The atmosphere, hydrosphere and climate system
This domain tests the structure and composition of the atmosphere and hydrosphere and the energy balance that drives climate. Expect quantitative items on lapse rate, albedo or the greenhouse effect, plus qualitative items on circulation and climatic variability.
Atmosphere
Composition by volume: N₂ 78 %, O₂ 21 %, Ar 0.93 %, CO₂ about 0.04 %, plus variable water vapour. Layers: troposphere (average lapse rate 6.5 °C/km, all weather), stratosphere (ozone absorbs ultraviolet, temperature rises with height), mesosphere, thermosphere and exosphere.
Energy balance and greenhouse effect
Incoming short-wave radiation balances reflected short-wave radiation (albedo, Earth average about 30 %) plus outgoing long-wave radiation. Greenhouse gases absorb and re-emit long-wave radiation, so the surface stays warmer than the effective radiating temperature. Feedbacks: water vapour and ice–albedo amplify warming; cloud behaviour and ocean CO₂ uptake remain uncertain.
Circulation and hydrosphere
Uneven insolation plus the Coriolis effect produce Hadley, Ferrel and polar cells, trade winds, the westerlies, the polar front and the jet stream. The hydrological cycle moves water between ocean, atmosphere and land through precipitation, evaporation, runoff and infiltration. Thermohaline circulation is driven by temperature and salinity; El Niño–Southern Oscillation and the North Atlantic Oscillation explain much Iberian rainfall variability.
Worked example
Air at 20 °C and 500 m rises over a 2,500 m mountain. Using 0.65 °C per 100 m, an ascent of 2,000 m gives a drop of 13 °C, so the summit air would be 7 °C if condensation is ignored. In practice condensation releases latent heat, roughly halving the saturated lapse rate, so the real summit temperature is nearer 13–14 °C.
Traps that cost marks
- Saying temperature falls with height in every layer; it rises in the stratosphere.
- Confusing the greenhouse effect with the ozone hole: different gases, altitudes and mechanisms.
- Treating weather and climate as synonyms, or citing one cold winter as evidence against warming.
- Confusing El Niño with La Niña, or placing the North Atlantic Oscillation in the Pacific.