Why hurricanes spin counterclockwise

Q: Why do hurricanes in the Northern Hemisphere spin counterclockwise? Why don't hurricanes in the Southern Hemisphere spin counterclockwise also? Why in the S. Hemisphere do they spin clockwise?
HurricaneGeek, West Palm Beach, Florida

A: A hurricane is a system of violent thunderstorms with high winds — circulating about a central low-pressure area, called the eye. "Air pressure flows from higher pressure towards lower pressure, although not directly so. Instead, it spirals inwards," says meteorologist Sharanya J. Majumdar, professor at the University of Miami. And this takes us to the reader's question. Although a hurricane is a fearfully complex phenomenon, why it spins in the direction it does is comparatively simple.

Why does the air spin? The air masses that make up a hurricane move towards the low-pressure area, pushed by surrounding higher- pressure air. But, because Earth's surface spins at different speeds (faster at the equator, slower near the poles), air doesn't move in a straight line from high to low pressure.

For example, at about 39 degrees of latitude north and south (about the latitude of Denver in the USA and Hastings in New Zealand), Earth's surface moves about 810 mph (1300 k/h) from west to east. At the equator it moves about 30 percent faster at about 1040 mph (1670 k/h).

Why do hurricanes spin counterclockwise in the Northern Hemisphere? Suppose the hurricane's low-pressure area is off the coast of Florida at the spot marked 'L' in the diagram. Higher-pressure area from the north, west, east and south will move towards the low pressure L as depicted by the white arrows. But the air to the north moves eastward slower than the low-pressure area; moreover, the air to the south moves faster than L. The different speeds cause the air to circulate counter clockwise about the low. The diagram illustrates the action:

· The north air mass (north arrow) flows toward L but it moves eastward slower than L. So the north air mass will lag behind (west) the faster-moving low.

· Likewise the south air mass moves eastward faster than L so it will move ahead of L.

· The resulting air mass movement is counterclockwise about L.

It is easy to see how different rotational speeds at different latitudes cause a counterclockwise rotation, when we examine the speeds themselves. Please see the spin-speed diagram. The latitude of Hurricane Ivan's low is about 29 degrees north; so, the low rotates west to east about 908 mph (1460 k/h). Hurricane Ivan extends north to about 35 degrees north (about the latitude of Spartanburg, SC). The air at this latitude spins about 852 mph (1370 k/h) — slower than the low. Ivan sweeps south to about 24.5 degrees north; those air masses spin at about 945 mph (1520) — faster than the low.

The diagram shows these two most northerly and most southerly air masses, and some points in between. I subtracted the speed of the low (908 mph) from each of the W to E speeds, to give the speeds relative to the low. The winds north of the low blow from east to west, as indicated by the little red arrows. The winds south of the low blow from west to east, relative to the low. The result is counterclockwise motion. (To illustrate the concept, I read the latitudes off the figure and they are, therefore, approximate.)

Thus the air masses circulate counterclockwise in the Northern Hemisphere — because of the direction of Earth's spin.

It's much the same in the Southern Hemisphere. Suppose the cyclone's low-pressure area is off the coast of Samoa in the South Pacific at the spot marked 'L' in the diagram. Higher-pressure area from the north, west, east and south will move towards the low pressure L as depicted by the white arrows. But the air to the south moves slower than the low-pressure area (since it's closer to the pole), and lags to the west. Moreover, the air to the north moves faster, and speeds ahead to the east. The different speeds cause the air to circulate clockwise about the low. See the cyclone diagram.

By the way, almost all the planets in our solar system have hurricane-like cyclonic storms. In 1999, an immense (almost four times the size of Texas) hurricane gyrated counterclockwise just off the Martian North Pole.

Further Reading

How hurricanes form, Aerospaceweb.org

Do hurricanes stay near where they form?, WeatherQuesting

When is the hurricane season?A>, WeatherQuesting

How does the energy associated with hydrogen bonds fuel hurricanes?, A>WeatherQuesting

FAQ about hurricanes, National Weather Service

Tropical cyclones, National Weather Service

Do other planets have weather? If so, what kind, what planets?, WeatherQuesting

(Answered March 10, 2008)