Why giant storms start over warm sea, and why they spin.
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A hurricane is a giant spinning storm of wind, cloud and rain, hundreds of kilometres across. Its winds blow at 119 km/h or more. Every one starts over a warm tropical sea, like this one heading for the Caribbean.
Hurricanes feed on warm sea water. The sea has to be about 26.5 °C or warmer. Push the slider to warm up the tropical seas and see where storms can start. (The warm patches are drawn roughly.)
Warm, wet air rises off the sea. As it rises it cools, and the water vapour in it turns into huge storm clouds. More wet air rushes in along the sea to take its place, and rises too.
The spinning Earth twists the air rushing in, so the whole storm turns. North of the equator (the dotted line), storms spin anticlockwise. South of it, they spin clockwise.
The sea near the equator is plenty warm. But hurricanes almost never start there, because the Earth’s twist is zero right on the equator. A storm has to start at least about 500 km away from it to get spinning.
It’s the same kind of storm everywhere, with different names. Over the Atlantic and the north-east Pacific it’s a hurricane. Over the north-west Pacific it’s a typhoon. Over the South Pacific and the Indian Ocean it’s a tropical cyclone.
A hurricane needs warm sea to keep going. Push this one out of the Gulf of Mexico and onto land. What happens?
Here’s the whole Atlantic Ocean, from top to bottom. The storms in the north are hurricanes.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).