The Atmosphere and Climate: Composition, Dynamics, Climate Change and the Canarian Context
Domain 6 tests atmospheric composition and structure, the greenhouse effect, general circulation, and climate change evidence - then applies it to the Canarian climate, which is dominated by the trade winds and the subtropical anticyclone. Exam items often ask you to explain why the islands are mild, humid on the north and arid in the south.
Composition and structure
Dry air: ~78% N₂, 21% O₂, 0.93% Ar, 0.04% CO₂, plus variable water vapour (0–4%). Layers: troposphere (weather, lapse rate ~6.5 °C/km), stratosphere (ozone layer absorbs UV), mesosphere, thermosphere, exosphere. Pressure falls with altitude.
Energy balance and the greenhouse effect
The Earth absorbs shortwave radiation and emits longwave infrared; greenhouse gases (H₂O, CO₂, CH₄, N₂O, O₃, CFCs) absorb and re-emit IR, raising surface temperature. Albedo (ice, clouds, snow) reflects shortwave radiation. Positive feedbacks: water vapour, ice–albedo, permafrost methane. Negative: increased cloud albedo, CO₂ uptake by ocean and biosphere.
Circulation and the Canarian climate
Hadley, Ferrel and polar cells; the ITCZ; the Azores subtropical anticyclone drives the NE trade winds (alisios) over the Canaries, producing a temperature inversion at ~1000–1500 m capped by the 'mar de nubes', which feeds the monteverde on north-facing slopes. Result: windward north is humid and mild; leeward south is arid; summits above the inversion are dry and sunny. Ocean upwelling and the Canary Current keep temperatures equable. Calima brings Saharan dust and heat. Climate change: warming trends, sea-level rise, ocean acidification, more heatwaves and drought, altered Saharan dust transport.
Worked example
Why is Anaga green and the south of Tenerife arid at the same altitude? Reasoning: the trade winds arrive moisture-laden from the NE; Anaga faces the wind, forcing orographic ascent and condensation below the inversion, giving cloud and drizzle. Southern slopes are in the rain shadow and lie below the subsidence inversion, so air sinks, warms and dries. Rain therefore falls mainly on the windward side, and the south depends on groundwater and desalination.
Traps that cost marks
Saying the greenhouse effect is caused by the ozone hole. Placing the ozone layer in the troposphere. Claiming the alisios come from the Sahara. Confusing weather with climate.
- CO₂ is only ~0.04% of air but climatically decisive.
- The trade-wind inversion keeps cloud below ~1500 m in the Canaries.
- Calima = Saharan dust intrusion, worsening air quality and reducing visibility.
- Albedo and water vapour are the main feedback multipliers.
- Canary Current upwelling moderates coastal temperatures and supports fisheries.