Climatology: Atmosphere, Temperature, Winds and Precipitation (NCERT Geography Digest)

Composition and structure of the atmosphere

• Composition (by volume of dry air): nitrogen about 78 percent, oxygen about 21 percent, argon, carbon dioxide and trace gases; plus variable water vapour and dust (which aid condensation).
• Layers by height:
  • Troposphere: lowest layer; all weather occurs here; temperature falls with height. Extends to about 8 km at the poles and 18 km at the equator.
  • Stratosphere: contains the ozone layer (absorbs ultraviolet radiation); temperature rises with height; stable, ideal for jet aircraft.
  • Mesosphere: temperature falls again; the coldest layer; meteors burn up here.
  • Thermosphere / Ionosphere: temperature rises sharply; ionised layers reflect radio waves; auroras occur here.
  • Exosphere: the outermost, merging into space.
• Normal lapse rate: temperature falls about 6.5° Celsius per 1000 metres in the troposphere.

Insolation, heat budget and temperature

• Insolation is incoming solar radiation. The atmosphere is largely transparent to short-wave solar radiation and is heated mainly from below by terrestrial (long-wave) radiation.
• Heat budget: incoming and outgoing radiation balance over the year, keeping global temperature broadly stable. The greenhouse effect traps outgoing long-wave radiation and warms the surface.
• Controls on temperature: latitude, altitude, distance from the sea (continentality), ocean currents, prevailing winds, cloud cover, and slope/aspect.
• Inversion of temperature: temperature rises with height (reversing the normal pattern), common on clear, calm winter nights, especially in valleys (cold-air drainage), with implications for frost and air pollution.

Pressure and winds

• Air pressure falls with height. Global pressure belts: equatorial low (the doldrums), subtropical high (horse latitudes), subpolar low, and polar high.
• The Coriolis force (from the Earth's rotation) deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Planetary (prevailing) winds: trade winds (subtropical high to equatorial low), westerlies (subtropical high to subpolar low), and polar easterlies.
• Periodic and local winds: monsoons (seasonal reversal), land and sea breezes, mountain and valley breezes, and named local winds (Loo, Chinook, Foehn, Mistral).

Weather systems

• Cyclones: tropical cyclones (warm-core, forming over warm tropical seas above about 27° Celsius, very destructive, with a calm eye) and temperate (mid-latitude) cyclones (frontal, in the westerlies). An anticyclone has high pressure at the centre with diverging winds and fair weather.
• Air masses and fronts: when contrasting air masses meet they form fronts (warm, cold, occluded, stationary), the basis of temperate-cyclone weather.

Water in the atmosphere

• Humidity: absolute humidity (actual water vapour per unit volume) and relative humidity (percentage of saturation at that temperature). The dew point is the temperature at which air becomes saturated.
• Condensation forms: dew, frost, fog, mist and clouds. Cloud families: cirrus (high, wispy), cumulus (heaped), stratus (layered) and nimbus (rain-bearing), with combinations such as cumulonimbus.
• Precipitation types by cause:
  • Convectional: intense surface heating lifts moist air (equatorial regions, afternoon thunderstorms).
  • Orographic (relief): moist air forced up a mountain barrier rains on the windward side, leaving a dry rain shadow on the leeward side.
  • Cyclonic / frontal: associated with fronts in temperate cyclones.

World climate (introduction)

• Koppen's classification uses temperature and rainfall to define groups: A tropical, B dry, C warm temperate, D cold (continental), E polar, and H highland.
• Climate change: greenhouse gases (carbon dioxide, methane, nitrous oxide, CFCs) intensify warming, raising sea levels and shifting climate zones; past climate is reconstructed from tree rings, ice cores, pollen and sediments.

How CAPF asks this

• Order of atmospheric layers and where weather and ozone occur. The normal lapse rate. Coriolis deflection direction.
• Orographic rainfall and the rain-shadow effect (a perennial favourite, linked to the western Ghats and Cherrapunji in india climate).
• Composition percentages (nitrogen and oxygen) and the cause of temperature inversion.

Authored practice

1. In the troposphere the normal lapse rate is about: (a) 1 degree Celsius per 1000 m (b) 6.5° Celsius per 1000 m (c) 10° Celsius per 1000 m (d) it does not change. Answer: (b) 6.5° Celsius per 1000 m. Authored practice, not a verbatim PYQ.
2. Rainfall caused by moist air being forced to rise over a mountain barrier is called: (Answer: orographic / relief rainfall.) Authored practice, not a verbatim PYQ.