Who, What, Why: What is thundersnow?

Snow falls in Omagh, Northern Ireland Image copyright PA

Thundersnow has been reported in parts of the UK. What exactly is this dramatic-sounding weather event?

First there was the frostquake. Then the firenado. Now another weather-related portmanteau has seized the headlines. Thundersnow has been reported in parts of Cornwall, South Wales, Tayside and Teesside. It sounds dramatic. What does it entail?

Basically, it is the same as a thunderstorm, except that snow falls instead of rain. It occurs when the atmosphere is unstable and the layer of air closest to the ground is cold enough to create snow, but still warmer than the air above it. "In this instance, the heating is coming from the sea which is still quite warm," says BBC weather presenter John Hammond. As the warm air rises, water droplets condense to form cumulonimbus clouds. Lightning occurs when these rub against each other, and thunder is the sound of the lightning.

It can look more spectacular than a normal electric storm. When thundersnow occurs during night time, the lightning appears brighter because it is reflected against the snowflakes. But the snowfall also serves to muffle the thunder, which will typically be heard no more than three miles away. We hear it after the lightning strike because sound moves more slowly than light.

Thunderstorms are more likely to occur in warm, humid conditions and during the summer. But while thundersnow is relatively unusual, it is not unheard of, either (and thundersleet - its slushier, drabber cousin). "It's less common in the winter, but it's still quite typical," says Hammond. It was reported in Glasgow as recently as February 2013, and he thinks the UK could witness some more this weekend.

Reporting by Jon Kelly

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