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How do you forecast snow?

Forecasting snow will always be a challenge in the UK. This is because of the UK's geographical position between the Atlantic Ocean and continental Europe and the variable weather patterns we experience.

The type of weather we get in the UK depends on where the air has come from. The coldest directions in the winter are generally when the wind comes from the Arctic Circle to the north or from the continent to the east.

When the UK sees these conditions, it brings a higher chance of colder weather - and the possibility of snow.

When will it snow?

For most of us it’s usually pretty exciting to see snow in the UK, because it doesn’t happen all that often. Unlike, say, Canada or mainland northern Europe, which are geographically continental, the UK being an island surrounded by water causes all sorts of problems. The main reason for this is because sea temperatures in winter are higher than the land, so you either have to be up a mountain or have seriously cold air in place to see snow. With our dominant wind being a mild south-westerly, seeing cold air itself is a bit of a rarity too.

What are the ingredients for snow? Essentially, cold air and moisture. Let’s tackle the cold bit first.

To get cold across the UK we need winds from the north or east. Northerly winds are normally cold whatever time of year because they come from the Arctic. In winter, easterly winds are cold because they arrive from Russia, Siberia, eastern Europe etc. all of which get cold in the winter months. However, there is another way which requires very little wind at all - high pressure that becomes established across the UK for a long time in winter. Provided that skies are clear, temperatures can fall gradually day-on-day because the sun is weak and there is little cloud to keep in the heat.

What about the moisture? Well, to get any type of precipitation we need air to rise, cool, condense and form clouds so it has somewhere to fall from. This can either happen in the form of bands of rain moving in, from showers, or even from the effects of hills and mountains. However, if our cold air doesn’t move over any water it has a limited supply of moisture available. This is most obvious with cold easterly winds, which are dry because they’ve moved over a huge land mass and only a relatively small stretch of the North Sea.

As it’s so cold high up in the atmosphere, most precipitation either starts off as snow or supercooled raindrops. As it falls to earth, it moves through warmer air most of the time and melts. Depending on the temperature of the air near the ground we either see rain or sleet or hail. If the air temperature stays below 2 °C, then we’ll normally see snow.

Sounds doable so far, but because of that sea surrounding us, there’s usually time for any rain-bearing weather to warm up a bit before it reaches us. And it only takes a bit; sometimes a fraction of a degree is the difference between rain and snow. That’s what makes forecasting snow difficult (and often frustrating!).

Aside from moving to a colder country, there are things that make forecasting snow a little more straightforward. If you live on a hill then you’re more likely to see snow simply because the air is colder the higher you go. We’re not talking mountain tops either; sometimes you only need to be 100 to 200 metres up. Also, living away from the coast helps because the warm seas help to keep adjacent land and air temperatures that bit higher.

One thing forecasters look for when forecasting snow is something called the “freezing level”. This is a part of the atmosphere where the air temperature is at 0 °C. The lower the freezing level, the less time and distance there is for precipitation to turn to rain before it reaches the ground. Even if the freezing level is quite high, it turns out that heavy precipitation can drag down cold air and keep snow falling lower down. If the precipitation stays heavy the freezing level can fall lower and lower.

The difficulty comes when you have a band of rain moving in. Weather systems tend to have warm air and moisture wrapped up in them. This helps them develop as part of their life cycle, and while the moisture part is good, the warm air isn’t as it’ll start moving the freezing level upwards. Remember, a weather system arises from different types of air coming together, and often a band of rain meeting cold air will give snow. For a time. This is because as the warm air arrives it slides up over the cold air. The precipitation is falling slightly ahead into the colder air lower down but in time the air mixes and rain becomes more likely. You’ll often find that there’s a fine line between who sees snow and who sees rain.

So although there are fairly simple ingredients for snow, the UK’s position on the Earth creates a load of obstacles that make forecasting it quite complicated.

Did you know?

Thirty centimetres of fresh fallen snow has about the same water equivalent as 25 mm of rainfall.

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