Energy balance and general circulation

The tropics receive a surplus of solar energy and the poles a deficit, so heat is redistributed by horizontal and vertical transfers in the atmosphere and by ocean currents.

The tri-cellular model describes three circulation cells in each hemisphere — Hadley, Ferrel and Polar — producing surface pressure belts (equatorial low, subtropical high, subpolar low, polar high) and prevailing winds.

High in the troposphere sit the jet streams and upper westerlies, whose speed and position reflect the pressure gradient and the Coriolis force (deflection: right in the Northern Hemisphere, left in the Southern).

Descending air

Factors influencing air temperature

CCEA lists five controls on air temperature:

• Latitude — the angle of the Sun and depth of atmosphere traversed.
• Distance from the sea / continentality — the sea moderates temperatures; interiors are more extreme.
• Altitude — temperature falls with height (the environmental lapse rate averages about 6.5 °C per km).
• Ocean currents — warm currents (e.g. the North Atlantic Drift) raise coastal temperatures.
• Seasonality — the tilt of the Earth changes day length and Sun angle through the year.

Calculate — lapse rate

Calculate — annual temperature range

How precipitation forms

Precipitation forms when air is forced to rise, cool and condense. Three mechanisms:

• Convectional — surface heating makes air rise; common in the tropics and in summer thunderstorms.
• Orographic (relief) — air is forced up over high ground, giving wet windward slopes and a dry rain shadow to leeward.
• Frontal — warm air is forced to rise over cold air at a front, as in a depression.

Sort the rainfall type

Tap a cause, then tap the type of precipitation it produces.

Depressions and anticyclones

The weather of the British Isles is dominated by mid-latitude systems, steered by the Polar Front Jet Stream and built from contrasting air masses.

• Depressions — areas of low pressure; warm and cold fronts bring cloud, rain and changeable, windy weather as air rises.
• Anticyclones — areas of high pressure; sinking air gives settled, dry, stable weather (heatwaves in summer, frost and fog in winter).

You interpret these on surface pressure (synoptic) charts and satellite imagery.

Match the weather feature

Tap an item on the left, then its matching partner on the right.

Reading a synoptic chart

ENSO and tropical cyclones

El Nino Southern Oscillation (ENSO): in an El Nino year, weakened trade winds let warm water spread east across the Pacific, shifting rainfall and disrupting weather worldwide; La Nina is the opposite, cooler phase.

Tropical cyclones (hurricanes/typhoons) form over oceans warmer than about 26–27 °C, drawing energy from latent heat as moist air rises around a calm central eye, ringed by the eyewall and spiralling rainbands.

CCEA requires a case study of one recent hurricane, tropical cyclone or typhoon (for example Hurricane Katrina in 2005 or Cyclone Haiyan in 2013) — its impacts and the management strategies in place to reduce them.

Keep figures honest: describe impacts and evaluate management qualitatively; do not invent death tolls or damage costs.

Cyclone formation

Anticyclone in winter

The big ideas to know

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