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.

"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 ^,,^

Bob Harrington's reply to this post is correct. I'd like to add 2
additional bits of info
a) Here in the UK Cumulonimbus can be identified by shape alone.

The "tall towers" as Americans like to call them :-) can be seen from
many miles away- often too far to be sure if there is actual rain
falling from the base of the cloud. Cumulonimbus also often has a
distinctive "anvil" shape at the top, which helps identify them.
Because of their importance in terms of local flooding, wind shear,
damaging gusts, etc, we don't wait until we see rain (or lightning)
before classifying the cloud as Cumulonimbus (Cb).
We identify 2 types of CB - with anvil, or without anvil.

b) Your description of the 2 clouds together sounds very like a Cb and
"daughter cell".

Cold, dense, downdraughts from the Cb hit the ground and spread away
from the originating cloud. If the cold air collides with warmer air on
its way to being sucked in to the Cb, the result can be an uplifting of
the warm air. If it rises to the point at which the original cloud
formed, a second cloud (the daughter cell) might be formed.

As the inflow of air to the original cloud has been shut off, the
original often decays, and the daughter cell takes over as the main
cloud.

Hope this helps.

Colin

"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).