What Is the Structure of a Hurricane?

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Hurricane Igor - way too early for Halloween. Image courtesy of NASA

Hurricane Igor – way too early for Halloween. Image courtesy of NASA

A well-developed tropical system, with a closed low pressure center, has a characteristic look on satellite imagery. It is symmetrically round, which means there is no vertical shear. The highest clouds, which show up as bright white on the image, closely surround the center.

At low levels, wind is circulating counterclockwise with a component towards the center. This motion is hidden on satellite imagery by the higher clouds. What we see is counterclockwise motion of high clouds on the periphery of the storm with an outward component. This is air exiting the system and causing the pressure to remain low or fall further near the center.

The Eye of a Hurricane

A well-developed hurricane has a clear area near its center known as an eye. The eye can be clearly visible on satellite imagery, and normally measures between three and thirty miles across. The formation of an eye in a developing hurricane forebodes strengthening and the disappearance of an eye due to clouds indicates weakening of the storm.

Tornadoes Spin off Hurricanes

Just as nature dislikes motion in an absolutely straight line, it also is not fond of any sort of regular motion such as a circular rotation around a low pressure center. There are bound to be perturbations, and these can spin in various ways. The hurricane is surrounded by areas of higher or lower winds, and even little spirals of their own. When the hurricane hits land, some of these are likely to spin off as tornadoes.

Hot Towers and Hurricanes

Just as the horizontal flow can vary, so can the vertical structure. With all the chaotic motion associated with a strong hurricane, unusually powerful updrafts can occur, which can build into thunderstorms that reach to unusual heights. The part of the atmosphere that produces weather has a lid on it called the tropopause, a surface of demarcation between the troposphere, with its instabilities, and the stable stratosphere.

Weather rarely invades the stratosphere, but on occasion a thunderstorm can poke a hole in the tropopause. Conditions for this to happen are right in a hurricane, and the result is a hot tower — an updraft that breaches the tropopause. Because of the turbulence associated with hot towers, they are locations in the hurricane where the strongest surface winds occur.

To put it simply, if the updraft is strong enough to power through the tropopause, there must be powerful inflow at low levels.

The Future of Hurricanes

There is little doubt that air and water temperatures are rising worldwide. Other things being equal, warmer water means more, and more intense, hurricanes. However, the feedback mechanisms of climate change have not been established, and other things may not be equal. The summer of 2013, for example, was characterized by very dry African air over much of the Atlantic Ocean, and it was a summer with fewer strong storms than normally occur.

On the other hand, in 2013 the Pacific typhoon season was active, and the strongest cyclone ever recorded in the Indian ocean struck India a devastating blow. So regardless of trends or forecasts (all the ‘expert’ hurricane forecasts for the Atlantic for 2013 were wrong), those who live in areas where hurricanes historically have struck should always be prepared.

Further Reading:

Sirvatka, P. Hurricanes and Tropical Meteorology. College of DuPage. Accessed October 25, 2013.

NOAA. Hurricane Basics. Accessed October 25, 2013.

National Hurricane Center. Hurricane PreparednessAccessed October 25, 2013.

Weather Underground. Tropical Weather — Easterly Waves. (2010). Accessed October 25, 2013.

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