"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—"