Depressions: birth, life and movement
1The lesson in plain words
A depression grows on the polar front as a wave with a warm and a cold front. It deepens, occludes, then slowly fills.
What a depression is
A depression, or low, is an area of low pressure with closed isobars. In the Northern Hemisphere the wind circles it anticlockwise. Mid-latitude depressions bring most of the wind and rain to north-west Europe.
Birth on the polar front
It usually starts as a wave on the polar front. Warm air pushes towards the pole and cold air pushes towards the equator. Pressure falls at the tip of the wave. A warm front forms ahead of the warm air and a cold front behind it. Between them lies the warm sector.
Deepening and occlusion
A low is deepening when its central pressure falls. Winds often strengthen if the pressure gradient steepens. The cold front moves faster than the warm front and catches it. Warm air is lifted off the surface and an occluded front forms. The low is usually deepest near this stage. It later fills: central pressure rises. It often slows and winds ease, but check the local gradient and forecast.
Movement
A young low usually moves in the direction of the isobars in its warm sector, in the North Atlantic often east or north-east at 20–30 knots. Old, occluded lows slow down and may stall. New secondary lows can form on a trailing cold front and develop fast.
Where the wind is
The wind is strongest where the isobars are closest together. Explosive deepening means a central-pressure fall of at least about 24 hPa in 24 hours at 60° latitude, with a lower threshold farther south. Such rapid development can turn a moderate forecast into a storm. Always check the latest warnings.
2Key data
- Deepening / filling
- Central pressure falling / rising
- Usual movement
- Along the warm-sector isobars; often E or NE at 20–30 kn
- Occlusion
- Cold front catches warm front; warm air lifted aloft
- Explosive deepening
- At least about 24 hPa in 24 hours at 60°; lower farther south
- Strongest wind
- Where the isobars are closest together
3Key points
- 1
A depression usually starts as a wave on the polar front.
- 2
It has a warm front, a warm sector and a cold front.
- 3
Deepening: central pressure falls; filling: it rises. Winds often strengthen or ease respectively.
- 4
The cold front catches the warm front to form an occlusion.
- 5
A young low moves along its warm-sector isobars.
4Frequent mistakes
Wrong: A deepening low means the pressure is rising.
Right: Deepening means central pressure falls; filling means it rises. Winds often strengthen or ease respectively, but local wind depends on the pressure gradient.
Wrong: The strongest wind is always at the centre of a low.
Right: Near the centre the wind can be light. The strongest wind is where the isobars are tightest.
Wrong: Once a low occludes, the danger is over.
Right: A low is often deepest around occlusion, and secondary lows can form. Keep checking the warnings.
5Practice case & exam keys
At 1200 today, the synopsis reads: "Low 1002, 300 miles west of Ireland, deepening rapidly, expected 975 Irish Sea by 1200 tomorrow." What does that tell you about the next day?
Show the answer
The low is forecast to deepen by 27 hPa in 24 hours, meeting explosive-deepening criteria at this latitude. It is heading east towards the Irish Sea. Severe winds may develop, but their location depends on the pressure gradient. Check the wind forecast and warnings, and arrange shelter before conditions deteriorate.
Exam keys
- Depressions form as waves on the polar front, with warm and cold fronts and a warm sector.
- Deepening means central pressure falls; filling means it rises. Winds often strengthen or ease, but check the local gradient and forecast.
- Occlusion: the cold front overtakes the warm front; young lows move along warm-sector isobars.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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