Highs, ridges, troughs and cols
1The lesson in plain words
A high brings sinking air, light wind and settled weather, but also haze and fog. Ridges, troughs and cols are the shapes between systems.
Anticyclones
A high, or anticyclone, is an area of high pressure with closed isobars. In the Northern Hemisphere the wind circles it clockwise, spiralling slightly outwards. Its isobars are usually wide apart, so wind near the centre is light. Highs move slowly and can stay for days, steering lows around them.
Weather in a high
Air sinks and warms, making it stable: vertical mixing is limited. An inversion, a layer where temperature increases with height, can trap haze and moisture below. Fair weather is common, but persistent low cloud can bring anticyclonic gloom. On calm nights, cooling near the surface can form fog. Strong winds can blow at the edge where a high lies beside a low.
Ridges
A ridge is a tongue of high pressure stretching out from a high; its isobars bulge outwards from the centre. It often brings fair weather and lighter winds. Between travelling lows, this may be a short lull. Check the forecast for how long it will last.
Troughs
A trough is a tongue of low pressure stretching out from a low. Many troughs carry a front or a line of showers. As one passes, the wind often veers in the Northern Hemisphere and showers or squalls may follow.
Cols
A col is the area between two highs and two lows, arranged like a cross. Pressure gradients are weak, so wind is light and variable. Cols can bring fog in winter, or thunderstorms in summer. Wind direction can change without warning when you leave one.
2Key data
- High (NH)
- Clockwise, slightly outwards; light wind near centre
- Sinking air
- Stable air: fair, hazy or cloudy; calm-night cooling can form fog
- Ridge
- Extension of high pressure; often fair with lighter winds
- Trough
- Tongue of low pressure: showers, wind shift
- Col
- Between two highs and two lows: light, variable wind
3Key points
- 1
Northern Hemisphere: wind circles a high clockwise.
- 2
Sinking air in a high is stable: fair, but hazy or foggy.
- 3
Strong wind can blow on the edge of a high next to a low.
- 4
A ridge often brings fair weather and lighter winds; a trough brings showers and a wind shift.
- 5
A col gives light, variable wind, fog or thunderstorms.
4Frequent mistakes
Wrong: A high always means sunshine and perfect visibility.
Right: Sinking air traps haze and can give persistent low cloud or fog, especially in winter.
Wrong: There is never strong wind under high pressure.
Right: Where a high lies beside a low, the isobars can be close together and the wind strong.
5Practice case & exam keys
In October a large high sits over France for several days. The forecast for the Gulf of Lion gives light variable winds, "fog patches" at night and visibility "moderate or poor, occasionally very poor". You plan a night passage to the Balearics. Does the high make this an easy night passage?
Show the answer
No. Light wind does not guarantee good visibility. Stable air can trap haze; fog can drift off the cooled land or form where moist air crosses cooler water. Expect poor visibility and possible motoring. Choose a better departure window if your crew or equipment cannot manage those conditions, and check for stronger wind near the high's edge.
Exam keys
- Northern Hemisphere highs: clockwise circulation, light winds near the centre, slow movement.
- Sinking air is stable: fair weather, but haze, low cloud and fog are common.
- Ridge = extension of high; trough = extension of low; col = weak gradient between two highs and two lows.
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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