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.