winds are deflected right in the northern hemisphere and left in the southern by the Coriolis effect, caused by the earth's rotation.
The loo is the hot dry summer wind of north India and it is asked by name, since it affects Bihar directly in May and June. Note that a wind is always named after the direction it comes from, not the one it blows towards.
Cyclones and Rainfall
A cyclone is a system of low pressure with winds spiralling inward — anticlockwise in the northern hemisphere. An anticyclone is high pressure with outward, clockwise winds and brings clear weather.
| Rainfall type | Cause | Example |
|---|---|---|
| Convectional | Heated air rises and cools | Equatorial afternoon rain |
| Orographic or relief | Air forced up over a mountain | Western Ghats, Meghalaya |
| Cyclonic or frontal | Warm and cold air masses meet | Temperate regions |
Orographic rainfall explains why the windward side of the Western Ghats is wet and the leeward side dry — the rain shadow. It is also why Meghalaya receives the world's heaviest rainfall while the plateau beyond is far drier. The rain-shadow concept is asked in some form almost every year.
पवन उसी दिशा के नाम से जानी जाती है जिससे वह आती है।
TRE pointer: Two reasoning questions carry this topic: why deserts lie at 30 degrees (sinking air, not heat) and the rain shadow behind a mountain range. The troposphere cools with height, stratosphere warms contrast is the factual item. For Bihar, the loo is the local wind to know by name. Remember that winds are named after where they come from, which is a distinction the options exploit.
60-Second Recap
- Air is 78 per cent nitrogen, 21 per cent oxygen.
- Temperature falls with height in the troposphere and rises in the stratosphere.
- Four pressure belts: equatorial low, subtropical high, subpolar low, polar high.
- Deserts lie under the subtropical high because sinking air prevents rain.
- Winds are named after the direction they come from; the loo blows over north India.
- Orographic rain wets the windward slope and leaves a rain shadow beyond.
Frequently Asked Questions
Why do the world's great deserts lie near 30 degrees latitude?
Because the subtropical high pressure belt sits there, where air descends. Sinking air warms and holds its moisture rather than releasing it, so rainfall is very low. The Sahara, the Arabian desert and the Thar all lie in this belt.
What is a rain shadow?
The dry area on the leeward side of a mountain range. Moist air rises up the windward slope, cools and gives heavy rain there, then descends the far side warm and dry. The Western Ghats and the Deccan plateau are the standard Indian example.
Why does temperature rise with height in the stratosphere?
Because the ozone layer there absorbs ultraviolet radiation from the sun and warms the surrounding air. In the troposphere below, heat comes from the earth's surface, so temperature falls as you move away from it. The reversal is asked directly.
What is the loo?
A hot, dry local wind that blows over the plains of north India, including Bihar, during May and June. Local winds affect only a small region, unlike the permanent trade winds and westerlies or the seasonal monsoon that reverses direction with the year.