Re: What keeps clouds up?

[William Beaty <billb@eskimo.com>]

On Sun, 23 Mar 1997, David Bowman wrote:

> I didn't quite follow all of Leigh's post on the falling cloud problem.  I
> did not see Hewitt's TPT explanation so I'm not sure what you are objecting
> to.  Did Hewitt neglect the role of evaporation at the cloud base or what?

Here's Hewitt's answer.  (His question was in my previous message):

     The presence of clouds has to do with updrafts and small terminal
  speeds of falling droplets.  A typical cumulus cloud is in the region of
  updraft speeds of at least 1 m/s -- more than the terminal speed of
  typical droplets and more than enough to support the droplets.  But
  even for a cloud in no updraft, the droplets drift so slowly out of the
  bottom of the cloud, and evaporate so quickly, that they have little
  chance of reaching the earth.  They're replaced by new droplets forming
  above. 
     Raindrops are huge compared with typical cloud droplets.  Their
  terminal speed is greater than the speed of most updrafts and they
  indeed fall from the cloud -- and they evaporate slowly enough to reach
  the ground. 


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