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Emily - Weatherview

Do You Have The Foggiest Idea About Fog?

There’s nothing that makes you feel more wintery than looking out of your window to a thick white blanket of fog, the streetlights look hazy, everyone is wrapped up and it has that definite Christmas feel to it. However many people don’t really know what fog is or how it’s formed.

Fog is when water is cooled at dewpoint (the temperature that air can no longer hold all of its water vapour,) and condenses to form droplets suspended in mid air, it is essentially a cloud that is lying at ground level, however fog is specifically defined as when visibility is reduced to less than 1km for pilots and 200m for drivers. Anything less than this is officially termed mist. There are many different types of fog depending on how they form. When warm air passes over a cold surface advection fog forms, it is most common around coasts where there is a large land-sea temperature difference, for example there is commonly a thick shroud of fog that hugs the California coastline due to the condensation over the cold California current in the Pacific. Widespread coastal fog also forms where moist winds blow across a cold ocean current. In polar areas cold air from land surfaces may chill the vapour evaporated from the sea surface to produce steam fog. A similar phenomenon can be seen over rivers and lakes after a cold night.

Upslope fog forms when light winds push humid air up a slope until it reaches dewpoint and condenses. This type of fog develops quite far from the peak and can sit over large areas. Freezing fog occurs when the water droplets in the air become supercooled which means that are actually below freezing point, and they then condense onto a nucleus such as dust particles which results in any obstacle that comes into contact with freezing fog to become covered in a coat of ice.

Radiation fog is probably the most common type, especially here in the UK. It forms during clear, calm, cold nights when the earth’s surface is emitting long wave radiation back into space. Condensation first occurs on the ground and then the air above it is cooled by conduction, radiation fog can be anywhere between 3ft and 1000ft high. As cold air sinks into valleys and hollows these become reception areas, other than that fog remains stationary, this is why when driving you can often dip in and out of areas of particularly thick fog.

So why has the last week been so foggy? Over the last weekend (19th/20th Nov) there was a high pressure system sat over the central and eastern areas of England with a warm humid southerly air flow flowing in a clockwise direction around the centre. Overnight the clear skies and cold temperatures resulted in the air reaching dewpoint, creating the moisture in the air to condense and form the fog and mist patches. The winds remained light into the week which has caused the fog to linger in places and some rain has only increased the moisture in the air. However winds have now got slightly stronger and the winds have developed into south easterly which brings drier air through, resulting in many areas of fog beginning to clear.


However we are likely to experience more fog through the winter so it is important to remember to stay safe, especially when driving, keep an eye on your speed as fog can give the illusion of driving slower than you actually are, don’t hang on other cars tail lights, always drive with dipped headlights as full beam will reflect off the fog and create a white wall effect and most importantly beware of freezing fog creating ice patches on the road. 
Emily Quiggin
Contributing Weather Forecaster
Friday November 25th 2011
Disclaimer.  The opinions expressed by the above forecaster are not necessarily those of Positive Weather Solutions.