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Winds Around Depressions

On a weather map, a depression appears as a pattern of roughly concentric – and more or less circular – isobars.

About 500 m above the surface the wind blows very nearly parallel to the isobars. This is called the geostrophic wind. Using a hand bearing compass to measure the clouds’ movement gives it direction.
Over water, surface drag reduces the Coriolis effect, so the surface wind is angled inwards by about 10-20°.
Over land, there is even more drag, so the surface wind is angled inwards by about 20-40°.

This helps depressions to fill more quickly over continental land-masses than over open ocean.

Wind strength is determined by the differences of pressure within the depression.

Where the pressure changes a lot in a short distance (shown by closely-spaced isobars) the winds will be strong. Where the isobars are far apart, winds will be light.

  • In general, the surface wind over the sea is about 70% of the wind speed at 500 metres.

  • Over land, it is more variable, but may be only about 50% of the wind speed at 500 metres.

Frontal systems

A front is the word used to describe the leading edge of a moving air mass. A warm front is the front of a warm air mass, and a cold front is the front of a cold air mass, so as a depression passes to the north of you, you are likely to experience eacht in turn.

On a weather map, a warm front is usually marked with blobs or spikes point in the general direction in which the front is moving.

Why fronts produce rain

When warm air catches up with colder air (at a warm front) it slides upwards. At a cold front (where cool air catches up with warmer air) the cold air slides underneath, forcing the warm air upwards.

As the warm air rises, it expands and cools. Any moisture in the air condenses to form clouds and then drops of rain, hail and snow.

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