Pentrating Towering Cumulus Clouds

"Mike Rapoport" wrote in message
link.net...

"Icebound" wrote in message
...

"Mike Rapoport" wrote in message
ink.net...

"Ron McKinnon" wrote in message
newscD8e.1052899$8l.772250@pd7tw1no...

"Mike Rapoport" wrote in message
nk.net...

TCU have, as you would expect, characteristics between CU and CB
clouds. All three can be thought as different stages of the same
think, a cloud pruduced by convection. Obviously three seconds before
a TCU starts producing lightning and becomes a CB, it is going to a
lot like a thunderstorm inside.

A minor point: A TCU will not 'produce lightning to become a CB'. If
it produces lightning it *is* a CB, and has been for some time, but it
is
not the production of lightning that makes it a CB.


That is what I said ( I think) A thunderstorm becomes a thunderstorm
when the thunder starts. Three seconds before the first lightning, it
is still a TCU.

Well, no. A TCU becomes a cumulo-nimbus when the rounded califlower-like
shape of the TCU begins to top off with a wispy fibrous top, often
stretching downwind like an anvil. This is ice crystals forming at the
very top of the cloud.

That is the classic definition used by weather services. Observers will
not call it a CB if they continue to see the hard-edged form at the
tops.... not until they see the formation of the wispy fibrous top.
However, once lightning, or hail, or funnel-clouds are observed, the
observer will almost surely class it a CB, regardless.


Do you have a source of this definition? I can't find one but while
looking, I found several sites with pictures of CBs that don't all have
the wispy tops

Here is one: http://www.chitambo.com/clouds/cloudshtml/calvus.html


I chose the definition from the Canadian Manual of Observation which defines
CB:

DEFINITIONS OF CLOUDS
Cumulonimbus: Heavy and dense cloud with
aconsiderable vertical extent, in the form of
amountain or huge tower. At least part of its
upper portion is usually smooth, or fibrous or
striated, and nearly always flattened; this part
often spreads out in the shape of an anvil or vast
plume.

-----

But some more insight can be obtained from the World Met Organization
code-table:
http://badc.nerc.ac.uk/data/surface/code.html (Low cloud type)

Code 3 allows a CB to be defined without the "clearly fibrous" tops, but
which must "at least partially, lack sharp outlines", ....which shows the
the icing of the tops has begun.

Code 9 CB requires the "clearly fibrous" top.

The key is that to meet the classic definition of CB, the icing of the tops
(and hence the smoother fibrous shape)... has at the very least, begun to
form.




It is not necessary that a CB produce lightning, nor hail, nor heavy
precipitation, nor Mammae, Funnel Clouds, Tornados or Waterspouts.
It can do none of these things and still be a CB. But if any of these
things happen it is necessarily a CB.

I am not sure what you are saying here, If it doesn't produce lightning
it isn't a thunderstorm (CB) so I would say that it is nessesary for a
CB to produce lightning.

The production of the ice-crystals at the top will normally be
accompanied by lightning, but ever if it is not, it would still be called
a CB if the fibrous ice-crystal anvil-shaped top exists.



Conversely, when a TCU is only slightly taller than a CU, it is going
to be more like a CU inside. There is a relationship between the
vertical height of a convective cloud and turbulence but it is not
absolute. I have never heard of large hail coming from anything other
than a big CB.

If it hails, it's a CB, by definition. A TCU can produce snow
pellets, however (and ice pellets, I think (it's been a while)),


This is the first time that I have heard that hail defines a
thunderstorm. A thunderstorm is defined by lightning (and therefore
thunder). Hail is produced by updrafts in a cloud through the freezing
level allowing frozen precipitation to remain aloft and grow. I don't
see why this couldn't happen without lightning. I was hailed upon
yesterday and there was no thunder.


Hail (hard "real" hail, not the soft stuff in some cold-weather cumulus)
... Hail does not define a thunderstorm, but because the conditions
required for it are typically exactly the same that produce lightning and
the rest of the CB symptoms, you will be hard pressed to find an observer
who will not call a hailing cloud a CB, just because he hasn't seen
lightning or heard thunder.



I agree that most observers would call a hailing cloud a CB but the hail
that fell on me yesterday was pea sized and mostly clear yet fell from a
cloud that had a top of perhaps 15,000' and was not producing thunder.


Where, geographically? And did it fall on you on the ground, or did you
encounter it in flight? And how was the cloud top determined.

"Icebound" wrote in message
...

Snip


I chose the definition from the Canadian Manual of Observation which
defines CB:

DEFINITIONS OF CLOUDS
Cumulonimbus: Heavy and dense cloud with
aconsiderable vertical extent, in the form of
amountain or huge tower. At least part of its
upper portion is usually smooth, or fibrous or
striated, and nearly always flattened; this part
often spreads out in the shape of an anvil or vast
plume.

-----

But some more insight can be obtained from the World Met Organization
code-table:
http://badc.nerc.ac.uk/data/surface/code.html (Low cloud type)

Code 3 allows a CB to be defined without the "clearly fibrous" tops, but
which must "at least partially, lack sharp outlines", ....which shows the
the icing of the tops has begun.

Code 9 CB requires the "clearly fibrous" top.

The key is that to meet the classic definition of CB, the icing of the
tops (and hence the smoother fibrous shape)... has at the very least,
begun to form.


Thanks! I have been using "CB" and "thunderstrom" interchangably, perhaps
this is not strictly true.


I agree that most observers would call a hailing cloud a CB but the hail
that fell on me yesterday was pea sized and mostly clear yet fell from a
cloud that had a top of perhaps 15,000' and was not producing thunder.


Where, geographically? And did it fall on you on the ground, or did you
encounter it in flight? And how was the cloud top determined.



North Idaho. I was on the ground (bicycling in hail of all things) The
tops I estimate at 10-12k based on my experience flying in the area (I am
confident that I could easily top them VFR). I gave 15,000' as a very
conservative estimate, I have a high degree of confidence that they were
lower. The highest terrain around is 6200' and this was sticking to the
bottom of the cloud so the vertical height was about 6000'.

Mike
MU-2

"Mike Rapoport" wrote in message
ink.net...

"Icebound" wrote in message
...


Thanks! I have been using "CB" and "thunderstrom" interchangably, perhaps
this is not strictly true.


Ron has correctly described the difference. CB is a cloud, thunderstorm is
a condition reported only when thunder is heard or lightning seen. Being
the "thunderstorm cloud", the terms CB and thunderstorm are often used
interchangeably in generic descriptions, but we can and should be more
precise.


I agree that most observers would call a hailing cloud a CB but the hail
that fell on me yesterday was pea sized and mostly clear yet fell from a
cloud that had a top of perhaps 15,000' and was not producing thunder.


Where, geographically? And did it fall on you on the ground, or did you
encounter it in flight? And how was the cloud top determined.



North Idaho. I was on the ground (bicycling in hail of all things) The
tops I estimate at 10-12k based on my experience flying in the area (I am
confident that I could easily top them VFR). I gave 15,000' as a very
conservative estimate, I have a high degree of confidence that they were
lower. The highest terrain around is 6200' and this was sticking to the
bottom of the cloud so the vertical height was about 6000'.


I don't pretend to be an expert in mountain meteorology. But because the
direction of the wind plays such an overwhelming part in the lift equation,
it would be my guess that a TCU was developing, the vertical currents were
there, the coalescence of water was occurring and freezing. But in a
typical flatland TCU/CB this build-up keeps going for perhaps up to couple
of hours and great height, here I would guess that a wind change killed the
vertical currents rather quickly... and the precipitation, no longer
supported, simply fell out.

My guess, don't know.