posted on Mar, 13 2011 @ 03:19 PM
11- Virga clouds
In meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground. At high
altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because
the air pressure increases closer to the ground. It is very common in the desert and in temperate climates. In North America, it is commonly seen in
the Southern United States and the Canadian Prairies.
Virga can cause varying weather effects, because as rain is changed from liquid to vapor form, it removes heat from the air due to the high heat of
vaporization of water. In some instances, these pockets of colder air can descend rapidly, creating a dry microburst which can be extremely hazardous
to aviation. Conversely, precipitation evaporating at high altitude can compressionally heat as it falls, and result in a gusty downburst which may
substantially and rapidly warm the surface temperature. This fairly rare phenomenon, a heat burst, also tends to be of exceedingly dry air.
Virga also has a role in seeding storm cells whereby small particles from one cloud are blown into neighboring supersaturated air and act as
nucleation particles for the next thunderhead cloud to begin forming.
Virga can produce dramatic and beautiful scenes, especially during a red sunset. The red light can be caught by the streamers of falling
precipitation, and winds may push the bottom ends of the virga so it falls at an angle, making the clouds appear to have commas attached.
The word virga is derived from Latin, twig or branch. A backronym sometimes found in amateur discussions of meteorology is "Variable Intensity Rain
Cirrus Virga 1
Cirrus Virga 2
Altocumulus Virga 1
Altocumulus Virga 2
Altocumulus Virga 3