The structure and energy budget of the atmosphere, air masses, the formation of depressions and anticyclones, and the weather they bring to the British Isles.

What this dot point is asking

CCEA wants you to explain the atmospheric energy budget, identify the air masses that influence the British Isles, describe and account for the formation and weather sequence of mid-latitude depressions and anticyclones, and apply this to the weather experienced in Northern Ireland, using located events such as Storm Ciara (February 2020) and the December 2010 cold spell.

The atmospheric energy budget

Solar radiation heats the Earth unevenly because the curvature of the Earth spreads insolation over a larger area at high latitudes. There is a net energy surplus between roughly $40^\circ$ N and $40^\circ$ S and a deficit poleward of those latitudes. To balance this, heat is transferred polewards by winds, ocean currents (the North Atlantic Drift keeps Northern Ireland mild for its latitude) and air masses. Some incoming short-wave radiation is reflected (albedo), some absorbed by clouds and gases, and the surface re-radiates energy as long-wave radiation, some of which is trapped by greenhouse gases.

Air masses

The dominant air mass over Northern Ireland is tropical maritime, arriving on the prevailing south-westerlies, giving mild, moist, cloudy weather and much of the annual rainfall. Polar maritime (Pm) air from the north-west brings cold, showery, bright conditions. The rare but severe polar continental air brings the cold, dry, snowy spells of winter, as in December 2010.

Depressions

Mid-latitude depressions form along the polar front where warm tropical maritime air meets cold polar maritime air. A wave develops on the front; warm, less dense air is forced to rise over the denser cold air; surface pressure falls; and air converges and rises, spiralling anticlockwise in the northern hemisphere.

As a depression passes, the warm front arrives first with gradually thickening cloud (cirrus to nimbostratus) and steady rain; the warm sector brings milder, drizzly conditions; then the cold front brings a narrow band of heavy, thundery rain from cumulonimbus, falling temperature and clearing skies. Eventually the faster cold front catches the warm front to form an occlusion, ending the system.

Anticyclones

Anticyclones are areas of high pressure with gently descending, stable air, which warms and dries as it sinks, suppressing cloud formation. They bring settled weather: in summer, hot, dry and sunny conditions, sometimes ending in thunderstorms; in winter, cold, clear nights with frost and fog because strong radiation cooling and a temperature inversion trap moisture near the surface.

Examples in context

Example 1. Storm Ciara, 8 to 9 February 2020 (a deep depression). Ciara was a violent mid-latitude depression with a central pressure below $950 \text{ hPa}$. The steep pressure gradient produced damaging south-westerly winds and a sequence of fronts that dropped heavy rain across already saturated ground. In Northern Ireland the River Bann, Faughan and Camowen caused localised flooding, transport was disrupted and power lines came down. The event illustrates the formation and impact of a fast-moving, deep depression along the polar front.

Example 2. The December 2010 cold spell (a blocking anticyclone and polar continental air). In December 2010 a persistent blocking high to the north and east drew bitter polar continental air across the British Isles. Aldergrove (Belfast International) recorded temperatures below $-10^\circ$C and Northern Ireland saw weeks of lying snow and ice, with burst pipes and major disruption to water supplies. This located example shows how high pressure and a cold, dry source region combine to give a prolonged severe winter spell, the opposite of the changeable depression weather.

Try this

Q1. Define the term air mass. [2 marks]

• Cue. A large body of air with broadly uniform temperature and humidity inherited from its source region.

Q2. Describe the weather changes as a cold front passes. [3 marks]

• Cue. A narrow band of heavy, sometimes thundery rain, a sharp fall in temperature, the wind veers, then clearing brighter showery skies.

Q3. Explain why an anticyclone in winter can bring fog and frost rather than sunshine. [4 marks]

• Cue. Descending stable air, light winds, a surface temperature inversion, strong overnight radiation cooling, moisture trapped near the surface.