FOG can make the landscape look magical – but also causes travel chaos for drivers.
But what exactly is it? Not many people actually know how fog is formed and how it differs from mist.
What is fog?
Fog is like a low-level cloud that originates from a local body of water – such as a lake, marsh or the sea.
It differs from regular clouds because it contains water droplets from a number of different sources.
When water vapour in the air condenses into tiny droplets of liquid water, they stay suspended in the air and fog is formed.
Fog contains up to 0.5ml of water per cubic metre.
If you were to fill an Olympic sized swimming pool with fog and then somehow condense it, you would be left with around 1.25 litres of water – just over 2 pints.
The foggiest place in the world is an area of the Atlantic Ocean called Grand Banks off the coast of Newfoundland, Canada.
The area forms the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south.
This creates over 200 days of fog every year.
Fog warnings are often issued when driving conditions are difficult and dangerous.
What different types of fog are there?
Fogs are generally named after the physical process that takes place to produce them.
The main types of fog are as follows:
This type of fog usually appears in the winter and is aided by clear skies and calm weather.
The cooling of land overnight by thermal radiation cools the air close to the surface.
This reduces the ability of the air to hold moisture and allows condensation and fog to occur.
Radiation fogs usually disappear as the ground warms after sunrise.
Freezing fog is made up of supercooled water droplets which remain liquid even though the temperature is below freezing point.
One of the characteristics of freezing fog is that rime – a frost formed on cold objects made-up of feathery crystals – is deposited on the windward side of surfaces such as lampposts, pylons and transmitting masts.
Valley fog forms where cold dense air settles into the lower parts of the valley condensing and forming fog.
It is usually a result of warm air passing over the valley.
It can last for several days during calm conditions in winter.
This type of fog occurs when most air passes over a cool surface and is cooled.
A common example of this is when a warm front passes over an area with snow cover.
It is also common at sea when moist tropical air moves over cooler waters.
If the wind blows in the right direction then sea fog can become transported over coastal land areas.
Upslope fog or hill fog forms when winds blow air up a slope.
The air cools as it rises and allows moisture in it to condense.
This type of fog is caused by cold air passing over warmer water or moist land.
It often causes freezing fog and sometimes frost.
When warm water evaporates into the low air this type of fog occurs.
What is the difference between mist and fog?
The only difference between mist and fog is visibility.
It is called fog when visibility is 1km or less and called mist when visibility remains above 1km.
In driving terms it tends to be referred to as fog when visibility is 100m or less.
How do I drive in fog?
According to the Highway Code, you have to use headlights when visibility is "seriously reduced" – generally defined as less than 100 metres (about the length of a football pitch).
The AA recommends you take your time, keep a greater distance between you and the car in front and keep your front and rear fog lights on.
You should be aware of how to operate your fog lights, and take care not to rely too much on automatic lighting.
If you get to a junction and you can't see properly due to fog, stop and roll your windows down to listen to traffic.
What else does fog do?
Fog is responsible for a rare weather phenomenon called a fog dome.
The white bulbs are caused by a source of heat close to the ground that forces fog to lift away into a dome as warm air rises like a hot air balloon.
Fog also has its own rainbows called a fog bow, which look like a normal rainbow but are just plain white.
They are made up of tiny water droplets that are so small, they white-wash the fog bow.
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