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Cloud Formation
Name: Bob
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
What holds a cumulous cloud together as it travels with the
air currents? I seem to recall that it is formed by rising warm air that
has reached the dew point, starting at a particular altitude. I live in
Cleveland, Ohio, and can often see Lake Erie "lake-effect" creating
clouds as winds (especially NW winds) pass over the lake. Why doesn't the
rising, moist air create one long sheet of cloud - more like a
stratus-type? I think I have read that a cumulous cloud has rising air at
its center and falling air currents at its sides. If that is so, then how
can it possibly maintain that circulation pattern while passing over so
many different types of surface terrain? Is the circulation pattern
really that persistent in spite of attempts at disruption? Is there a
different force at work that is keeping the clouds "together" and
creating cumulous clouds with clear (sometimes predictably consistent)
distances between adjacent, neighboring clouds? Shouldn't some other
types of rising, falling, etc. air currents disrupt the cumulous cloud
and cause it to run into neighboring clouds or pull apart as its
circulation or the forces that maintain its integrity are disrupted?
Replies:
Bob,
A cumulous cloud is a "convective" cloud, meaning that it
forms from rising air that is initially warmer than the
surrounding air. This air rises rapidly and cools as it rises,
condenses, and forms a cloud. These are often called cell
clouds as they do not normally form a sheet (except in stratocumulus,
altocumulous, and cirrocumulous) like stratus. Stratus clouds are
formed by the lifting of an entire layer of air. The three
cloud types that I named in parentheses combine the two
processes (the layer is lifting and individual parcels of air
are being lifted in the layer). Sometimes rising air dominates
and the cumulous clouds fill in and result in stratocumulous.
As the cumulous cloud rises, the air around it must descend
(assuming that the entire layer of air is not being lifted)
to make way for it. Thus, cumulous clouds are often well spaced.
SODAR (SOund Detection And Ranging, using acoustics) and
LIDAR (LIght Detection And Ranging, using lasers) remote
sensing instrumentation show that on a cumulous cloud day, such as
a nice day in summer) rising air from a large area of
the surface is interspersed with falling air. These rising and
falling air volumes can be amazingly persistent and can move
with the cloud. A very small difference in temperature change
with height can create either of these preferential areas.
Terrain can indeed disrupt cumulous cell health. Once formed, the
cumulous cloud is it's own environment (a wet one) and will not
dissipate unless the air that surrounds it drags it downwards
(so that the air can warm above the dew point) or it is eroded
by drier air from around it (called entrainment).
As for the Lake Erie clouds, the different areas of warmer and cooler
water (for different depths of water, etc.) act in the same way as
the land, with preferential warm and cooler areas. Cumulous cell
development over the water usually is not as strong because the
differences in water surface temperatures are not as great as those
over land areas. During winter, when water temperatures are low,
you will see more stratus over any of the Great Lakes than during the
summer; layer lifting is more common in winter.
David Cook
Argonne National Laboratory
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Update: June 2012
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