By Tom Frame
Following on from Chris Westbrook’s excellent exposition on the beauty of snow flakes, nature has been kind enough to provide many of us with our first snowfall of the winter this weekend – Reading included (if only a few flakes). This has been the first real cold spell since the last winter, and the abruptness of the change will have made it stand out to many.
So why such an abrupt change from what so far has been an unusually mild November?
The answer here is relatively straightforward and has to do with the location of the British Isles on the boundary between the large maritime expanse of the Atlantic Ocean and continental Europe. This location makes the UK particularly sensitive to changes in wind direction. In winter when the wind is coming from the west or south-west the air will tend to be relatively warm and moist, when the wind is coming from the east or north-east, the air will tend to be relatively cold and dry, and when the wind is coming from the north or north-west it will tend to be cold and moist.
The reason for these differences lies in the surface that the air must travel over to reach the UK. The temperature and humidity of the air is constantly being modified by the surface of the Earth. But this modification does not happen instantly, so the air arriving in the UK carries with it the temperature and humidity properties given to it from the surface it was over previously.
As dry air passes over oceans its humidity increases due to evaporation of water from the ocean surface. Therefore air arriving in the UK from the Atlantic will tend to be relatively humid. Over land there is less liquid water available for evaporation, so dry air tends to remain dry and humid air may lose humidity through rainfall. So if the air arriving in the UK has travelled over land it will tend to be drier.
The temperature of the air also depends on the type of surface it has travelled. Air travelling over a warm surface will tend to be relatively warm as it is heated by the surface, whereas air travelling over a colder surface will receive less heat and therefore be relatively colder. How warm or cold the surface is depends a lot on the speed with which it heats up and cools down due to radiative heating and cooling. The surface of dry land tends to heat up and cool very rapidly, so its temperature varies a lot between day (when it is heated by solar radiation) and night (when it cools by emitting radiation). The surface of the ocean however does not change as much because it is able to store large amounts heat. This is why the sea feels warmer at night and in winter. In winter the surface of the ocean tends to be warmer on average than the surface of the land, therefore the air travelling over the ocean will be warmer than average compared with air travelling over land. Temperature also varies significantly with latitude, with the pole being much colder than the equator, therefore the air coming from the north is always colder than air coming from the south.
So far this November, the UK has largely been receiving air from the west and south-west, making temperatures relatively mild for the time of year. However this weekend winds changed to a northerly direction, so that the air travelled down from the Arctic bringing very cold and fairly moist air, and snow and sleet to southern England for the first time this winter.