Pressure differences tend to push winds in straight paths. Yet winds follow curved paths across the Earth. In 1835, Gustave-Gaspard Coriolis, a French scientist, first described mathematically what's going on, giving his name to the Coriolis force.
In simple terms, as air begins flowing from high to low pressure, the Earth rotates under it, making the wind follow a curved path. In the Northern Hemisphere, the wind turns to the right of its direction of motion. In the Southern Hemisphere, it turns to the left. The Coriolis force is zero at the equator.
You might be wondering: If the Coriolis force turns winds to the right in the Northern Hemisphere, why do winds go counterclockwise around large systems, such as hurricanes, north of the equator?
This happens because Coriolis is only one of the forces acting on air to cause winds. In simple terms, in the Northern Hemisphere, while the Coriolis force is pushing the wind toward the right, the pressure-gradient force, caused by air pressure differences, is pushing the air toward the center of the area of low pressure, and for various reasons is stronger then the Coriolis force. This is nicely described, with a drawing on the Coriolis FAQpart of the Web site of Alistair Fraser of Pennsylvania State University. (More below about this site)
A USATODAY.com graphic shows how pressure differences start the air moving to become wind. You'll find a lot of information about air pressure and the winds by going to our Guide to the science of the atmosphere, scrolling down to the "What and why of wind" ribbon and following the links under it.
Flush this nonsense down the drain
Don't believe what you hear about Coriolis making the water in a sink or toilet rotate one way as it drains in one hemisphere, the other way in the other hemisphere. The Coriolis force is noticeable only for large-scale motions such as winds.
The best Web material we've found on the Coriolis force is the Bad Coriolis part of the Bad meteorology Web site of Alistair Fraser of Pennsylvania State University.
His Coriolis FAQ answers a lot of questions, beginning with why winds move counterclockwise around storms in the Northern Hemisphere while the Coriolis force pushes moving objects to the right, and why it is correct to call it the "Coriolis force," instead of the "Coriolis effect." The text and diagram with the first question in the FAQ clear this question up.
Coriolis force and hurricanes
The fact that the Coriolis force is zero at the equator and very weak near the equator explains why tropical cyclones such as hurricanes and typhoons won't form on the equator even though the other factors there, such as warm ocean water, would make them likely. But, the idea that you don't have to worry about tropical cyclones in the zone about 180 miles north and south of the equator isn't a sure thing, as Typhoon Vamei showed in December 2001.
More on Coriolis
One of the answers in our Answers archive: History of meteorology has quite a bit on Coriolis, how long it took scientists to realize his idea applies to the weather, and the correct spelling of his name. (Scroll down to the question about the spelling of his name.)
Understanding the Coriolis force is difficult, in addition to the material on Alistair Fraser's Coriolis FAQ, you'll find a different way of looking at things by going to the Rotating Frames of Reference in Space and on Earth section of NASA's From Stargazers to Starships Web book.