The UK remains in the grip of a cold snap, with temperatures falling as low as -10ºC overnight in parts of the country.
However, the snowfall of last week has largely been replaced by rain, with the Environment Agency issuing dozens of flood warnings.
On Wednesday morning there were 30 flood warnings in effect, with the most serious in the South West and the Midlands.
The heavy rainfall – despite temperatures being around freezing – has led people to question whether it is possible for it to be too cold to snow. Here’s the science behind snowfall explained.
Can it be too cold to snow?
Many people believe that the temperature must be below freezing for snow to fall, but the Met Office explains that this is not actually true.
“Precipitation falls as snow when the air temperature is below 2°C. It is a myth that it needs to be below zero to snow,” the forecaster says.
“In fact, in this country, the heaviest snowfalls tend to occur when the air temperature is between 0-2°C. The falling snow does begin to melt as soon as the temperature rises above freezing, but as the melting process begins, the air around the snowflake is cooled.”
If the temperature is warmer than 2°C then the snowflake will melt and fall as sleet rather than snow, and if it’s warmer still, it will be rain.
It can, however, occasionally be too cold to snow – or at least too cold to make snow likely.
Matt Peroutka, a meteorologist at the US National Weather Service’s Techniques Development Laboratory in Maryland, explains to Scientific American: “The atmosphere must contain moisture to generate snow – and very cold air contains very little moisture.
“Once the air temperature at ground level drops below about -20ºC, snowfall becomes unlikely in most places. Therefore, significant snowfall at such very low temperatures is rare.”
What do we need for snow to fall in the UK?
The most important ingredients for snowfall are the air being cold enough and a supply of moisture.
The Met Office said: “To get cold air across the UK we need winds from the north or east. Northerly winds – air travelling from north to the south – bring the air straight from the arctic and over a cold sea to reach the UK. In winter, easterly winds – travelling for the east to the west – are cold because they arrive from the cold continental interior of mainland Europe.
“The most common wind direction in the UK is south-westerly though, so more often than not we get relatively mild air from the Atlantic bringing rain, rather than this cold air from the north and east which often turns any rain to snow.”
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.
If skies are clear, temperatures can fall gradually day by day because the sun is weak and there is little cloud to keep in any heat at night.
The Met Office adds: “What about the moisture? Often with the cold easterly winds, and the air travelling over so much dry land, there is very little moisture in it to form the snow and we end up with some crisp winter sunshine instead.
“We either require the cold air to meet a rain-bearing weather front and turn it into snow, or for the cold air to pick up enough moisture from its short journey across the North Sea, to form showers.”
The effects of air rising up hills and mountains must also be considered. As it’s normally colder higher up in the atmosphere, when the air rises up a hill, it becomes colder, and condenses to form cloud and precipitation.
This precipitation will either be rain or snow, depending on just how cold the air is, and where the “freezing level” is.
This is the part of the atmosphere where the air temperature is at 0°C. This can be as low as 200ft or so above sea level on some days, the Met Office explains.