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On January 2, 2006 these lanes of
interesting altocumulus clouds formed late in the morning. The puffy altocumulus
clouds of water droplets began to crystallize in the cold air above Phoenix, AZ
and as the water droplets quickly solidified into growing ice crystals they
began to drop. This precipitation never reached the ground and could therefore
be considered a form of "virga". In the background are much higher
cirrus clouds. Following are more
pictures of this same phenomena.
Click on the smaller thumbnail images to
get the larger images.
 
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This image show more precipitation
with the super cooled cumulus associated with the surrounding cirrus clouds.
The super cooled water droplets of the cumulus cloud quickly release energy as
they crystallize to snowflakes. Each snowflake grows as more and more of the
water droplets contribute to the growth of the ice crystals by releasing heat
energy to drop to the more stable, lower energy level of the solid state. This
cloud was seen over the wide open spaces in New Mexico on route 60 east of
Springerville, AZ. |
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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. It is also
common in the Southern United States during summer.
Virga can cause very interesting 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 meaning twig or
branch and a popular backronym in meteorology is
"Variable Intensity Rain Gradient Aloft."
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