What is fog?

You may be familiar with fog, but have you ever wondered what it is, or how it forms?

Put simply, fog is very low lying cloud. Like a cloud, fog is made up of millions of tiny water droplets or ice crystals that form when air is cooled close to the Earth’s surface and water vapour within it condenses. These droplets are light enough to remain suspended in the air There are several different types of fog, each forming under different conditions. Radiation fog forms in situ at night under clear, cold and relatively calm conditions, when water vapour close to the cold ground condenses as it cools. Radiation fog can be very slow to clear during the winter months when the sun is weak. Advection fog forms when moist air flows over a relatively cold surface and is cooled. When this happens the air can no longer hold moisture and so condenses, forming fog. A common example of this is coastal fog where relatively warm moist air passes over the cool surface of the sea. Coastal fog usually occurs in the spring and summer months when conditions begin to warm up but the sea which warms more slowly, stays relatively cold

You can find more information on different types of fog, on the Met Office website:

http://www.metoffice.gov.uk/learning/fog

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