|Road transport disrupted by poor visibility in northern India [AP]|
Fog has been affecting huge swathes of Europe, northern India and northern China over the last few days, causing major disruption to road and air transport. But this is no coincidence; fog is often a feature of November weather in the northern hemisphere.
This particular type of fog is known as radiation fog – fog which is formed by the cooling of air near the ground by long wave radiation out into the atmosphere.
Whilst radiation fog can form in any month of the year, it is, for many parts of the northern hemisphere, particularly common in the month of November. But why should this be?
Put simply, fog is low cloud which forms at the surface. Radiation fog forms in the evenings and nights as the air cools. Now all air holds a certain amount of water vapour. The greater the temperature, the more moisture the air can hold.
So if the air is cooled sufficiently, the water vapour will condense to form droplets. Further cooling will cause the air to become saturated and fog will form. The temperature at which this occurs is referred to as the fog point.
Under perfectly still, clear conditions any fog will be short-lived. It tends to drop out of the atmosphere, because of gravity, to give a heavy dew. So for fog to persist or develop there must be a light wind to mix the air and bring further moisture down from higher up in the atmosphere.
So it is clear that foggy conditions are more likely to form under clear skies, with light winds, moist air and during a prolonged period of nighttime cooling.
All these conditions point towards weather patterns we typically experience in November. Late autumn is a time when high pressure is often established across the interiors of the large continental land masses of North America, Europe and Asia.
Under these areas of high pressure winds are often very light and cloud is notable by its absence, favouring nighttime radiation.
But high pressure is usually associated with high summer, when fog is notable by its absence, so there must be some additional factors at play in November.
The first of these is that we are moving into the time of year when the nights are long and daylight is at a premium. In other words, there is more time for cooling to take place. But this cannot be the sole factor, as nights are longer in December and just as long in January.
The second factor is that the air in November is still relatively warm with the cold air of winter still to arrive. Therefore, the air can hold a lot of moisture. But when ‘radiation’ conditions are good, that moisture is turned into fog droplets. As the winter months arrive and the air turns colder, there is less moisture held in the air available for fog formation.
So whilst radiation fog conditions can occur throughout the year, the perfect combination of long nights, moist air, clear skies and light winds is most likely to be found in November.