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The air around us can hold a certain amount of water vapour. The warmer the air the more water vapour it can hold. When the air can't hold any more water vapour, tiny floating water droplets form. We see these tiny floating water droplets as fog (or clouds when they are higher up).

You can think of the air around us as being a bit like a sponge; it can only hold so much water. When the air is warmer the sponge is bigger; when the air is cooler the sponge is smaller. When the sponge is full, you see the excess water around it – fog.

Relative humidity tells us how full the air is of water vapour, so we know when to expect fog.


Sea Fog (Advection Fog)

Fog forms at sea when very humid air is blown from a warm region of water to a cooler region of water. As the air cools it can't hold as much water vapour and so fog is formed. Take care on days when humidity is high. To see the current sea temperatures click here. It's worth noting that the sea is cooler very close to the coast at night, especially in Summer.


Radiation Fog

Radiation fog forms over the land but may drift or be carried a short distance out to sea. It usually forms on a clear night with light winds during Autumn, Winter and Spring. The clear night means the ground gets very cold, which in turn makes the few centimetres of air above the ground cold too. On a still night the effect is dew or frost. On a night with a light breeze the effect is a fog - the light breeze gently mixes the cold air on the ground with warmer air higher up. On a windy night the fog is lifted clear from the ground and becomes low cloud.


Frontal Fog

Frontal fog is quite rare. It occurs when rain falling along a weather front saturates the air causing a fog along the front.


Fog forecasts

The shipping and inshore forecasts describe the visibility in the last part of the forecast for each area, using the following scale:

Forecast Word Description
Fog Visibility less than 1,000m
Poor Visibility between 1,000m and 2 nautical miles
Moderate Visibility between 2 and 5 nautical miles
Good Visibility more than 5 nautical miles

Forecast examples

Inshore forecast:

From Cape Wrath to Rattray Head including Orkney.
24-hour forecast: Wind: east to southeast 6 to gale 8 gradually decreasing 4 or 5. Weather: cloudy, periods of rain. Visibility: moderate or good. Sea State: rough to very rough decreasing moderate.

Shipping forecast: