Hurricanes derive their energy from warm, moist air. The best source of such "fuel" for this heat engine is the air above a very warm (at least 80F) and sufficiently deep (at least 150 feet) layer of water. As air being drawn into the hurricane travels over this warm water, seawater evaporates ensuring that moisture levels are at very high levels as the air enters the storm.


As the water vapor in the rising air condenses to form liquid droplets, latent heat is released into the surrounding atmosphere. This warming of the column allows rising air parcels to proceed to even higher levels in the atmosphere. Barometric pressure at the surface falls in response to the overall warming and decreasing density of the air column. In turn, lower pressure at the surface encourages an increase in the volume of warm, moist air entering the hurricane. A chain reaction is now underway as the influx of fresh marine air condenses and warms the air column even further. Of course, whether or not this chain reaction persists and results in an intense hurricane is dependent upon the presence of many other favorable environmental factors, such as relatively low vertical wind shear.


Tropical cyclone structure from Jetstream: An Online Weather School (NOAA).

Although size can vary dramatically, hurricanes are typically about 300 miles in diameter. A hurricane is an extremely complicated structure comprised of three main features; outer rainbands, an eyewall and the eye. These components are clearly evident in the cut-away diagram of a tropical cyclone to the right.



Rainbands are rings of thunderstorms that spiral in towards the eye from the outer boundary of the storm and provide hurricanes with their signature appearance. Their distance from the hurricane's center makes them the first direct evidence of the system's approach. Rainbands are responsible for most of the rain and tornadoes associated with a hurricane, and are thought to contribute to the overall intensification of the system.


The thunderstorms comprising rainbands are air columns characterized by rising air parcels. As these air parcels reach the top of the column, they spread out and slowly descend outside of the column in the inner and outer lanes between the rainbands. The process of descending air parcels is referred to as subsidence, and results in compressional warming between the rainbands. In addition to creating a generally calm and rain-free region, this warming contributes to lower surface barometric pressure which further promotes the overall intensification of the hurricane.


The relationship between rainbands and the eye in a rapidly intensifying hurricane is not completely understood and its study has largely been limited to computer simulations. A new project called the Hurricane Rainband and Intensity Change Experiment (RAINEX), hopes to unravel some of the mystery by analyzing Doppler radar images and other data that is harvested during aircraft reconnaissance of hurricanes. While the accuracy of the forecast of the track of a hurricane has improved dramatically over the past ten years, the accuracy in predicting the intensity of a hurricane has lagged behind. The goal of the RAINEX study is to narrow this gap. 



The eyewall is a ring of towering thunderstorms that defines the hurricane's eye. Similar to a spinning ice skater, the winds spiraling inwards toward the center of a hurricane have angular momentum. As the air parcels approach to the storm's center, their distance from the center of rotation decreases. The laws of physics require that these parcels maintain their angular momentum so their speed must increase as they continue their approach towards the eye. Eventually, as the force acting on these air parcels comes into balance with the centrifugal force at the storm's center, the inward journey of the air parcels halts and they rise to form the eyewall. The hurricane's most violent winds and deepest convection is associated with the eyewall.


Image of Hurricane Wilma on October 17, 2005 showing a "hot tower" in the eyewall. Graphic from NASA's Goddard Space Flight Center.

Data from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite suggests that features within the eyewall of hurricanes called "hot towers" may be associated with rapid intensification. Hot towers are rain clouds that extend to a height of nearly 9 miles and are significantly taller than most of the clouds within the eyewall. It is the immense amount of latent heat associated with condensation that results in the term "hot" being applied to these air columns.

A joint study completed in early 2004 by researchers at NASA and George Mason University found that tropical cyclones containing a hot tower within in their eyewall were twice as likely to intensify in the six-hour period following the emergence of the tower than those cyclones without such a feature. Ongoing research in this area may allow forecasters to better predict periods of rapid intensification such as that exhibited by Wilma.

The eyewall does not simply form and remain static during the hurricane's life. The diameter of the eyewall may decrease resulting in intensification of the storm and an increase in wind speeds. It is also possible for a rainband to organize into a ring surrounding the eyewall. This usually results in the disintegration of the original eyewall as access to its energy source--moist air--is impeded. Eyewall replacement cycle is the term given to this process where an existing eyewall surrenders to a new outer band and fades away. Although the overall intensity of a hurricane declines during an eyewall replacement cycle, the area of hurricane force wind usually expands. In short, a weaker hurricane with a broader impact is the short-term consequence of this process.


In stark contrast to the eyewall, a hurricane's eye is characterized by light winds and is usually, but not always, cloud-free. As air parcels are vented from the top of the thunderstorms comprising the eyewall, some diverge into the eye. Lacking active convection, these air parcels descend and the temperature within the eye increases due to compressional warming. It is not uncommon for aircraft reconnaissance to record significant temperature differences outside and the inside the eye. For example, the flight early on October 19th observed a 14C difference between the regions. Descending air tends to suppress the development of clouds and precipitation, hence the relatively calm conditions within the eye. The diameter of the eye is directly related to the storm's intensity, with very small eyes associated with the most powerful hurricanes.


The eye is not a consistent diameter from the surface to the upper level of the hurricane. In fact, in some instances the diameter of the eye expands significantly as altitude increases resulting in the "stadium effect". This tendency for the eye to expand with increased height is the result of the forces affecting the winds located within the eyewall to change as altitude increases. Friction decreases with height, so wind speeds increase as air parcels within the eyewall ascend. The Coriolis force and centrifugal force, both related to wind speed, increase as the magnitude of the wind increases. In contrast, the pressure gradient force affecting the parcels decreases with increased altitude. The interaction of these forces allow the parcels to move outward and they move upwards and the diameter of the eye increases with altitude.


Passage of the eye over a location is a dramatic event. As the violent winds associated with the eyewall pass, the weather conditions improve rather dramatically, only to deteriorate just as rapidly as the opposite side of the eyewall passes overhead. Deadly consequences result for those who are unfamiliar with the nature of a hurricane and venture outside to survey the damage before the entire storm has passed.


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© 2005-2006 Mark A. Thornton