Cirrus Cloud Formation
Name: Bob T.
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
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.
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.
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
Wendell Bechtold, meteorologist
National Weather Service Forecast Office
St. Louis, MO
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
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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Update: June 2012