Cindy Elliott, age 10, of High Point, North Carolina for her question:
What causes a whirlwind?
A dizzy little whirlwind is a dancing breeze. It may dance down a street, twirling leaves and bits of paper up to the roof tops. Summer is a good time to see a dusty whirlwind in the desert, wobbling from side to side as it goes. It seems to do just as it pleases. But actually, every whirlwind must obey certain rules. It is caused by the sun, the air and the spinning earth—and it must obey their regulations.
A whirlwind is born when the beaming sun warms up a patch of ground. The ground warms the air above it. Warm air expands and spreads out because it needs more room. It is also lighter than cooler air. Now picture a column of light warm air, sitting on that patch of extra warm ground. It needs more room—and there is only one direction in which it can go.
It cannot push sideways through the cooler, heavier air around it. But it can rise upward—and this is just what it does. A whirlwild begins to form when this stream of warm air rises above a warm patch of ground. As it rises, it may be strong enough to pull up some dust and dry leaves.
But this is not the whole story. A whirlwind is made of airy gases and it must obey certain rules. It is a fluid and in some ways it behaves like runny water. We can see for ourselves that water flows downhill. Air is invisible but it also tends to flow or blow in a certain direction. As a rule, masses of cooler heavier air blow toward pockets of warm light air. The masses mingle and merge to even things out, so to speak.
The whirlwind is a rising pocket of light warm air, naturally the cooler surrounding air breezes in toward it. More and more air blows in to fill and refill the rising updraft. And this is not all. Because the earth spins around like a top, it gives a twist to things moving above the ground. It twists the breezy whirlwind.
North of the equator, the big winds of the world are curved to blow slightly toward the right. The winds of a howling hurricane also spiral toward the right. South of the equator these winds are twisted toward the left. But for reasons unknown, a dizzy little whirlwind does not have to obey these global rules. Its twisting breezes can swirl around either left or right whether it is north or south of the equator.
A busy little whirlwind can rise as high as 300 or 400 feet and some¬times as high as 1,000 feet. As it blows and goes, it mixes and mingles a pocket of extra warm air with the cooler air around it. This helps to cool things off a little. On a scorching hot day, things are slightly cooler when dozens of whirlwinds dance over the desert.