A depression passes: the sequence of a front
1The lesson in plain words
If a young Atlantic low passes north of you and both fronts cross your position, expect a warm front, a warm sector, then a cold front.
Ahead of the warm front
Here an eastward-moving Atlantic low passes north of you in the Northern Hemisphere, and both fronts cross your position. Watch for high wisps (cirrus), a thin veil (cirrostratus) sometimes with a halo, a grey sheet (altostratus), then thick rain cloud (nimbostratus). The barometer falls steadily. The wind backs (turns anticlockwise), for example from south-west to south, and strengthens. Rain becomes continuous and visibility falls.
At the warm front
As the warm front passes, the wind veers (turns clockwise), for example from south to south-west. The fall of the barometer slows or stops. The rain eases to drizzle and the temperature rises.
In the warm sector
In the warm sector the wind stays fairly steady from the south-west. Pressure is steady or falls slowly. The air is mild and moist: low sheet cloud (stratus), drizzle and poor visibility, with fog where it crosses a cold sea.
At the cold front
The cold front often passes quickly. The wind veers sharply, for example from south-west to west or north-west, often in a squall (a sudden, strong increase in wind). Heavy rain falls, sometimes with thunder or hail. Then the barometer starts to rise, often quickly, and the temperature drops.
Behind the cold front
Cooler air brings heap clouds, showers, gusts and very good visibility between the showers. An occluded front gives a mix of both fronts: a long band of rain and a wind veer, but no warm sector. If an eastward-moving low passes to your south instead, the wind backs steadily, for example from east to north-east to north, with no warm sector.
2Key data
- Before the warm front
- Pressure falls; wind backs and freshens; rain sets in
- Warm front passing
- Wind veers; fall slows; drizzle; temperature rises
- Warm sector
- Steady SW wind and pressure; stratus, drizzle, poor visibility
- Cold front passing
- Sharp veer, squalls, heavy rain; pressure rises, colder
- Behind the cold front
- Showers, gusts, very good visibility
- Eastward-moving low passing south (NH)
- Wind backs steadily; no warm sector
3Key points
- 1
Ahead of a warm front: falling pressure, backing wind, thickening cloud.
- 2
At each front the wind veers (low passing north, Northern Hemisphere).
- 3
Warm sector: mild, steady wind, low cloud, poor visibility.
- 4
Cold front: sharp veer, squalls, then rising pressure and showers.
- 5
Eastward-moving low passing south of you in the north: the wind backs instead.
4Frequent mistakes
Wrong: When the rain stops in the warm sector, the bad weather is over.
Right: The warm sector is often a lull. The cold front, with squalls and a sharp wind shift, is still to come.
Wrong: A rising barometer after a cold front means the wind will drop at once.
Right: A quick rise often comes with strong, gusty wind behind the front. Wait for the wind to ease before increasing sail.
Wrong: Every front gives exactly the textbook sequence.
Right: Real fronts vary from weak to violent. Use the sequence as a guide and confirm with the forecast and barometer.
5Practice case & exam keys
You are at anchor in a bay open to the north-west. The forecast gives a low passing to your north tonight: "Southwest 5 to 7, veering northwest 6 to 8 after the cold front." Is the anchorage safe?
Show the answer
No. The bay is sheltered now from the south-west, but after the cold front the wind will veer to the north-west and blow straight in at up to force 8, often in squalls. The wind would push you towards the shore at night. Move before dark to a harbour or anchorage sheltered from the north-west.
Exam keys
- Warm front ahead: cloud thickens and lowers, pressure falls, wind backs and freshens.
- Warm front passes: wind veers, pressure steadies, drizzle; cold front: sharp veer, squalls, pressure rises.
- Eastward-moving low in the Northern Hemisphere: passing north, wind veers at the fronts; passing south, it backs.
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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