Fog at Sea — Types, Forecasting and What to Do

Sea (advection) fog forms when warm, moist air blows over a colder sea; it is common in spring and early summer, can last for days and is not cleared by wind. Radiation fog forms over land on clear, calm nights; it drifts into harbours and estuaries but usually burns off by late morning. If fog comes down at sea, slow to a safe speed, sound your fog signal, post a lookout, use radar and AIS, and get out of shipping lanes.

A sailing yacht with an orange sail almost hidden in thick fog off Brixham, Devon — A yacht sailing in fog off Brixham, Devon. In fog, visibility can drop below a cable within minutes. Photo: Forbes Johnston from Winchester, UK, CC BY 2.0.

Two panels: sea fog forming as warm moist air blows over a cold sea, and radiation fog forming over land on a clear calm night and drifting onto the water — Sea fog and radiation fog form in different ways and clear in different ways.

Main fog types for sailors.
Type	How it forms	When it clears
Advection (sea) fog	Warm, moist air moves over a colder sea	Only when the air mass changes; persists in wind up to force 5 or 6
Radiation fog	Land cools on a clear, calm night	Usually burns off by late morning; rarely goes far offshore
Frontal fog	Rain falling into cold air ahead of a warm front	When the front passes
Sea smoke	Very cold air over relatively warm water	As the air warms

Forecasting fog

Watch the dew point. When the air temperature is close to the dew point and the air is moving over colder water, sea fog is likely. A warm-sector south-westerly over the cold spring sea of the western Channel is a classic recipe.

Sea fog over grey water with rocky islands fading into the mist — Sea fog hiding a coastline. Marks you could see a minute ago disappear. Photo: Theo Crazzolara, CC BY 2.0.

If fog comes down

Fix your position immediately and note it in the log.
Switch on navigation lights and start your fog signal: one prolonged and two short blasts every 2 minutes under sail.
Slow to a safe speed (Rule 6) and post a lookout, listening as well as looking.
Use radar and AIS, and hoist a radar reflector. Make sure you can be seen.
Everyone on deck wears a lifejacket. Have the liferaft ready and the engine running.
Head for shallower water, out of shipping lanes, or anchor if safe.
Under Rule 19, if you hear a fog signal forward of the beam, slow to bare steerageway and navigate with extreme caution.

The fog rules in full

Related guides

Frequently asked questions

Does wind blow fog away?

Not sea fog. Advection fog can persist in winds of force 5 or 6. Only a change of air mass, such as a cold front, clears it.

What is the sound signal for a sailing boat in fog?

One prolonged blast followed by two short blasts, at least every 2 minutes.

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