Module 13 - Meteorology
Sea Fog
Sea fog is most commonly advection fog: warm, moist air moving over a colder sea surface. The air is cooled to its dew point from below, causing water vapour to condense into fog. Around UK coasts this is common in spring and early summer when the sea is still cold but milder moist air arrives from the south or west.
Sea fog can form and persist even when the gradient wind is not calm, because the wind is transporting the moist air over cold water. It is often stubborn on windward coasts and near headlands, and it may thicken quickly after a wind shift brings moist air over a cold sea area.
Warning signs include a small temperature-dew point spread, moist warm air over cold sea, poor visibility in nearby reports, and low stratus over the water. Treat sea fog as a passage-planning hazard: prepare radar/AIS, sound signals, navigation lights, safe speed, extra lookout, and a realistic diversion plan before visibility collapses.
Key points
- Sea fog often forms when warm moist air passes over colder sea
- It is common in spring and early summer around cold coastal waters
- It can persist in a breeze because the wind supplies moist air
- Watch temperature-dew point spread, nearby visibility reports, and low stratus
- Prepare restricted-visibility procedures before entering fog
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