George and I are two ordinary people. He studies the Weather Reports every
day; I do occasionally. He thinks he understands meteorology; I don't. But
lately I felt that I _must_ have some explanation of the weather, so I
asked George to explain it.
He said, "Certainly; it's quite simple. Take wind. Wind is caused by
differences of _pressure_."
"What _is_ pressure? Who is pressing what?"
"Pressure is what the barometer tells you--not the thermometer; you must
keep the thermometer out of this. Suppose it is very hot in London--"
"Don't be ridiculous."
"Well, suppose it is very hot at a place A--"
"I thought we were keeping the thermometer out of this."
"It comes in indirectly. But don't keep interrupting. If it is very hot at
the place A, the air at A rises. You see?"
"No."
"Obviously it does. If you light a candle--"
"Yes, yes, I do see that. Don't begin about candles."
"Well, the result of that is that there is less _pressure_ at A. In other
words, there is more room for the air to move about. When that happens the
air at the place B--"
"Where is that?"
"Oh anywhere. I told you to think of two places, A and B."
"No, you told me to think of a place A, and I am still thinking of it,
because it is very hot there."
"Well, this is another place, where the pressure is simply frightful. When
the air rises at A the air from B rushes over to A to fill up the gap, and
that is what we call wind."
"I see."
"No, you don't. It isn't quite so simple as that. Now, the atoms of air
rushing from B to A don't go _straight_ there, but they travel in--in sort
of _circles_."
"Why do they do that?"
"Well, the fact is that these atoms are so keen to get over to A, where
there is plenty of room, that they jostle each other, and that makes them
go round and round. If they go round and round _against_ the clock, like
that, they are called cyclones, or depressions, or low-pressure systems. If
they go with the clock, like that, it is an anti-cyclone."
"Oh!"
"What do you mean--'Oh'?"
"What I said; but go on."
"Now suppose this air--"
"Which air?"
"The air from B. Suppose it is travelling in a cyclone--"
"But isn't a cyclone a low-pressure thingummy?"
"Yes."
"And didn't you say that B was a high-pressure place?"
"Yes."
"Then how does the air coming from B manage to be low-pressure stuff?"
"I see what you mean. There _is_ an explanation, but it would take too long
to hazard it now. Suppose the air is coming from B in an anti-cyclone, then
..."
"All right. I'll suppose that."
"... it rushes over to A and fills up the gap. There is more pressure at A
and the barometer goes up--"
"Is it fine then?"
"No, it rains. You see, the air from B is colder than the air at A was
before the air came from B."
"I _don't_ see."
"Well, obviously it _must_ be."
"How 'obviously'?"
"Well, the whole thing started with it being very hot at A, you remember,
so that the air rose. If it had been hotter still at B just then the air
would have risen at B instead, and it couldn't have rushed over to A.
There'd have been a frightful muddle."
"There is."
"Well, it's your own fault for interrupting. This air, then--"
"Which air is this?"
"The air from B. The air from B cools the air at A--"
"But I thought the air at A had risen."
"Not all of it. And that makes it rain."
"Why?"
"Oh, well, I can't go into that. It's something to do with condensation.
Air absorbs more moisture when it is hot than when it is cold--"
"So do I. I understand that."
"When the air cools the water condenses."
"Is it fine then?"
"No, it rains, you fool."
"When is it fine?"
"Wait a bit. The falling of the rain of course generates heat--"
"Why 'of course'?"
"I can't explain _exactly_, but you know perfectly well that it's always
warmer on a cold day after the rain."
"Yes, but not on a hot day."
"Yes, it is."
"No, it isn't."
"It is, really. Anyhow, this is a cold day."
"No, it isn't. You said it was very hot at A."
"I'm not going to argue. You must take it from me that rain generates
heat."
"All right. Is it fine then?"
"No. Heat being generated the air rises. The result of that is that there
is less _pressure_ at A--"
"Is it fine then?"
"I've explained already what happens then. The air from B--"
"Do we begin all over again now?"
"More or less, yes."
"So that at this place, A, it's always raining or just going to rain?"
"Yes, if it starts by being hot there, as it did just now, I suppose it
is."
"What happens if it starts by being cold?"
"It rains. I've explained that. The cold air can't contain so much
moisture--"
"Don't begin that again. What about B? Is it any good going there? We had
frightfully high pressure there at one time."
"Yes, but it rains so much at A that more and more air rushes from B to A
to fill up the gap caused by the air rising on account of the heat
generated by the rain falling, and very soon you get frightfully low
pressure at B--"
"Is it fine then?"
"No, it rains."
"You surprise me. But suppose it had started by being low pressure at B?"
"Why, then of course it would have been raining the whole time at B."
"Where would A have got its rush of air from then?"
"From the place C."
"Is it fine there?"
"No, it's raining. It is like B was after the air rose at A."
"Oh. Then whatever happens at these places, A, B and C, it _must_ rain."
"More or less, yes. More really."
"Are there any more places? I mean, if I am at A where ought I to go?"
"There is a place, D--"
"What happens there?"
"Conditions are favourable for the formation of secondary depressions."
"Then where do you advise me to go?"
"I'm not advising you. You asked me to explain the weather, and I have."
"I think you have. I understand it now."
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