Archives For nimbostratus

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.

As a warm front approaches the cloud thickens and decreases in altitude from a high cirrostratus veil through to altostratus and finally stratus. Fracto-stratus are the low level, ragged fragments, of stratus which appear dark and forbidding, below rain clouds such as nimbostratus. Despite its’ angry looks, this particular warm front is weakening on its way across the UK and will not bring any significant rainfall to Reigate. News on the exciting cold weather this week coming soon!

Today illustrated how observing changing cloud types can help predict an approaching front and rain. If you look up, you can often have a good shot at forecasting the weather several hours away. Today, a warm front approaching Reigate rapidly from the west was heralded by a series of cloud formations. In case you missed the drama, here is a round up of the best bits:

A bright blue sky and a frosty calm morning quickly gave way to a milky sky with the appearance of high cirrus cloud building into a thicker blanket of milky cloud called cirrostratus. These indicate increasing moisture in the upper atmosphere, which is often a sign of deteriorating weather and frontal rain approaching. A “halo” might appear around the sun with thickening cirrostratus: again, a sign of deteriorating conditions aloft. Cirrostratus clouds continued to thicken and lower into a blanket of middle altitude clouds called altostratus. These turn the sun into a fading white disc. Altostratus clouds are rarely thick enough to cause rainfall. As the front approaches low level stratus clouds appeared scudding above Reigate Hill in the 50mph winds aloft. Persistent rainfall started in Reigate mid-afternoon when thicker featureless nimbostratus clouds moved overhead, marking the arrival of the front proper. The temperature rose throughout the day, from 0°C at 8am to 10°C by 10pm, another indication of an approaching warm front.  The front travelled 200 miles in about 4 hours, a 50mph race across the UK and, despite the gathering wind and gloom, brought a wonderful lesson in meteorology, hope you had a moment to enjoy it!