Marine fog and visibility: what small craft should know

How sea fog forms

Air always contains some invisible water vapour, and warm air can hold more of it than cold air. If air is cooled far enough, it reaches its dew point, the temperature at which the vapour starts to condense into tiny droplets. When that happens at the surface, the result is fog. The types of fog differ in what does the cooling.

Advection means horizontal movement. Advection fog forms when warm, humid air is carried over a colder surface and chilled from below until it reaches its dew point. Over the sea this is often called sea fog. It is typical of spring and early summer in many temperate regions, when the air has warmed but the sea is still cold, and of coasts with cold currents or cold water welling up from below. Where and when it occurs varies widely.

Two features make sea fog awkward. First, wind does not necessarily clear it. The wind is what delivers the supply of moist air, so sea fog can persist in a moderate breeze. Stronger winds tend to lift it into low cloud, but not always. Second, the sun does not necessarily clear it either. Fog over land goes when the sun heats the ground, but the sea surface temperature hardly changes over the course of a day, so the cooling from below continues. Sea fog can last for days, until the wind changes direction or a different air mass arrives.

Sea fog is often patchy, lying in banks with clear gaps between them. Along the shore it may draw back out to sea as the land heats up during the day and return in the evening, so a clear beach is no proof of a clear sea.

Radiation fog and other types

Radiation fog forms over land on clear, calm nights, when the ground loses heat to the sky and cools the air resting on it. It collects in valleys and low ground and can drift over rivers, estuaries and harbours. It usually thins and clears during the morning as the sun warms the ground and the breeze picks up, though in winter it can linger for much of the day.

A third type, sometimes called steam fog or sea smoke, appears when very cold air passes over much warmer water, for example on lakes and rivers on cold mornings. It is often shallow but can still hide the surface.

Visibility in air is not visibility underwater

The visibility in a weather forecast is atmospheric: how far you can see through the air. It says nothing about how far a diver or snorkeller can see underwater. Underwater visibility depends on sediment stirred up by waves and currents, plankton, river runoff and the state of the tide, none of which appears in a weather forecast. A perfectly clear day can have murky water.

What fog means on the water

In fog the shoreline, the horizon and other vessels disappear together. Without a compass or an electronic position it is easy to lose all sense of direction. Sound becomes unreliable: an engine or a horn can be heard but its direction is hard to judge. A kayak, paddleboard or small open boat sits low, shows up poorly or not at all on radar, and may never be seen by the crew of a larger vessel that cannot stop or turn quickly in any case.

If fog is forecast or visible offshore, the simplest decision in a small craft is not to go, or to stay very close to shore in shallow water where larger vessels cannot operate.

Slow down. Travel at a speed that lets you stop within the distance you can see.

Show the correct navigation lights and be ready to make sound signals. Carry a horn or whistle. The requirements depend on the type of craft and the waters, so check the rules that apply where you are.

Make yourself easier to detect. A radar reflector mounted as high as practical helps, but never assume you have been seen.

Keep out of shipping channels, ferry routes and busy harbour entrances.

Know where you are before the fog arrives. Carry a compass, know the bearing back to shore, and do not depend on a single electronic device.

Wear lifejackets, and make sure someone ashore knows your plan.

Visibility forecasts are coarse

Fog is among the harder things to forecast. Whether it forms depends on small differences in temperature and humidity near the surface, and it is patchy on a scale far finer than a weather model can represent. Marine forecasts usually describe visibility in broad categories such as good, moderate or poor, or mention fog patches, and the exact terms and thresholds differ between weather services. A visibility figure from a model is a rough indication for a wide area.

The weather forecast on a CatchWindow location page gives the general picture, but it is not a navigation tool and cannot say where a fog bank will sit. For that, use the official marine forecast, any warnings in force, and your own eyes.

In short

Sea fog forms when warm, moist air meets cold water, and it can persist through wind and daylight.

A hazy or vanishing horizon, or a grey bank offshore, is a reason to stay in or come in early.

In a small craft the main danger is not being seen. Stay shallow, stay close and stay out of the way of larger vessels.