Wednesday, November 14, 2012

Watch for falling virga!

Some interesting cloud formations floated over the Memphis metro this morning as an upper-level disturbance moved through the area. Below is a great picture taken by Angela High along the north loop of I-40.
Virga descending from clouds over north Memphis on Wednesday morning. Photo credit: Angela High.
With the disturbance passing by, these clouds had enough moisture to generate precipitation.  However, due to very dry air in the layer beneath the clouds and above the ground, the precipitation evaporated as it fell.  This evaporating precipitation, which looks like rain falling but never reaches the ground is called virga.  Due to the evaporation process, some very interesting cloud formations can result - streaks that reach towards the ground, "balls" of cloud that appear to be descending, wispy clouds, etc.

Below is a "sounding" of the atmosphere over Memphis at 9am this morning.  Temperature is the red line, dewpoint is the green line and the top of the image is at about 26,000' while the earth's surface is at the bottom.  When the green and red lines are close together, the humidity is high; when they are far apart, humidity is very low, allowing evaporation to occur.

This morning, the base of the clouds were at about 15,000' and the precipitation was falling into a very dry area from a few thousand feet off the ground up to 10,000'. This is where the evaporation took place and virga formed.

Atmospheric sounding over Memphis Wednesday morning, showing how virga formed.
Did you see the virga, and did you know what it was when you saw it?  From now on, you will!

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