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Fog at Christmas

Snow is often less of a hazard during the festive period than poor visibility.

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Fog is, in effect, low cloud. Like all clouds it has a base (the ground) and a top (here just below the top of the Wilson Bridge over the Potomac River in Washinton DC).
By Richard Angwin
Published On 5 Jan 2015

Christmas is a time of year usually associated with snow, at least in the Northern Hemisphere. Christmas cards often depict scenes of Santa pulling his sleigh through the snow. In fact, this is largely a myth created during the 19th century

December is often more associated with dull, foggy weather conditions. Santa is likely to require front and rear foglights on the sleigh if he is to make his rounds across much of Europe and North America.

Significant snowfall is usually reserved for later in the winter. In a typical winter, much of the northern hemisphere is still cooling off. This means that the air can still hold a relatively large amount of moisture. As the nights become longer (21st December was the winter solstice, the longest night) so there is more time for night-time temperatures to fall to what meteorologists refer to as the ‘fog point’.

When the fog point is reached, the air cannot hold any more moisture without condensation taking place. If there is a lack of breeze, the moisture will soon drop out of the air forming a heavy dew, or, if the air temperature is within a few degrees of freezing, a white frost, known as hoar.

But any breeze will keep the fog droplets in suspension and further cooling, or an injection of more moisture will see the fog develop and persist.

The current thick fog in northern India and Pakistan is the result of a cold wave. Lahore, in Pakistan, has had to contend with poor visibility. 
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Low temperatures across northern India have caused travel and transport problems in recent days. Temperatures in Amritsar have fallen as low as 3C, so this Sikh worshipper at the Golden Temple was certainly suffering for his religion.
As air temperatures usually decrease with height, cold air from the top of a valley will sink towards the bottom, allowing fog to form, as seen here in Izbasesti, Romania.
Cold air draining from the surrounding mountains overnight will settle over the warm waters of a lake, such as Lake Lucerne, Switzerland. The resulting fog follows the countours of the valley within which the lake is located. 
Fog in the northern cities of China is made worse by pollution. As winter begins to bite, people light fires to keep warm, often with highly-polluting lignite coal. This can result in very dense, persistent fog, or smog.
Fog in much of Europe has a major impact on travel and transport. Intelligent monitoring systems allow authorities to alert motorists to the presence of fog, thereby reducing the risk of serious accidents.
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Major sports stadia have undersoil heating to overcome the risk of losing a fixture because of a frozen pitch. Some even have covers to keep out the rain. But fog is one hazard it is difficult to legislate against.