Everybody knows, or thinks they know, that the Earth’s daily 24-hour spin around its axis causes the Coriolis force, which in turn causes hurricanes to rotate and household drains to swirl in a counter-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Actually, none of that is true. Some of it is completely false, and the rest is not exactly what is really going on.
The Earth doesn’t make the Coriolis force, which will show up on any rotating frame of reference. However, on such a large and fluid world, one simple force can have some seemingly very complicated effects, and so myth and reality have become interwoven over time. To understand things better, let’s go back to simpler days, when this remarkable force was first discovered.
♦ The gang that couldn’t shoot straight
In the early 19th century, warring countries began to notice that their new long-range cannons kept missing targets in a consistent way. The cannon ball would always veer off to the cannon’s right, whether the battle was south of the equator or north of it. In 1835, Gaspard Gustave de Coriolis, a French scientist, figured out why this was happening.
He saw that technology had changed the overall situation. Up until then, enemy lines had always been close enough to each other for their cannon shots to follow a straight line from Side A to Side B. Now Side A’s cannons were powerful enough to hit Side B from a very long distance, which was nice for Side A because they could be safely out of range of Side B’s guns and still shoot, but the longer distance between the two sides brought a new factor into things: the radius of the Earth.
Coriolis pointed out what everybody already knew, that Earth is a huge sphere that rotates around its axis in roughly 24 hours. The planet is a lot wider at the Equator than it is at the poles, so it has to really hustle down there for equatorial regions to complete the 24-hour spin at the same time as polar areas.
This increased rotation speed near the equator explains why it gets dark soon after sundown in the tropics, while twilight gets longer the further north and south you go, until day and night each lasts for months at the poles, depending on the time of year.
The Frenchman then explained that the increased rotation speed was why the cannons of Side A, closest to the Equator, were missing their targets. Side A was moving faster to the east, along with the Earth, than Side B was. From the viewpoint of a bird directly overhead, Side A’s cannon ball would travel in a straight line, but on the ground, the ball would curve to the right.
This is a little difficult to follow at first, but the University of West Florida has a very easy online Flash presentation that helps make it clear.
♦ The Coriolis force and the atmosphere
Earth’s atmosphere is a fluid and has currents just like the oceans, and the University of West Florida presentation also shows how the Coriolis force affects them.
Warm air at the Equator rises, cools and sinks down toward the surface again, forming vast circulation belts called Hadley cells from the Equator up to about 30 degrees north and south. At either pole, cold air flows into more temperate regions at around 50 or 60 degrees of north and south latitude because it is denser; then it warms up, rises, cools and repeats the cycle as a polar circulation cell.
Earth drags the atmosphere along with its rotation, so these closed circulations are affected by the Coriolis force. The Hadley cells are most strongly shifted to the right, because they are closest to the equator, and this sets up surface winds blowing out the east, the famous Trade Winds or Easterlies that explorers and merchants in sailing ships used to travel across the world’s oceans for centuries. The polar cells are minimally shifted by the Coriolis force, just enough to slip down a little ways from the pole and set the air in the mid-latitudes moving.
There is no closed circulation in the atmosphere between 30 and around 55 degrees north and south, but this mid-latitude air is in constant motion overall as eddies form in it from interaction with the Hadley and polar cells. It also is warmer nearest to the Equator and cooler near the poles, and this uneven heating also makes the mid-latitude air rise and fall.
The Coriolis effect on all this movement actually produces a westerly trend to winds known as the Prevailing Westerlies. This explains why the weather most often comes at you from the west, if you live in the mid-latitudes.
♦ Hurricanes and the Coriolis force
All this activity in the atmosphere means that some areas will have more air (and therefore a higher air pressure) while others have less air and a lower pressure. Air tends to move from an area of higher pressure to lower pressure. This, along with complex factors involving temperature and moisture, is what leads to tropical cyclones, or hurricanes as they are called in the Atlantic and parts of the Pacific.
If the Coriolis force alone acted on hurricanes, they would constantly move to the right, and end up going in circles. What it actually does is make the air rushing into a low pressure area spiral a little bit to the right. The faster the air moves, the more it accelerates to the right, and pretty soon it is spiraling in and rotating around an eye, always to the right. That motion is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
The Coriolis force also affects hurricane tracks in each hemisphere.
♦ The Coriolis force and water in our drains
The Earth is very big and its rotational speed everywhere is very fast in comparison to our own reference scales, so it seems that the Coriolis force would be very strong, too. This is probably the reasoning behind the mistaken belief that draining water spirals counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Check how it works in your own sink: sometimes the water will drain out one way, sometimes the other way. It all depends on how the water enters the sink.
What isn’t taken into consideration is the small scale of water draining from our sinks and bathtubs, compared to an Atlantic hurricane or a circulating Hadley cell. Also, the planet only rotates once a day. How many times does that water circle around on its way down the drain? As strange as it may seem, the Coriolis force is too weak to affect the direction in which water drains out of our sinks and tubs.
The Coriolis force only works on a rotating frame of reference, like the Earth. It’s a small force in a large and complicated fluid medium, and its effects that we see are both beautiful and puzzling. While it is powerful enough to help shape the direction of the winds, it is still too weak to make your sink drain out one way or the other. When fact and fiction about this force are separated, wonder still remains.