Fog, What is it?
Weather Forecast: Widespread frost and fog. Generally frosty, foggy start, with some fog persisting. Dense patches of freezing fog across Wiltshire. But what is Fog? In a way it is just a low level cloud on the ground, where visibility is less than 1,000 metres, others may say visibility of less than 200 metres with severe disruption below 50 metres.

On a still, cloudless night the ground and air in contact with it get colder and colder. There comes a time when that air cools to its dew point, the temperature at which the invisible water vapour it contains condenses into tiny water droplets.

This type of fog is most common in winter months. It needs a good dose of warm sunlight or a refreshing breeze to clear. In the olden days it combined with smoke to become London's infamous pea-souper smog's.

The Clean Air Act 1993 put paid to those. In fact, due to central London staying so much warmer these days, ordinary fog is also virtually unknown in the capital or major towns. There are other types of fog.

Freezing Fog
Worst is freezing fog, where the water droplets freeze to coat everything in rime ice, sometimes looking attractive and feathery. Unfortunately, it is also particularly dangerous to motorists and pedestrians, as roads are icy and you can't easily see where you are walking. This type of fog is often thicker and more reluctant to clear.

Advection Fog
Advection fog occurs when warm, moist air is blown over a cold surface, such as snow-covered ground or the sea. The thickest advection fog usually forms during nights with light winds. The reason for this is that humid air near the ground isn't mixed with drier air above. Usually, the fog burns off during the day, but it can last for many days if it is thick enough to block out the sun's light.

This type of fog is known as "advection" fog. "Advection" refers to horizontal air movements, such as those that bring warm, moist air into a region. Advection fog is very common during winter warm ups and early spring thaws and can occur almost anywhere in the country. The dense fog can wreak havoc on commuters and travellers and often causes major delays at airports.

Steam Fog
Then there is steam fog, often seen after a shower over warm roads and over ponds and rivers on cold winter nights. One of the signs of autumn fall, along with falling leaves, is the steamy look that ponds, lakes and rivers take on when chilly air blows across them. The "steam" is really fog, naturally enough often called "steam fog." This kind of fog is also known as "sea smoke" when it forms over cold oceans. The process begins when cold, dry air blows over warmer water.

Some of the water evaporates into the lower layers of the air and the air is warmed by the warm water. The warmed air rises, where it mixes with colder air above. The mixing cools the air enough to begin condensing some of the newly added water vapor back into tiny droplets - fog. If you look closely, you see that the bottom of the fog is at least a few inches, maybe a couple of feet, above the water.

The fog begins forming when the rising air is high enough to be cooled. Steam fog is most common in the fall because the winds begin turning chilly, but the water is slower to cool. Eventually the water cools enough that it no longer evaporates as much moisture into cool air. Another graphic explains radiation fog, which is much more common than steam fog.

Hill Fog
Finally there is hill fog, which is simply low cloud enveloping some high ground, but dangerous nevertheless. Hill fog or upslope fog, as its name implies, is formed as mild moist air is forced to ascend a hill or mountain range. As the air moves up the windward side of the mountain it cools down, and again if the air becomes saturated then cloud is formed which, if below the top of the hills, gives fog.