Tropical Cyclone

Several conditions are necessary to create a tropical cyclone. Warm sea surface temperatures, which reach a peak in late summer, are required to create and maintain the very warm, humid air mass in which tropical cyclones grow. This provides energy for storm Overhead view of the eye of a tropical cyclone. National Oceanic and Atmospheric Administration.
development through the heat stored in humid air called latent heat. It takes energy to change water into vapor; that is why one must add heat to boil a kettle of water. The reverse is also true: when vapor condenses back to form liquid water, heat is released that may heat up the surrounding air. In a storm such as a hurricane, many hundreds of tons of humid air are forced to rise and cool, condensing out tons of water droplets and liberating a vast quantity of heat. This warms the surrounding air causing it to expand and become even more buoyant, that is, more like a hot air balloon. More air begins rising, causing even more humid air to be drawn into the cyclone. This process feeds on itself until it forms a cyclonic storm of huge proportions. The more humid air available to a tropical cyclone the greater its upward growth will be and the more intense it will become.

For storm growth to get started some air needs to begin rising. Because tropical air masses are so uniformly warm and humid, the atmosphere over much of the tropics is fairly stable; that is, it does not support rising air and the development of storms. Thunderstorms occasionally develop but tend to be short-lived and small in scale, unlike the severe thunderstorms in the middle latitudes. During the late summer this peaceful picture changes. Tropical disturbances begin to appear. These can take the form of a cluster of particularly strong thunderstorms or perhaps a storm system moving westward off of the African continent and out to sea. Tropical disturbances are regions of lower pressure at the surface. As we have seen, this can lead to air rushing into the low pressure zone and setting up a vortex, or rotating air column, with rising air at its core.

An additional element is needed for tropical cyclone development: a constant wind direction with height throughout the lower atmosphere. This allows the growing vortex to stretch upward throughout the atmosphere without being sheared apart. Even with all these elements present only a few of the many tropical disturbances observed each year become hurricanes or typhoons. Some sort of extra kick is necessary to start the growth of a hurricane. This often comes when tropical disturbance near the surface encounters a similar disturbance in the air flow at higher levels such as a region of low pressure at about the 3 mi (5 km) level (called an upper low). These upper lows sometimes wander toward the equator from higher latitudes where they were part of a decaying weather system.

A computer enhanced image of Hurricane Diana at its strongest on September 11, 1984. The hurricane was just off the coast of South and North Carolina at the time, and winds within it were 130 MPH (209 km/h). U.S. National Aeronautics and Space Administration (NASA).

Once a tropical disturbance has begun to intensify a chain reaction occurs. The disturbance draws in humid air and begins rising. Eventually it condenses to form water droplets. This releases latent heat, which warms the air, making it less dense and more buoyant. The air rises more quickly off of the surface. As a result, the pressure in the disturbance drops and more humid air moves toward the storm. Meanwhile, the disturbance starts its cyclonic rotation and surface winds begin to increase. Soon the tropical disturbance forms a circular ring of low air pressure and becomes known as a tropical depression. As more heat energy is liberated and updrafts increase inside the vortex, the internal barometric pressure continues to drop and the incoming winds increase. When wind speeds increase beyond 37 MPH (60 km/h) the depression is upgraded to a tropical storm. If the winds reach 75 MPH (120 km/h) the tropical storm is officially classified as a hurricane (or typhoon, cyclone, etc., depending on location). The chain reaction driving this storm growth is very efficient. About 50-70% of tropical storms intensify to hurricanes.

A mature tropical cyclone is a giant low pressure system pulling in humid air, releasing its heat, and transforming it into powerful winds. The storm can range in diameter from 60-600 mi (100-1000 km) with wind speeds greater than 200 MPH (320 km/h). The central barometric pressure of the hurricane drops 60 millibars (mb) below the normal sea level pressure of 1013 mb. By comparison, the passage of a strong storm front in the middle latitudes may cause a drop of about 20-30 mb. The size and strength of the storm is limited only by the air's humidity, which is determined by ocean temperature. It is estimated that for every 1.8°F (1°C) increase in sea surface temperature the central pressure of a tropical cyclone can drop 12 mb. With such low central pressure, winds are directed inward, but near the center of the storm the winds are rotating so rapidly the Coriolis force prevents any further inward movement. This inner boundary creates the eye of the tropical cyclone. Unable to go in, the air is forced to move upward then spread out at an altitude of about 7.5 mi (12 km). Viewed from above by a satellite, the tropical cyclone appears as a mass of clouds diverging away from the central eye.