Why Columbus sailed south to go west, and how the winds make a giant loop.
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In 1492, Christopher Columbus set off from Spain to cross the Atlantic Ocean. He wanted to go west. So why did he sail south first, to the Canary Islands near Africa?
Sailing ships have no engines: the wind pushes them. You’re in Spain, and you want to sail west across the ocean. First, try the straight line. (The routes and winds are drawn simply.)
The North Atlantic has two big winds (drawn simply here). In the tropics, the trade winds blow steadily from east to west. Further north, the westerlies blow the other way, from west to east.
Hot air rises at the equator, flows north high up, and sinks near 30° north. There it spreads out along the sea: some blows back south as the trade winds, and some blows north as the westerlies.
Columbus sailed south first because of the wind. At the Canary Islands, his ships caught the trade winds, which blew them west across the ocean. They left on 6 September 1492, and on 12 October the sailors saw land: an island in the Bahamas.
But the trade winds blow the wrong way for going home. So in January 1493, Columbus sailed north, to where the westerlies blow. They carried him east, back across the ocean, and he was home in Spain by March. (The route is drawn simply.)
Put the two trips together and you get a giant loop: out in the south, home in the north. The winds blow round the same way, clockwise, around a big patch of high pressure in the middle (drawn simply). They even push the sea round with them, in the giant whirlpool from the Gulf Stream lesson.
You’ve crossed the ocean on the trade winds. Now your ship needs to get home to Europe.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).