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Re: What keeps clouds up?



Well, David Bowman's answer reveals some of the complexity of this
particular problem, but one has to recognize the superior simplicity
and conceptual accessibility of Hewitt's answer to the question. Is
it worth sacrificing those advantages just because that answer is
fundamentally incorrect and devoid of insight into the true nature
of the physics?

I read the complete Hewitt explanation in The Physics Teacher. The
terminal velocity of descent under gravity of the water droplets in
a cloud is indeed small; it is negligible compared to the evident
speed of bulk air motion in clouds. It is so small that I believe it
plays no role at all in the physics of the cloud. If one watches a
cloud edge with binoculars one can see that there is motion present.
Motion at terminal droplet velocity would be imperceptible in
binoculars. Moreover one will observe cloud both forming and
dissipating. The droplets are not eternal. If clouds have updrafts
in them as Hewitt says (and I believe they do), Why do clouds not
descend as the air is depleted below them? If there are updraft
regions in cumulus clouds are there not also downdraft regions? What
supports the droplets in those regions? Inquiring minds want to know,
and we owe them better explanations or else an admission of ignorance.

Bill Beatty's questions are apt; any bright student would appreciate
them in my view. An uncritical student might well be satisfied with
this explanation, but the brighter student will infer that authority
in physics should be questioned, in my view the only valuable lesson
taught by thgis example. Hewitt's conceptual grasp on this matter
seems to me to be seriously flawed. At the very least he should be
citing a source for such a controversial claim.

Hewitt blew it.

Leigh