Thunder snow!

Never heard of it before tonight, but it's awesome!

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.