Nimbostratus and shreds of fractus clouds forming in SE wind

February 10, 2013 — Leave a comment

How much colder is it 100 metres above you, right now? We tested this today by driving up the North Downs which stand 100 metres above Reigate, Surrey town centre.  A Kestrel 3500 was used to collect the data from the hill and temperatures were given a good while to stabilize.  The RGS weather station sits at the foot of the same hill at 96 metres. So, the (rather un-scientific!) results at 2.15pm were as follows:

Reigate 96m: Temp 3.1ºC; dew point 2.5ºC
Reigate Hill 200m: Temp 2.1ºC; dew point 1.5ºC

So Reigate Hill is a whole degree colder than the town, the wind chill was -1C. This was noticeable also in the heavy sleet on the hill.
The decrease in temperature with height is called “lapse rate” and it, usually, continues to drop for another 10,000 metres, the top of the troposhere (“weather atmosphere”). The 1C drop per 100 metres today is a steep lapse rate. We would normally expect around 0.6C drop per 100m, called the environmental lapse rate. The freezing level is currently only 300 metres above Reigate town centre and, with temperatures falling tonight the freezing level will almost certainly arrive at the surface.  The clouds were interesting: thick nimbostratus formed a solid cloud base at 300 metres; amazing shreds of fractus cloud formed on the stiff SE wind blowing up the hill.  Fractus or thicker Pannus clouds are those whispy shreds which appear under rain clouds and indicate deteriorating weather.

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