# What is the Mass of a Thunderhead?

by EskWIRED
 P: 97 ISTM that large clouds hold a lot of water, and that water is fairly heavy. Which got me to wondering just how "heavy" all the water in a large thunderhead would be? I'm assuming that many, many tons of water are floating around up there, but how many?
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 PF Gold P: 1,857 What does "ISTM" stand for? Do you have any idea or sources that you've already checked? Every thunderstorm is different, so I would say no one can say the amount of water, in tons, that a thunderhead contains. If you stand on your paved street during a thunderstorm, gather up all the rainwater running off in buckets, and then weigh it, what could you infer? Would those measurements tell anything about the total amount that the thunderhead contained?
 P: 566 There's a "what if" touching on this subject: http://what-if.xkcd.com/12/
P: 403

## What is the Mass of a Thunderhead?

 Quote by EskWIRED Which got me to wondering just how "heavy" all the water in a large thunderhead would be? I'm assuming that many, many tons of water are floating around up there, but how many?
Rogers and Yau, "A Short Course in Cloud Physics" on page 235 give the mean precipitation content of an isolated thunderhead as on the order of 109 kilograms. They also point out that the actual output during the life of a thunderhead can be more than five times that amount due to the fact that a thunderhead is a process, and not just an object.

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P: 548
 Quote by Bobbywhy What does "ISTM" stand for?
It seems to me that ISTM stands for It Seems To Me.
PF Gold
P: 1,857
 Quote by Enigman It seems to me that ISTM stands for It Seems To Me.
Enigman, thank you for your translation. Since I am an "OF" (Old Fart) I needed your help.
 P: 97 Klimatos that is exactly what I was wondering. Thanks. I find it amazing that so much mass can just float on air.
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P: 25,961
 Quote by Enigman It seems to me that ISTM stands for It Seems To Me.
This is why TSINA (text speak is not allowed) at PF, EskWIRED, please type out text speak.
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P: 6,386
 Quote by EskWIRED I find it amazing that so much mass can just float on air.
The density of water vapor is much less than the density of air, at the same pressure. Molecular weight of water = 18, compared with 28 for nitrogen, and 32 for oxygen.

If water vapor was not light enough to "float on air", thunderheads would never form.
P: 403
 Quote by AlephZero The density of water vapor is much less than the density of air, at the same pressure. Molecular weight of water = 18, compared with 28 for nitrogen, and 32 for oxygen. If water vapor was not light enough to "float on air", thunderheads would never form.
Water vapor is certainly lighter than dry air. However, the source that I was quoting was referring to the liquid condensate (cloud droplets) content of the thunderhead, not the water vapor content. These droplets are kept aloft by the force of the rising air balancing the downward gravitational force.

This is easier to picture if you think of the thunderhead (and all clouds) as the visible tops of invisible masses of rising air.

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