Most of us know, in general, what a hurricane is: it is a large storm that develops over water and sometimes moves onto land, it can be very dangerous, and it has its own name. But what other basic information do we need to know about hurricanes? Knowing these basic facts can help you prepare for future storms if you live close to a coastal area, and will also help you understand what hurricanes are all about.
The first basic fact you should know is that hurricanes require a warm, tropical ocean in order to develop. For hurricanes that begin in the Atlantic Ocean, the most favorable time of year is June 1st through November 30 – also known as hurricane season. Next, there must be some type of disturbance in the atmosphere above the tropics – an area of low pressure, for example, or the remnants of a frontal boundary. Hurricanes almost always form at least 300 miles from the equator, since the Coriolis effect which generates the hurricane’s circulation is not strong enough closer to the equator. Finally, the middle of the troposphere, which is the lowest layer of the atmosphere, must be relatively moist.
Once these conditions are met, convection can develop. The warm, moist air over the ocean rises, and as it rises it cools, condensing the water vapor into droplets just as in ordinary cloud formation. Now this basic fact is the key to the hurricane: when the water vapor condenses into clouds, it gives off heat to the air around it. The heated air is lighter and it rises further, creating an area of lower pressure beneath it. Air always moves from high pressure to low pressure, so at the surface the surrounding warm, moist air rushes in to take the place of the rising air. This is the very beginning of the hurricane’s eye – if conditions are favorable and the storm continues to become stronger, this first region of lowered pressure will become the calm, clear eye.
The warm water is the fuel that keeps the storm developing. If something changes – movement of ocean currents, for example – to cool the water, then the storm may fizzle out. If the water stays warm enough, the moist air will continue to take the place of rising air, causing the pressure at the center or eye to become lower and lower. The process may spread over a larger area of ocean, creating a circular wall of thunderstorms around the eye that continues to strengthen with time (and continued warm water fuel).
But why is the storm circular? This basic information is pertinent to all weather study but it can be difficult to understand. Earlier I mentioned the Coriolis effect (sometimes called the Coriolis force), which produces the circulation we see in hurricanes. This effect was first described by the French scientist Gaspard Coriolis in the 19th century. It’s caused by two things: one, the earth is round, and two, the earth turns on its axis. The atmosphere turns with the earth, but at the equator – since they must turn a much greater distance in 24 hours – both it and the earth move faster than they do at the poles. If a storm system is moving south from the North Pole, the surface below it is moving faster, and the storm appears to be pushed to the right (west). Vice versa, if a weather system (such as a hurricane) begins to move north from near the equator, it appears to move to the right (this time the east) because the earth below it is moving more slowly.
As convection continues, the potential hurricane begins to spin counterclockwise due to the Coriolis effect. There is one more element required for the storm’s development, however. Something has to happen to all the air that keeps rising! If it falls again near the center, it will produce high pressure to balance the lower pressure. To prevent this, there must be a high pressure system in the upper troposphere with winds that will disperse the rising air. This is the final factor to separate the mere tropical disturbance from the tropical depression, a storm that has clearly defined circulation, but with sustained winds less than 39 mph. With continued development, the tropical depression may go on to become a named tropical storm (winds 39-73 mph) and, eventually, a hurricane (winds 74 mph and above).
With this basic information in mind, you are better prepared to understand how and why hurricanes develop and progress the way they do. Hopefully you can share the basic facts about hurricanes with friends and family, especially your children!