Ask A Scientist Weather Archive

name         Christopher S.
status       educator
age          50s

Question -   Why are clouds discrete entities?  In other words, why does water vapor 
coalesce in "clumps" after rising and reaching the correct temperature and adiabatic 
pressure?  Why doesn't a uniform haze of water form.  As I look out my window, I see one 
place where there is "cloud" and another area, directly next to it, where there is no 
cloud.  Assuming that the atmospheric conditions which lead to cloud formation are the 
same in both places, why did one form in one place but not the other?  Is it that water 
vapor usually rises in discrete columns through the atmosphere, consequently forming clouds 
only in discrete locations or could there be something else?
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Chris,


You asked "Is it that water vapor usually rises in discrete columns through the atmosphere, consequently forming clouds only in discrete locations".  Yes and no.  In the situation that you saw out the window, the answer is yes, because you were apparently looking at cumulus clouds.  Cumulus clouds are produced by rising columns of air that often have enhanced water vapor (from surface evapotranspiration). Meteorologists call these air columns "plumes".  As the air ascends, it cools, and in your case, cools sufficiently to produce water droplets and thus a cumulus cloud.


In between these rising columns of air are descending columns of air.  The descending air warms as it drops, so it cannot produce a cloud. As that air reaches the surface, it is warmed further by the warmer surface and will begin to rise.  You can see that this produces a cycle of rising and descending air.  Meteorologists call this "mixing" of the Boundary Layer (the lowest couple of kilometers of the atmosphere).  As warming of the surface by the Sun continues through the daylight hours, mixing continues and the Boundary Layer grows higher. As evening approaches, the Sun can warm the surface less and the plumes weaken.  Eventually air stops ascending and the cumulus clouds often dissipate around sunset.


The other situation that can occur is if the entire lower part of the atmosphere is lifted, such as near a cold front.  This produces a layer of stratus clouds, which are not usually discrete. However, even these can develop some cumulus in them if warming of the air below them occurs non-uniformly, at which point they are called stratocumulus.



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

Part of your answer is that cumulus clouds are formed from thermals rising above warmed 
portions of the earth.  Yet, it is not a continuous stream of warmed air.  In the same 
way that the bubbler in a fish tank emits bubbles rather than a continuous stream of air, 
the air warmed by the earth moves up in bubbles.  When the bubble cools sufficiently 
(adiabatically) water condenses forming a cloud.   Horizontal winds move the cloud and 
the next bubble forms another cumulus cloud.  It would sure help if we could see these 
bubbles of warmed air the same as we see the bubbles in the fish tank!

Larry Krengel
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