Depressions in a nutshell
A depression is an area of low pressure. In the Northern Hemisphere the wind blows anticlockwise around it, slightly in towards the centre. UK depressions usually move from west to east, bringing a warm front first, then a warm sector of milder, moist air, then a cold front. The closer together the isobars, the stronger the wind.
What you'll see as it passes
| Stage | Wind | Pressure | Cloud and rain | Visibility |
|---|---|---|---|---|
| Warm front approaching | Backs and freshens | Falling steadily | High cirrus thickening and lowering to altostratus, then nimbostratus with steady rain | Good at first, worsening in the rain |
| Warm front passing | Veers | Fall slows or steadies | Rain eases to drizzle | Often poor |
| Warm sector | Steady in direction | Steady or falling slowly | Low, overcast cloud, drizzle, milder air | Moderate or poor, sometimes fog |
| Cold front passing | Veers sharply, often with squalls | Starts to rise | A band of heavy rain or showers, sometimes thunder | Poor in the rain |
| Behind the cold front | Gusty, often from the west or northwest | Rising | Clearer skies, cooler air, showers and cumulus | Good, except in showers |
This is the textbook sequence for a depression whose centre passes to the north of you. Real ones vary, and a centre passing to the south brings a backing wind instead. Use it to interpret what you see, alongside the forecast.
Warm, cold and occluded fronts
- Warm front: warm air rising gently over colder air. It brings a long, steady band of thickening cloud and rain, often many hours of it. On a chart it's a red line with semicircles.
- Cold front: cold air pushing in under warm air. It moves faster and gives sharper, shorter, more intense weather: squalls, heavy rain and a big wind shift. On a chart it's a blue line with triangles.
- Occluded front: where a cold front has caught up with a warm front and lifted the warm air off the surface. It can combine features of both. On a chart it's a purple line with both symbols.
What it means for a passage
- Watch the barometer. A steady fall with high cirrus spreading in from the west is the classic early sign of a warm front. Log the pressure every hour on passage.
- Plan for the shifts. Expect a veer at each front. Think about how a wind shift changes your lee shore and your tidal gates, not just your sail trim.
- Expect the cold front to be the rough bit. Reef early, before the squalls arrive, and brief the crew.
- Mind the visibility in the warm sector, especially near shipping lanes. See Rule 19 for fog.
For the words the forecast uses, see shipping forecast terms and the Beaufort scale.