There’s no denying that we have seen some cold weather this winter with plenty of frost, ice and in many cases, snow. However, the last week has been cold – arguably perhaps feeling colder than any other time this winter – but we haven’t seen any evidence of this on the ground in the way of frost. So how is this possible?
For a classic frosty night we need a few ingredients: low temperatures, clear skies, calm winds and moisture. A clear, calm night gives excellent radiation conditions – by this we mean that the heat absorbed by the Earth’s surface during the day escapes readily back into space and allows temperatures to fall. If the temperature falls to the dew point (the temperature to which air must cool for it to become saturated with water vapour) moisture will condense and form droplets on the ground’s surface. When temperatures fall below freezing the droplets freeze and we get frost.
So what about the last few days? They have been cold but there hasn’t really been any prolonged or hard frost. How come? Well, much of Scotland and Northern Ireland has had the required ingredients and been frosty, but the rest of the UK has only had low temperatures. Much of England and Wales have seen a fair amount of cloud and some brisk winds.
Surface pressure chart from 25 February 2013
Cloud acts as a blanket and although temperatures have fallen during the night-time, cloud cover has stopped them falling well below freezing and therefore made it difficult for a thick frost to form. The wind is also important as it mixes the lower part of our atmosphere. Rather than having cold air pooling in one place and causing low temperatures, the wind can bring less cold air from another location or even bring it down from the upper atmosphere. This also helps to keep temperatures from falling too low. However, easterly winds this week have certainly made it feel very cold indeed!
Visible satellite image from 25 February 2013
Lastly, the air near the surface has been relatively dry. This is important because it means the temperature of the air must fall very low in order to reach its dew point. The cloud and wind has stopped this from happening easily and therefore reduced the risk of frost.
Cold weather, then, brings lots of different tastes of winter, especially to the UK, and we have seen nearly all of them this season. More information on all types of weather can be found here.