Re: What keeps clouds up?

[PHRP@FHSUVM.FHSU.EDU (Roger A. Pruitt)]

"..why is it that all the water droplets in a cloud don't fall to the
ground?" Cumulus clouds are atop a column of rising air--a thermal. As the
air rises, it cools adiabatically about 10 degrees C per 1000 m. This is
called the dry adiabatic lapse rate. Eventually the temperature of the
rising air cools to the dew point temperature. If there are appropriate
condensation nuclei in the atmosphere, and there ussually are, condensation
takes place as the air is cooled to the dew point and below. The cloud base
marks the dew point temperature. Of course, the ascending mass of air
doesn't stop its upward motion as soon as it reaches the condensation level
but continues to rise for perhaps another 1000 m carrying the small cloud
droplets upward in the updraft. 

Inside the cloud rising air cools at a rate between 5 degrees per kilometer
and 9 degrees per kilometer. This is called the moist adiabatic lapse rate
and it is less than the dry adiabatic lapse rate because of the release of
latent heat of condensation. (Yes, let us hear from all you people who don't
like the word heat.)

Sometimes the atmosphere is very warm at the surface but very cold aloft.
The temperature profile for the atmosphere is called the environmental lapse
rate. If this is greater than 10 degrees C per 1000 m the atmosphere is
considered to be absolutely unstable. An ascending mass of air rising from
the surface at all times will be warmer than the surrounding environmental
air and continue to rise creating a tall cumulonimbus cloud. Warming the
surface on a hot summer day changes the environmental lapse rate and makes
the atmosphere more unstable.

There is a condition known as conditional stability of the atmosphere. For
this condition the environmental lapse rate is between the moist adiabatic
lapse rate and the dry adiabatic lapse rate. Thus, the atmosphere can be
quite stable near the surface of the earth, but parcels (large air masses)
lifted above the altitude where the parcel becomes less dense (warmer) than
the surrounding environmental air will continue to rise. In the summer we
often can see early in the day puffy cumulus clouds which billow upward in
the late afternoon as the surface air is warmed. The updrafts may become
strong enough to cause the cloud tops to penetrate above the level where the
ascending air becomes warmer than the surrounding air. When this happens the
cloud will continue to grow into a cumulonimbus. 

In the updrafts cloud droplets grow until they become too large to be kept
upward. As they fall they will continue to grow until they fall out of the
bottom of the cloud as rain. A typical rain drop is 100 times the size of a
typical cloud droplet. This makes the volume of the rain drop about a
million times the volume of the cloud droplet. Thus a cloud droplet must
make many collisions with other cloud drops in order to become a rain drop.
For coalescence to take place there needs to be electric fields present in
the cloud which of course there are.

Where I live, and most of you too, our precipitation is a result of ice
crystal formation, but that is another matter than the one asked about in
the following note.

Roger

> On p119 in the Feb. '97 issue of The Physics Teacher, Paul Hewitt treats
> the question "..why is it that all the water droplets in a cloud don't
> fall to the ground?"  He points out that individual droplets have small
> terminal velocity.
>
> Yet this can't be the whole answer, since the total mass of a cloud is
> quite large.  Yes, individual droplets fall very slowly.  But why don't
> the individual droplets fall as a group, dragging the air between along
> with them?  This behavior is seen when Ultrasonic-Humidifier fog is poured
> into clear air.  The mist-laden air is heavy and sinks rapidly, even
> though the individual droplets would fall very slowly.  The mist
> constitutes a denser fluid than the surrounding air, so it falls.
>
> What keeps clouds from pouring downwards like humidifier mist?  For some
> reason the density of humid air must not increase as cloud droplets form.
>
> A thought-experiment.  Take some air with non-zero absolute humidity.  Now
> move all the water molecules together in groups to form a population of
> droplets.  The droplets are heavy, but the air left behind is lighter, so
> the average density doesn't change and the cloud will not fall.  But I
> suspect that this reasoning is incomplete.  Is humid air lighter or
> heavier than dry air?  And if the humidity is suddenly removed from air,
> does the partial pressure fall, requiring that the parcel of air shrink
> and become denser?
>
> I'm confused.  Why do clouds stay up?
>
> .....................uuuu / oo \ uuuu........,.............................
> William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
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Roger A. Pruitt, PhD
Professor of Physics
Fort Hays State University
Hays, KS  67601