Katabatic and anabatic winds
1The lesson in plain words
Cold air slides down slopes, often at night: a katabatic wind. Warm air creeps up sunny slopes by day: an anabatic wind.
Katabatic winds
On a clear night, high ground cools quickly. The air touching it cools, becomes denser and slides downhill under gravity, like water. This is a katabatic wind. It gathers in valleys and pours out of their mouths onto the sea.
How strong
Ordinary night-time drainage is often light to moderate and strongest late at night or near dawn. Over snow or glaciers, cold air can drain downhill by day too. Steep valleys can concentrate the flow. Severe outflows can reach gale force and extend well offshore; there is no fixed safe distance.
Anabatic winds
By day, sunny slopes warm the air above them. It rises up the slope as an anabatic wind. Its strength depends on heating and terrain, and it often joins the sea breeze.
Fall winds
When a large pool of cold air builds up behind a mountain range and spills over, it can fall to the coast as a violent, gusty fall wind. The bora of the Adriatic is the best-known example in Europe; the Named winds lesson describes it.
At anchor
Look at the land around an anchorage. A valley or gorge opening onto the bay can send a cold, strong gust down at night, even in a calm forecast. Choose an anchorage away from the valley mouth, with a seabed your anchor can grip and room for your boat to swing. Set an anchor alarm.
2Key data
- Katabatic
- Cold air flowing downhill, mostly at night
- Anabatic
- Warm air flowing uphill by day
- Strongest katabatic
- Off snow, glaciers and steep valleys; can reach gale force
- Where
- Often concentrated at valley mouths; offshore reach varies
3Key points
- 1
Katabatic: dense cold air slides downhill, often at night.
- 2
Anabatic: warm air rises up sunny slopes by day.
- 3
Katabatic winds pour out of valley mouths and can be sudden and strong.
- 4
A fall wind such as the bora is cold air spilling over mountains.
- 5
Avoid anchoring off a valley mouth on clear nights.
4Frequent mistakes
Wrong: A calm forecast means a calm night in any anchorage.
Right: A katabatic wind can pour down a valley at night even when the gradient wind is light.
Wrong: Anabatic winds are the strong night winds off mountains.
Right: Anabatic winds blow uphill as sunny slopes heat the air. Katabatic winds flow downhill as air cools, often at night.
5Practice case & exam keys
In early spring you anchor in a Norwegian-style fjord inlet below snow-covered mountains. The forecast is light and variable and the sky will be clear. A steep valley opens just upwind of your spot. What should you consider?
Show the answer
Clear sky and snow cover make strong night cooling likely. Cold air will drain down the valley and can pour out across the anchorage as a strong, gusty katabatic wind, perhaps around dawn. Move away from the valley mouth, use enough anchor chain or rope for the depth and expected wind, with good holding, and set an anchor alarm.
Exam keys
- Katabatic: cold, dense air flowing downslope, mostly at night; strongest off snow and steep valleys.
- Anabatic: warm air flowing upslope by day; strength depends on heating and terrain.
- Fall winds such as the bora are cold air spilling over mountains to the coast.
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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