Sky fall over Reigate today!

August 8, 2013 — Leave a comment

Today, it snowed over Reigate!  The sky was actually seen falling at lunchtime.  Well, OK, more accurately, cirrus cloud was seen all over the sky at midday and wonderful virga fall streaks trailed behind . Virga is a beautiful treat for cloud watchers and today was an extra-special display.  Virga is any form of precipitation (snow, ice, rain), usually falling from medium or high cloud, that evaporates before reaching the ground.  So… it snowed over Reigate today but 10km up and it never hit the ground.

Whilst it was a warm day down in Reigate (100m, Tmax 25ºC), at 30,000 feet (10,000 m) it was a chilly -45ºC. A moist humid layer of air at 10,000 m moved in from the west filling the sky with cirrus clouds which are formed from ice subliming in humid high altitude air. Cirrus is a thin wispy cloud but can precipitate i.e. “rain”. The ice literally falls from those thin cloud veils. The cirrus today were being carried along by a brisk 40 mph westerly wind. The ice rapidly fell into slower wind speeds below: at about 5000m the wind was only 20mph. Hence, the cirrus appeared to leave veils of trailing ice particles: VIRGA.  Some were probably initiated as contrails which broke up in the brisk winds. A much lower layer of cumulus appeared from local convection at about 1000 m, where the air temp was still 20ºC.

Here are the charts for humidity at 10,000 m (300hPa) and 5000 m (500hPa): note the very humid air aloft and the very dry air at 5000 m.  The ice never of course hit the ground because it was evaporated / sublimed at great height.  Only passing aircraft might have experienced any virga precipitation.

virga conditions

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