Ask A Scientist Weather Archive

name         Bob T.
status       other
age          40s

Question -   Hi,
Is the source of moisture that forms cirrus clouds, ice crystals that come from below 
the cirrus cloud and move higher in the atmosphere  (like thunderheads) or does it come 
from above the cirrus clouds and move downward in the atmosphere (like rain)?
Are cirrus cloud ice crystals falling behind their source of origin due to drag in the 
atmosphere?
-----------------
Bob 

I would expect that the ice crystals that form to create cirrus clouds are sublimed, 
changing directly from water vapor to water solid.  Cirrus clouds have little vertical 
motion.  (More in cirrocumulus and less in cirrostratus)  It seems unlikely that the water 
present in these clouds would come directly from cumulus buildups.

Although there are different scenarios that might produce a cirrus cloud, they are often 
found in advance of a warm front.  The warm air rides up on the cooler air as it moves.  
The actual meeting of the air masses is higher in the atmosphere in advance of the ground 
location of the front.  As the warm air rises a combination of dropping pressure and 
dropping temperature (adiabatic cooling) causes the water vapor to change state.  Because 
warmer air is less dense, it tends to remain above the cooler air mass.

Larry Krengel
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Bob T.,

The highest altitude clouds in the atmosphere are cirrus clouds-icy clouds located at 
altitudes between 16,500 and 50,000 feet.  They are always composed of ice crystals.  
While the ice crystals in cirriform clouds often grow to precipitation size, and stream 
downwards in long, wispy tails, they always evaporate before reaching the ground. I hope 
that this helps.

Sincerely,

Bob Trach
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Dear Bob-

The sources of moisture for cirrus is generally BELOW the level of the cloud. 
Thunderstorms are one source of moisture for cirrus, but most cirrus develop as a result 
of air moving upward along isotropic surfaces, and then condensing.

The cirrus crystals fall DOWNSTREAM from their source, or location of condensation 
initiation.

Wendell Bechtold, meteorologist
National Weather Service Forecast Office
St. Louis, MO
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Bob,

All of the water that forms into ice crystals in cirrus clouds comes from lower in the 
atmosphere. Tall thunderstorms can inject a lot of water into the upper part of the 
troposphere, as well as into the lower stratosphere.

Cirrus cloud ice crystals can grow larger in saturated conditions, as they draw on the 
water vapor around them in the cloud. When they become too heavy  for the air motions to 
hold them up, they fall from the cirrus cloud, just as snow, rain, or hail falls from 
clouds because they are too heavy for the air motions to keep them in the cloud where 
they formed.

As the ice crystals fall, the wind below the cloud blows them away, forming a "mares' 
tail", a form of virga (which means precipitation falling from a cloud but appearing to 
be attached to it), that is sometimes seen under the cirrus cloud.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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