What are the perfect snow conditions?

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  • Snow over the North York Moors on Saturday 11th February 2012. Picture: Anna Gowthorpe/PA Wire

    Map to illustrate where the heaviest snow will be in an Arctic Maritime air mass

    Map to illustrate where the heaviest snow will be in an Polar Continental air mass

    Map to illustrate where the heaviest snow will be in an Polar Maritime air mass

  • What are the perfect snow conditions?
    11.02.2012 16:08

    It has been a fairly snowy week across the UK. However, compared to November and December 2010, it seemed like there was very little. This has led to questions such as what are the perfect conditions for heavy snow? When is snow most likely to fall? 

    Snow is usually associated with the winter season, with the snowiest month being January. Snow will fall when the air temperature is below 3C at ground level, but it is most common when air temperature is between -2C and 2C. It is important to understand the geography of Britain when forecasting snow as it usually influences where the snow will be heaviest. For example coasts are susceptible to snow showers due to the combination of moisture from sea surfaces and greater instability of air near to the relatively warm sea. This is often due to the direction from which the snow spreads.

    Of the six air masses that affect the UK, three are often associated with snow. The Arctic Maritime (AM) air mass, which sweeps in from the north and can spread snow showers onto north facing coasts. The Polar Maritime (PM) air mass travels from the north-west and the heaviest snow falls over the high ground in northern and western areas, but it can penetrate to the south coast. The Polar Continental (PC) air mass spreads from the east, which is often associated with the Siberian cold weather. The PC air mass is often fairly unstable causing snow showers along the east coast. As each air mass affects areas of the UK differently it is hard to say which air mass causes the greatest amount of snow. For example in the south-east, 27% of snowfall comes from AM, PC accounts for 40% and 27% occurs when a warm or occluded front from a PM air mass moves east. However, in general terms the heaviest snow will occur when the UK is being affected by the Arctic air mass by frontal systems.

    Snow will generally settle when the ground temperature is below 0C. However, there are a couple of exceptions. When there is heavy snowfall and the surface start above 0C, after a time, because melting snow flakes cool the road, it then begins to settle. Similarly it is often believed that snow will not settle on wet roads. However, this is not true because if the air temperature is below 0C, there will be snow, which when reaching a wet surface will become slushy and then freeze allowing the road to become snow covered.

    Forecasting snow is difficult for many reasons. The main reason is that snow does not occur very frequently in the UK, and therefore forecast models in the past did not have enough understanding of these events to be as accurate as with rainfall. However, models have improved in their treatment of snow over the last 4 winters, giving much more local detail. In addition to this a slight change in the temperature or humidity can determine whether there will be snow, rain or sleet. It is important to access carefully the chance of snow because local authorities and transport companies have to be prepared to grit roads and run extra services.  

    By: Sally-Jean Webb