Why is snow so challenging to forecast in the UK?
As predicted, some parts of the UK had their first snow of the winter season overnight and this morning, leading to problems with ice and travel delays.
Amounts of snow varied hugely, with some places having a covering, some just falling snow, and others rain or sleet.
If you ask any weather forecaster who works in the UK, they will tell you that it can be very challenging to forecast snow here. The conditions are often marginal, which means that, sometimes, predictions can go wrong.
I thought I’d take a little time to journey through and explain the variety of factors than can determine whether or not snow falls, which will hopefully show how complicated it can be.
Temperature of the air
Temperature is probably one of the most crucial factors when determining whether or not snow is likely to fall. As a general rule of thumb, snow in the UK tends to fall when the temperature is 2C or less.
It may sound odd that snow can fall when the temperature is above freezing, but you have to remember that the temperature of the air even a few tens of metres above the surface is colder than at ground level.
As a result of this, snowflakes can easily remain intact down to the surface, although they do start to melt. This is why snow that falls with a temperature above freezing tends to be fluffy and wet, as partially melted snowflakes stick together.
Snow that falls with a temperature of 0C or below doesn’t experience any melting. Therefore, it is drier and more powdery – like the snow that is great for skiing.
When the temperature is in the range of 2-4C, sleet or rain is likely. However, if rain falls for long enough, the temperature of the air will start to fall.
This is because water from rain drops will start to evaporate and cool the air – just like the process of evaporating sweat on your skin cools you down in summer.
If this evaporative cooling continues for long enough, the rain can eventually turn to snow.
So as you can see, the temperature is crucial as to what falls from the sky, with even half a degree making a huge difference.
The height of a location above sea-level is another highly influential factor on the likelihood of snow. In most cases, the higher up a hill or mountain you go, the colder it gets – roughly by 1C for each 100 metres.
This is why snow is always more likely over hills and mountains than lower down. What you may be surprised to know is by how much the chance of snow increases with altitude.
If there is a 20 per cent chance of snow at sea-level, this rises to 35 per cent at 100m, 60 per cent at 200m and around 75 per cent at 300m.
This shows how a journey that travels over hills can start off as a rainy one, but can easily turn into a snowy one with a relatively small increase in altitude.
Urban areas are renowned for being slightly warmer than rural areas during winter, due to the large amount of heat generated by city life – buildings, industry and transport.
However, to complicate things further, there can be temperature fluctuations within cities too.
As an example, a few years ago, I left work after a night shift at BBC Television Centre in Shepherd’s Bush and it was raining outside.
By the time I had reached Ealing – just five miles further west into the suburbs, it was snowing heavily, with a covering of snow on the ground. I phoned work to let them know that it was snowing, and they said that it was still raining in Shepherds Bush!
Such differences in precipitation over a short distance are extremely hard for weather computer models to capture, adding to the uncertainty in snow forecasts.
Distance from the coast
The UK is an island surrounded by relatively warm water, which means that in winter, temperatures in coastal areas are kept slightly higher than further inland.
Again, with temperature being so crucial in determining what falls from the sky, it means that whilst snow falls inland, a zone of around 5-10 miles from the coast, will see rain or sleet instead.
Wind strength and precipitation intensity
As mentioned earlier, if rain falls for long enough, evaporative cooling can gradually lower the temperature, resulting in it turning to snow.
The intensity of the falling precipitation is also influential. Light rain is generally less likely to turn to snow than heavier rain in this way, because less moisture is available for evaporative cooling to take place.
All of the above show just how many factors need to be taken into consideration when forecasting snow. It can be a real challenge and illustrates that whilst we get it right most of the time, sometimes, things don’t go as planned.
If you’ve got any questions about snow or winter weather, feel free to put them to me on Twitter – @liamdutton