How fog forms: advection, radiation and sea smoke
1The lesson in plain words
Fog forms when air cools to its dew point, the temperature at which water vapour starts to condense, or gains moisture. At sea, advection fog is common.
Advection fog (sea fog)
Advection fog forms when warm, moist air flows over a colder sea and cools to its dew point. It can cover large areas, last for days and persist in moderate or fresh winds. Sunrise does not guarantee clearance. It can thin or lift if the air warms or dries, or mixing increases. A change of air mass often helps; a wind shift helps only if conditions change.
Radiation fog
Radiation fog forms over land on clear nights with light wind, roughly 2–5 knots, when the ground cools and chills the moist air above it. It does not form over the open sea, because the sea hardly cools at night. But it drifts out of rivers and estuaries into harbours and a few miles offshore. It usually clears in the morning as the sun warms the land.
Sea smoke
Sea smoke, or steam fog, forms when very cold air flows over much warmer water. Water evaporates into the cold air and condenses at once, like steam over a hot bath. It is usually shallow, so mastheads may stick out above it. It is common in winter at high latitudes and in sheltered harbours on frosty mornings.
Frontal and coastal fog
Frontal fog can form when rain evaporates into colder air below and saturates it. Hill fog forms as moist air rises over hills and cools into cloud. Coastal headlands may disappear into low cloud while nearby water remains clear, or sea fog may spread ashore. Do not assume clear water means visible landmarks.
2Key data
- Advection (sea) fog
- Warm moist air over a colder sea; may persist in fresh wind
- Radiation fog
- Clear, light-wind night over land; usually clears as land warms
- Radiation fog wind
- Roughly 2–5 knots
- Sea smoke (steam fog)
- Very cold air over much warmer water; usually shallow
- Frontal fog
- Evaporating rain can saturate colder air below a warm front
3Key points
- 1
Advection fog: warm, moist air over a colder sea; sunrise does not guarantee clearance.
- 2
Sea fog can persist in moderate or even fresh winds.
- 3
Radiation fog forms over land on clear, calm nights and drifts into harbours.
- 4
Sea smoke: very cold air over much warmer water.
- 5
Hill cloud and sea fog moving ashore can hide coastal landmarks.
4Frequent mistakes
Wrong: Fog always clears when the sun gets up.
Right: Radiation fog over land usually clears as the land warms. Sunrise does not guarantee sea-fog clearance; warming, drying or increased mixing may help.
Wrong: Fog only forms in calm conditions.
Right: Radiation fog needs light wind, but sea fog can persist in moderate or even fresh winds.
Wrong: Radiation fog forms far out at sea on clear nights.
Right: The sea hardly cools at night. Radiation fog forms over land and drifts out from estuaries and harbours.
5Practice case & exam keys
In May off north-west Spain, a mild, moist south-westerly of force 4 blows over a sea that is still cool. Fog has covered the coast since dawn. A crew member says it will burn off by noon. Is that likely?
Show the answer
This is likely to be advection fog. It may persist while moist air continues over the cold sea. Do not rely on clearing by noon. Use updated forecasts and observations before expecting clearance, and plan for poor visibility.
Exam keys
- Advection (sea) fog: warm moist air over a colder sea; can persist in wind. Sunrise does not guarantee clearance.
- Radiation fog: forms over land on clear, light-wind nights; usually clears as the land warms.
- Sea smoke (steam fog): very cold air over much warmer water.
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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