The Coriolis Effect

Answering a Reader’s Questions

As posted last week, the Coriolis Effect steers the winds and ocean currents around the globe in huge spiral patterns, all because our Earth spins. A reader asked who discovered this pattern in nature and wanted to know what happens at the equator. Does the prevailing north counterclockwise and south clockwise pattern change abruptly at the equator?

In 1835, a French engineer named Gaspard Gustav de Coriolis was the first to describe this mysterious effect. He realized that the spinning of the Earth caused a unique movement pattern. Moving objects like the winds and ocean currents seemed to veer off their straight path. In the Northern Hemisphere, they seem to turn right. In the Southern Hemisphere, they appear to go left. But this veering is an illusion, the wind is going in a straight line, but the earth is moving under it.

The Earth spins faster at the Equator than at the poles. If you try to throw a ball in a straight line over a long distance, it’ll appear to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis Effect in action.

You can see this happen on a playground merry-go-round that is in motion. If you try to toss a ball to a friend directly opposite, the ball may seem to veer off to the side. The ball leaves your hand and continues to move in a straight line, just like Newton’s first law of motion says. Since the merry-go-round is moving, however, your friend has shifted position relative to the earth. Your friend directly across misses the catch. But it may go straight to the person sitting next to them, assuming the speed of the merry-go-round is at just the right speed.

When you look at hurricanes from a satellite, you see this amazing spiral. These intense storms spinning counterclockwise in the north and clockwise in the south are fun to watch on time-lapse weather maps. This motion isn’t random; it’s the Coriolis phenomenon at work.

However, at the Equator, things get interesting. The Coriolis effect is weakest here. You might think that would make for some wild weather, but it’s actually the opposite. Hurricanes almost never occur right at the Equator. In fact, the Coriolis effect nudges hurricanes away from the equator. They usually form when they’re a few degrees north or south of it.

So, the next time you see a storm spiraling on the weather report or watch the wind rustle through the trees, remember the Coriolis effect. It’s one of the many natural marvels that makes our world fascinating and beautiful.