Atmosphere: Composition, Insolation, Temperature, Pressure, Winds and Precipitation
This domain examines what the air is made of, how solar energy becomes temperature, and how pressure differences generate wind belts and rainfall.
Composition and layering
Dry air is about 78.08% nitrogen, 20.95% oxygen, 0.93% argon and 0.04% carbon dioxide, with variable water vapour. The troposphere holds weather and cools upward at the normal lapse rate of 6.5 °C per km. The stratosphere contains the ozone layer, which absorbs ultraviolet radiation so temperature rises with height. The mesosphere is the coldest layer; the thermosphere (ionosphere) carries auroras and reflects radio waves.
Insolation and temperature
High latitudes receive slanting rays, spreading the same energy over a larger area and a longer atmospheric path. Temperature also depends on altitude, distance from the sea, ocean currents and cloud cover. Isotherms bend poleward over land in summer and equatorward in winter. Valleys can suffer temperature inversion, trapping fog and pollution.
Pressure and winds
The pressure belts are the equatorial low (ITCZ), subtropical highs, subpolar lows and polar highs. The Coriolis force deflects winds to the right in the northern hemisphere. Planetary winds are trades, westerlies and polar easterlies; local winds include land–sea breeze and the daytime anabatic valley breeze.
Precipitation
Convectional rain falls in equatorial afternoons; orographic rain wets windward slopes such as Mawsynram; cyclonic rain comes with fronts and depressions. India's monsoon is a seasonal wind reversal driven by differential heating and the ITCZ shift.
Worked example
Sea level reads 30 °C; a hill station at 1,800 m would read 30 − (1.8 × 6.5) = 30 − 11.7 = 18.3 °C.
Traps
- Saying temperature falls with height in the stratosphere - ozone reverses that.
- Confusing relative humidity (percentage of saturation) with absolute humidity (mass per volume).