A rare 'thundersnow' warning has been issued by the Met Office today. Thundersnow - as the name suggests - involves heavy snowfall rather than rain as the primary precipitation, often accompanied by thunder and lightning. The unusual weather phenomenon occurs when the atmosphere is turbulent enough to form cumulonimbus clouds and typically happens around 10 times a year worldwide. Some areas - such as the North York Moors and the Scottish Highlands - expecting snow showers today and into