"OnePageQuest" wrote in message
...
Cumulonimbus or nimbostratus?

Recently I posted a question to the group concerning a phenomenon I had
observed in the vicinity of a thunderstorm. In that post I called the subject
cloud a "cumulus," and in the description I noted that it had extensive
vertical development, that there was another towering cloud nearby from which
heavy rain was falling, and that clear air prevailed in other areas of the
sky.
A few days after that posting I received an e-mail message from a reader of
the
group (who was not one of the respondents to the particular thread, which was
titled, "What was this STRANGE cumulus phenomenon?") advising me that I was
wrong, and that the subject cloud was NOT a cumulus but a nimbostratus.

This got me to thinking, and sent me to several "Meteorology 101" sites around
the Web. After further research, I concluded that I should have called the
cloud a cumulonimbus, but I came away with nagging doubts that my
corespondent's suggestion that it was a nimbostratus was correct.

I believe that that my description of the weather (in my original post) is
more
in keeping with air mass convective activity (the description of discrete
towering cumulus, and clear skies elsewhere), than it is with frontal
activity,
which I believe is where nimbostratus is found -- am I correct on this point?

Also, when I described it as a cumulus cloud, was I totally incorrect? I seem
to remember having been taught that cumulonimbus IS a TYPE of cumulus cloud,
that is, that cumulonimbus is a member of the broader family of clouds know as
cumulus. In any event, based on the description given, what should I have
properly called the cloud? Somehow, nimbostratus just doesn't seem to fit the
bill, but perhaps cumulonimbus is all wrong, too?

I thank the learned members of the group in advance for their answers.

If the cloud you saw was producing lightning, it was definitely a cumulonimbus.
As another person pointed out, cumulonimbus literally means a cumulus cloud with
precipitation. However, I think most meteorologist would reserve that name for
the deepest convective clouds only. This usually means that the cloud has
reached (or nearly reached) the tropopause, the upper boudary of the
troposphere. There can be towering cumulus that produce showers, but are not
high enough to be considered cumulonimbus. Also, some weak cumulonimbus might
not reach the tropopause, but then the cloud's top will often have a soft fuzzy
appearence, indicating the conversion of liquid droplets to ice crystals. This
doesn't happen until the cloud top has reached an altitude where its temperature
is far below freezing (usually below -20 deg F).