Climatology
This section covers Climatology: atmospheric structure, insolation and the heat budget, pressure and winds, humidity and condensation, cyclones, the monsoon and climate change. Expect definition, diagram and calculation questions.
Core rules
- The troposphere cools at the normal lapse rate of about 6.5°C per km; the stratosphere contains the ozone layer and warms with height.
- Insolation varies with latitude, angle of incidence, day length and albedo. The heat budget balances incoming shortwave and outgoing longwave radiation.
- Pressure belts alternate: equatorial low, subtropical high, subpolar low, polar high. Coriolis force is zero at the equator and maximum at the poles.
- Relative humidity = actual vapour ÷ saturation capacity × 100. Dew point is reached when air cools to saturation.
- Tropical cyclones need sea-surface temperature above about 27°C, latent heat and Coriolis force; they do not form within about 5° of the equator. Temperate cyclones have fronts. The Indian monsoon is a seasonal wind reversal driven by differential heating, the ITCZ shift, Tibetan Plateau heating and jet streams.
Worked example
Air at 30°C contains 12 g/m³ of water vapour; saturation capacity at 30°C is 30 g/m³. RH = 12 ÷ 30 × 100 = 40%. If the air cools to 20°C, where capacity is 17 g/m³, RH = 12 ÷ 17 × 100 ≈ 71%. Cool it to 10°C, capacity 9 g/m³: actual vapour exceeds capacity, so condensation occurs; the dew point lies between 10°C and 20°C.
Traps
- Saying RH is constant when temperature falls; vapour content may be unchanged but RH rises.
- Placing the ozone layer in the troposphere.
- Claiming Coriolis force is strongest at the equator.
- Describing the monsoon as only rainfall rather than a seasonal reversal.