Ice and smoke did make a cloak

21.02.08 16:59

Parts of the UK, especially northern England, were treated to a sharp but spectacular dose of winter this week, with the landscape turning white thanks to a combination of frost, rime and snow.

High pressure has been keeping the weather settled across the UK for some time now. However, it may be a surprise to some that it is the same high pressure system under which parts of the country saw daytime temperatures rising as high as 18 degrees Celsius last week, that led to Bramham in West Yorkshire seeing a daytime maximum of minus three Celsius on Tuesday this week.

Last week, the air circulating within the high pressure system was relatively dry, leading to fine sunny days with little cloud. This week, an injection of moisture into the level of the atmosphere closest to the surface, known as the boundary layer, led to the formation of fog and low cloud, and it was this that helped create the wintry scenes.

It wasn’t cloudy everywhere this week but the moister atmosphere helped hoar frost to form under the clearer skies across large parts of the country. Hoar frost is the feathery white frost that forms readily on cars and grass, and on untreated roads and pavements.

Where it did become foggy, rime formed as the fog droplets froze onto objects such as trees and fences. Rime is more spectacular than hoar frost as it denser and harder, and often creates beautiful icy structures on objects such as trees and fences.

The last ingredient of the winter wonderland was snow. Tuesday’s reports of falling and lying snow initially may have surprised some forecasters because the weather setup, with high pressure firmly in charge, is not normally conducive to great amounts of any precipitation, let alone snow. However, as photographic evidence proved, it was the genuine article. Despite being under high pressure, the low cloud was of sufficient depth to allow ice crystals form in enough quantity that they clumped together into snowflakes and fell to earth.

The snow appeared to be concentrated in areas of the country where heavy industry releases particulates into the atmosphere in great quantities. Particulates create a surface onto which cloud water droplets can condense, and in this case they froze and aggregated to create the snowflakes. The snow that resulted, known as ‘industry snow’, is fairly common on continental Europe and can give moderate falls.

The high pressure system has now declined and mild weather has already returned to the UK, with cloudier and windier conditions expected across the country over the next few days. The first significant rain for many days has arrived in Scotland and Northern Ireland, and will move to England and Wales during Friday.

By: Laura Gilchrist