Tropical Cyclone

In some ways tropical cyclones are similar to the low pressure systems that cause weather changes at higher latitudes in places like the United States and Europe. These systems are called extratropical cyclones and are marked with an "L" on weather maps. These weather systems are large masses of air circulating cyclonically (counterclockwise in the northern hemisphere and clockwise in the southern hemisphere). Cyclonic circulation is caused by two forces acting on the air: the pressure gradient and the Coriolis force.

In both cyclone types air rises at the center, creating a region of lower air (barometric) pressure. Since air is a fluid, it will rush in from elsewhere to fill the void left by air that is rising off the surface. The effect is the same as when a plug is pulled out of a full bathtub: water going down the drain is replaced by water rushing in from other parts of the tub. This is called the pressure gradient force because air moves from regions of high pressure to lower pressure. Pressure gradient forces are responsible for most of our day-to-day winds. As the air moves toward low pressure, the Coriolis force turns the air to the right of its straight line motion (when viewed from above). In the Southern Hemisphere the reverse is true: the Coriolis force pushes the moving air to the left. The air, formerly going straight toward a low pressure region, is forced to turn away from it. The two forces are in balance when the air circles around the low pressure zone with a constant radius creating a stable cyclone rotating counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

All large scale air movements such as hurricanes, typhoons, extratropical cyclones, and large thunderstorms tend to set up a cyclonic circulation in this manner. (Smaller scale circulations such as the vortex that forms in a bathtub drain are not cyclonic because the Coriolis force is overwhelmed by other forces. You can make a bathtub drain vortex rotate clockwise or counterclockwise simply by stirring the water the right way. The larger a system is the more likely that the Coriolis force will prevail and the rotation will be cyclonic.) The Coriolis force is a consequence of the rotation of Earth. Moving air masses, like any other physical body, tend to move in a straight line. However, we observe them moving over Earth's surface, which is rotating underneath the moving air. From our perspective the air appears to be turning even though it is actually going in a straight line, and it is we who are moving.

The Coriolis effect can be demonstrated by two people riding across from each other on a merry-go-round. If one person throws a ball straight at his friend she will rotate out of position while the ball is moving and will be unable to catch it. To the two observers the ball seemed to curve away from the catcher as if some force pushed it. Of course the ball actually went perfectly straight but the observer's rotating frame of reference made it appear that a force was at work. On the surface of the rotating Earth this apparent force—the Coriolis force—makes moving air masses curve with respect to the surface and sets up cyclonic circulation.

In both tropical and extratropical cyclones, the rising air at the cyclone center causes clouds and precipitation to form. A fully developed hurricane consists of bands of thunderstorms that grow larger and more intense as they move closer to the cyclone center. The area of strongest updrafts can be found along the inner wall of the hurricane. Inside this inner wall lies the eye, a region where air is descending. Descending air is associated with clearing skies, therefore, in the eye the torrential rain of the hurricane ends, the skies clear, and winds drop to nearly calm. If you are in the eye of a hurricane the eye wall clouds appear as just that: towering vertical walls of thunderstorm clouds, stretching up to 7.5 mi (12 km) in height, and usually completely surrounding the eye. Hurricanes and other tropical cyclones move at the speed of the prevailing winds, typically 10-20 MPH (16-32 km/h) in the tropics. A hurricane eye passes over an observer in less than an hour, replaced by the high winds and heavy rain of the intense inner thunderstorms.