24 Sep 2013

What is fog?

With the autumn equinox having passed us by on Sunday, we are now well and truly in the season of mists and mellow fruitfulness.

As the days shorten and the nights get longer, the weather is starting to show its autumnal face, which in recent days has meant that mist and fog are making more frequent appearances.

Fog is no stranger to us at this time of year, but if someone asked you to define fog in terms of how far ahead you can see, would you be able to do it?

Did you also know that there are actually four types of fog that occur in the UK – each named based on the way that it forms?


What is the definition of fog?

The official definition of fog is when visibility is less than 1000m, which is often used with reference to aviation.

However, for the general public and motorists, thick fog is considered to be a visibility of 200m or less, with dense fog 50m or less – the point at which severe disruption to transport occurs.

How does fog form?

The mass of air over a particular location has its own characteristics, based on where its origins lie. So, in autumn, air that arrives across the UK from the south west is warm and holds a lot of moisture.

For each given mass of air, it will have something called a dew point. This is the temperature at which the air, when cooled, will become saturated.

fog_drops_g_wpWhen the air becomes saturated, it is no longer able to hold any more moisture, so the moisture condenses – producing tiny water droplets that become suspended in the air around us.

It is this abundance of tiny water droplets suspended in the air that reduces the visibility, forming fog. As you would imagine, the greater the number of water droplets, the lower the visibility and the denser the fog.

Radiation fog

This type of fog occurs mainly during autumn, winter and early spring in the UK under clear skies. As the nights are longer, there is more time for temperatures to lower, as heat from the earth’s surface radiates back into space.

With a greater amount of cooling taking place, the chance that the dew point will be reached increases, with fog likely to form.

Advection fog

This type of fog occurs most often in winter and spring, when the land can be very cold and then warm, moist air from the Atlantic moves over it.

fog_coast_g_wpAs the warm, moist air moves over the cold land or water, it is chilled from below, causing it to condense into tiny water droplets, resulting in fog formation.

This is how sea fog forms and if the wind blows in the right direction, this sea fog and be blown on to the land – especially around coastal areas. This is also the cause of fog over snow when a thaw takes place.

Upslope fog

This type of fog forms when warm, moist air is blown up the side of a hill or mountain.

As the temperature decreases with height, the warm, moist air is cooled as it travels further and further upwards.

The moisture in the air is eventually cooled enough to condense, forming hill fog – something that is hazardous for hill walkers or drivers travelling over higher routes.

Evaporation fog

This type of fog is caused by cold air passing over warmer water or moist land. In a way, it’s like an upside down version of advection fog.

As the cold air moves over the moist surface, some of the water evaporates into the air, which then cools and condenses, causing fog to form.

This type of fog is more common in autumn, when the sea is still warm from summer, yet has cold air moving over it from the east or north east.

So, the next time that you see some fog, have a think and see if you can work which of the four types it could be.

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I also post regular updates on Twitter – @liamdutton

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