Atmosphere, Climate and Meteorological Processes
This domain covers atmospheric composition and structure, the dynamics that create winds and precipitation, the climate system and the evidence and mechanisms of climate change. PAU questions often ask you to explain a weather situation in terms of pressure, humidity and stability, or to separate natural from anthropogenic forcing.
Composition and structure
Dry air is about 78% N₂, 21% O₂, 0.93% Ar and roughly 0.04% CO₂, plus variable water vapour. The atmosphere is layered by temperature gradient: troposphere (temperature falls, average lapse rate about 6.5 °C/km, all weather), stratosphere (temperature rises because of ozone absorption), mesosphere and thermosphere. Pressure falls roughly exponentially with altitude.
Dynamics
- Forces on air: pressure gradient (from high to low), Coriolis (deflection, zero at the equator), friction (near the surface). Balanced flow gives geostrophic wind aloft.
- Global cells: Hadley, Ferrel and Polar, producing the ITCZ near the equator, subtropical highs around 30° (including the Azores High that dominates Catalan summers) and the polar front with its jet stream.
- Local and regional winds: sea breezes; in Catalonia the Tramuntana (cold, dry, northerly), the Mestral and the Llevant (humid easterly bringing coastal rain).
- Stability: an air parcel is unstable when its lapse rate exceeds the environmental lapse rate; conditional instability produces cumulonimbus and hail. A DANA (isolated depression at high levels, formerly called gota freda) brings torrential autumn rainfall to the Catalan coast.
Climate system and change
Climate depends on latitude, altitude, distance from the sea, ocean currents and atmospheric circulation. Catalonia has a Mediterranean climate (Csa) with dry summers and a second rainfall maximum in autumn. Forcings and feedbacks: greenhouse gases absorb outgoing infrared radiation; water-vapour feedback amplifies change; ice–albedo feedback does the same; aerosols and cloud changes partly offset it. Evidence comes from instrumental records, ice cores, tree rings and sea-level measurements.
Worked example
It is October and the western Mediterranean is unusually warm, with a cold, isolated depression aloft over the area. Reasoning: warm sea surface supplies latent heat and moisture; the cold air aloft increases the environmental lapse rate and therefore instability; weak upper-level flow means the system moves slowly and rain falls repeatedly over the same catchment. Prediction: convective storms with high rainfall intensity and flash floods on small coastal basins, plus a high risk on the Ebro Delta and in rambles.
Traps that cost marks
Saying temperature always decreases with altitude above the troposphere; confusing the ozone layer with the greenhouse effect; treating the greenhouse effect as wholly anthropogenic (it is natural; human emissions enhance it); forgetting Coriolis is zero at the equator.