"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.

From your descriptions, I believe cumulonimbus was accurate. 'Cumulo' -
heaped, 'nimbus' - raining (or snowing, hailing, frogging,
cats'n'dogging...)

Cumulus clouds form in unstable air in a cellular form, with local
vertical development surrounded by sinking air which helps keep it
clear. When the cumulus develops to the point that precipitaion begins,
it is classified as cumulonimbus - at least here in the Pacific
Northwest where lightning is rare, the term may be used elsewhere only
once lightning has begun.

Nimbostratus ('raining layer') clouds form when the whole air mass is
being lifted en masse, such as when a warm front pushes moist stable air
over a receding dome of colder air; or through orographic means when
moist stable air is forced to rise over a mountain range.

Bob ^,,^