Wednesday, 12 December 2012

Cold as ice...

There was some suggestion that last night temperatures might have plummeted to -14 degrees but although cold it didn't quite reach that temperature. At 7.30am this morning my car was recording -5 and that was certainly cold enough for me.

Just before going to bed last night I went outside to my car. It was freezing and the air was full with fog but no ice had formed on my car. This morning it was a very different picture. The Sun rose to a near cloudless sky and there was a still crisp feel about the day. The fog had lifted and the world was covered in a marvellous frost.

As you are probably aware frost occurs when air containing water vapour comes in contact with surfaces that are below the freezing point of water as well as the dew point.

The dew point is the temperature at which water vapour in the air condenses to form liquid water. If the surface that the dew is forming on is colder than the freezing point of water then the dew will form frost.

The frost forms as spiky crystals called spicules. Last night the temperature was cold for the whole night and because of the fog there was a lot of water vapour in the air, this allowed the frost to form in abundance. The direction of these frosty spikes also gives an indication of the direction of any breeze that might have been blowing in the night.

This type of frost was a thick hoar frost, more precisely air hoar relating to the frost that occurs on tree branches, spider webs and telephone wires for example.

According to my research the word hoar has Old English roots relating to an adjective describing signs of old age, as the trees look like they are covered in old white hair.

At work today I discussed with an esteemed colleague whether what had occurred today was in fact rime. Apparently this is not the case as rime is formed as a continuous think layer of ice rather than individual frozen droplets, www.weatheronline.co.uk/reports/wxfacts/Hoar-Frost.htm. Rime would be formed for instance on a ships rigging as it passes through freezing fog, the ice building up in layers.
What was amazing about today is that with the Sun out the frost sparkled in the trees and the cold weather meant that it lasted most of the day.
Jack Frost did good would you agree?
 

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