"You mean both in summer and winter?"
"Yes," answered Tom, "they're more from the northwest in winter, I think, but they're generally westerly."
"If the winds are due to the position of the equator and the poles," the old farmer said shrewdly, "I don't see why summer and winter ought to make any difference."
"That," said the Forecaster, "is due to an entirely different set of conditions. It's due to the difference in radiation. There's much greater change in temperature over the land than over the sea. Take an island like Bermuda, for example. From the hottest day in summer to the coldest day in winter there isn't a change of more than forty degrees, because Bermuda is surrounded by water and is near warm ocean currents. In Arizona, on the other hand, there's a change of as much as fifty degrees of temperature in a single day. That is because land absorbs heat quickly and lets it go equally quickly. The interior of a continent in summer time heats and expands the air in the same way that the air is heated over the equator, and, in the same manner, sets in motion another system of winds, for cold air comes rushing down from all sides and forces up the rising warm air.
"Take Asia, for example, where the continental mass is large and the plateaus high. The interior becomes so hot that the air is sent up like the draught in a big chimney, and cool winds from the sea blow toward the interior from all sides in the summer time, and away from it, to all sides, in the winter time. That's what causes the famous Indian monsoons, which blow steadily to the north-east for the six months of summer and just as steadily to the south-west for the six months of winter. The native boats, there, are built on purpose for the monsoon, so that they can only sail with a fair wind and they make one round trip a year, going south with the monsoon in winter and returning with the summer monsoon."
The old farmer scratched his head.
"There's more to this than I thought," he said; "I always supposed that winds just happened."
"No, indeed," the Forecaster answered, "every place in the world has its own system of winds, though in some parts there are so many variations that it isn't always easy to distinguish between the regular and the irregular currents. In the United States the surface winds are very irregular, for we live in one of the stormiest regions of the entire world. Still, that doesn't alter the general rule that all our weather comes from the west."
"And yet," said the farmer, in a puzzled manner, "I don't see why it comes from the west."
"I think I can explain it to you," the weather expert replied. "You know that when water is running down a hole at the bottom of a basin, if it is in motion it doesn't go down straight but with a circular movement, finally making a whirlpool?"