What is freezing fog?

Fog has affected many areas over the last couple of days, which in some places has been freezing.

Freezing fog forms in the same way as ‘normal’ fog and is typical in winter, aided by clear skies and calm conditions. The cooling of land overnight under clear skies means that any heat radiates back into space cooling the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation of water vapour into millions of tiny water droplets to occur and fog in to form.

When temperatures are well below freezing, the tiny water droplets suspended in the air are made up of supercooled water droplets – which remain liquid even though the temperature is below freezing.  This occurs because the liquid needs a surface to freeze upon, such as a dust or pollution particles. However if there are not enough of these particles about then the water can stay as a liquid.


However droplets from freezing fog can freeze to surfaces on contact, forming rime. Often confused with frost, rime is a rough white deposit formed of feathery ice crystals. It can often be seen on vertical surfaces exposed to the wind – like lamp posts and pylons – as the supercooled water droplets freeze on contact as they drift past.

With warnings in force for fog people are advised to take extra care when driving in affected areas with journeys taking longer than usual. By being ‘weather aware’ our warnings help you prepare, plan and protect yourself from the impacts of severe weather.

Visit our website for more information on different types of fog.

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