How cold does it have to be to snow?

​Schools have closed and trains have been cancelled across the UK following the coldest night of the year so far, with temperatures dipping to below -15C in parts of Scotland.

Much of the British Isles is now covered in snow, with the Met Office predicting ice and further snowfall for every day this week. Yellow weather warnings, which mean there could be travel disruptions, are in place across the UK – and reports of Brits spending hours stuck in cars due to snowy or icy conditions have been making headlines in the last 24 hours.

Met Office meteorologist Rachel Ayers said there is a possibility that conditions will become slightly milder by the weekend, but it is “too early to be certain”, reported The Guardian.

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What is snow and how does it form?

The Met Office defines snow as “solid precipitation” that forms in “a variety of minute ice crystals at temperatures well below 0C but as larger snowflakes at temperatures near 0C”.

When these ice crystals join together, they form the unique and intricate shapes we know as snowflakes. The joining process occurs when crystals “descend through moist air that is slightly warmer than 0C” meaning they “melt around the edges and stick together to produce big flakes”.

Rain, snow and sleet are all closely related and the presence of one rather than another depends on the air temperature. Snow falls as sleet when temperatures are above 2C and as rain when temperatures are warmer than 5C.

For snow to form the deep blanket of white that can be shaped into snowmen and snowballs, ground temperatures need to be below 2C and the air temperature between 0 and 2C, explained Metro. Any warmer and the snowflakes will melt immediately upon contact with the ground.

It is a myth that temperatures need to be below zero to snow. In the UK, the heaviest snowfalls generally occur when the air temperature is between 0C and 2C, rather than below freezing.

Can it ever be too cold to snow?

Rather surprisingly, it can be too cold for snow. As the Maryland meteorologist Matt Peroutka told Scientific American, “the atmosphere must contain moisture to generate snow – and very cold air contains very little moisture”. As a result, snowfall becomes unlikely when ground temperatures drop to around -20C or colder.

That said, although snow is unlikely to form at extremely cold temperatures, there is “no such thing as too low a temperature for some sort of ice crystal to form and for such crystals to settle out and land on the surface”, added Peroutka.

However, without the necessary moisture in the air, those ice crystals cannot link together to form snow. Crystals that fall to the ground without bonding are typically described as “ice fog” rather than snow.

Are there different types of snow?

Broadly, snow can be described as either wet or dry, depending on the composition of the icy crystals. Dry snow, described by Snow Guide as the “sweetest type of snow there is”, forms when the crystals struggle to bind properly in cool air with little moisture. It’s the most coveted variety for winter sports and is also known as powdery snow.

Wet snow, which occurs when the temperature is just above freezing and so is more compact because the snowflakes melt and stick together, is best for snowball fights, said the Met Office.