Anticyclones in summer and winter

Overview

Surface winds in an anticyclone in the Northern Hemisphere, blocking anticyclone and why does it matter and weather of a summer anticyclone in the British Isles.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an anticyclone, and what are its typical pressure, size and speed of movement?

   Model answer

   An anticyclone is an area of high atmospheric pressure in which air is subsiding from above and diverging outwards at the surface. Central pressure is typically 1020 to 1040 millibars, it may be 2000 to 3000 kilometres across, and it moves only slowly or stays almost stationary, often persisting over the same region for days or even weeks. Its isobars are widely spaced and roughly circular.

2. 2

   Why do anticyclones give clear skies and dry weather?

   Model answer

   Air in an anticyclone is descending. As it sinks it enters higher pressure and is compressed, so it warms at the dry adiabatic rate of about 10 degrees Celsius per 1000 metres. Warming raises the air's capacity to hold water vapour, so its relative humidity falls, it moves further from its dew point and any cloud in it evaporates. There is no mechanism forcing air upwards to cool and condense, so precipitation is very unlikely and skies stay clear, though in summer strong surface heating can break the subsidence inversion and set off thunderstorms.

3. 3

   Describe the surface winds in an anticyclone in the Northern Hemisphere.

   Model answer

   Air arriving from above must spread outwards at the surface, so it diverges from the centre. The pressure gradient force pushes it outwards and the Coriolis effect deflects it to the right of its path, turning the flow until it circulates clockwise while still drifting outwards. Because the pressure difference between the centre and the edge is small and spread over a great distance, the isobars are widely spaced, the pressure gradient is gentle and the winds are light or even calm. In the Southern Hemisphere the deflection is to the left, so the equivalent flow is anticlockwise and outwards.

4. 4

   Why do anticyclones last so much longer than depressions?

   Model answer

   A depression is driven by a temperature contrast between two air masses that it destroys as it occludes, so it burns out its own energy source within days. An anticyclone contains a single uniform air mass with no fronts and no contrast to consume, and the subsidence that maintains it is self-sustaining, since warming and drying prevent any cloud or uplift that might disturb it. Anticyclones are also slow-moving and can block the path of approaching depressions, forcing them to divert, which allows the same settled conditions to persist for a week or more.

5. 5

   What is a blocking anticyclone and why does it matter?

   Model answer

   A blocking anticyclone is a slow-moving or stationary high that sits across the normal west-to-east track of Atlantic depressions and forces them to divert to the north or south of it. Because it can remain in place for one to three weeks, the same weather type is repeated day after day. Over the British Isles this can mean a long summer heatwave and drought, or in winter a prolonged spell of hard frost and fog, while the diverted depressions bring persistent heavy rain to whichever region they are steered towards.

6. 6

   Describe the weather of a summer anticyclone in the British Isles.

   Model answer

   Clear skies allow strong insolation through the long summer day, so temperatures climb rapidly and can exceed 25 to 30 degrees Celsius in southern and eastern districts. Winds are light and variable, humidity is often low, and the sky is blue or hazy with only shallow fair-weather cumulus that fades by evening. Nights are short and, because some warmth is retained and the air is often humid, they can be uncomfortably warm rather than cold. Coastal areas may develop a sea breeze in the afternoon that holds temperatures several degrees lower than inland.

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