Mr. Cloud knows his chemistry

Feb. 17, 2003

I am sorry, folks, but I do not know where you can find elderberries around here. I do not know how to determine how many degrees off plumb half a bubble would be. I do not know what those little dots on your ceiling are. I suspect they are flyspecks, but I don't know.

I also don't know why for sure I took up today's question. It involves numbers and formulas and stuff and little good ever comes from meddling with powerful forces such as those. Nonetheless, it had a certain appeal, I guess.

If clouds are full of water, they must be very heavy. How are they able to float in the sky like they do if they are so waterlogged?

I feel a need to sigh deeply at this point, but I am not sure why. Perhaps it is because we are going to have to discuss weight vs. density, and I always have trouble with that. Had I known it was going to come to this, I would have paid more attention during physics class or whatever class it was that that came up in.

First of all, you are correct. Clouds weigh a lot. A nice, fluffy, ordinary cumulus cloud of no particular distinction might weigh as much as 500,000 pounds. Big thunderheads can weigh tons and tons and tons.

If even a little innocuous-looking cloud were suddenly to plunge out of the sky and hit you on the head, you would be squished.

However, clouds do not just fall out of the sky. For one thing, they are held aloft by updrafts. Your ordinary raindrop is exceedingly small, so it doesn't take much of an updraft to suspend it in air. A raindrop doesn't fall until the water droplet or ice crystal has been bounced up and down in a cloud to the point where it gloms onto enough other drops or crystals and gets heavy enough to overcome the updraft.

Next: the density thing. Density is mass divided by volume. Write that down. Moist air - the air in a cloud, for example - is less dense than dry air.

Way up high in the sky where Mr. Cloud lives, there is a lot of oxygen and nitrogen, but up that high, they are both what you call diatomic. Instead of O and N, they are O{-2} and N{-2}, which means they have atomic masses of 32 and 28 units, respectively.

Water vapor, on the other hand, is made of one oxygen atom and two hydrogen atoms. Hydrogen is very light, just one atomic unit, and oxygen, the non-diatomic brand, is 16 atomic units, which adds up to 18 atomic units.

Hence, assuming the temperature is equal, moist air is less dense than dry air and hence water vapor can float along in the air.

Reach Thompson at clay. or (602) 444-8612.

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