Hurricane Basics 2
2.6. Hurricane Structure. Contrary to how many weather maps appear, a hurricane is more than a point on a weather map, and its path is more than a line. It is a large system that can affect a wide area, requiring that precautions be taken far from where the eye is predicted to come ashore. This section talks about the different parts of the hurricane and will help you better understand hurricane hazards. The main parts of a hurricane (Figure 2.4.) are the rainbands on its outer edges, the eye, and the eyewall. Air spirals in toward the center in a counter-clockwise pattern, and out the top in the opposite direction. In the very center of the storm, air sinks, forming the cloud-free eye.
2.6.1. The Eye. The hurricane's center is a relatively calm, clear area usually 20-40 miles across. People in the midst of a hurricane are often amazed at how the incredibly fierce winds and rain can suddenly stop and the sky clear when the eye comes over them. Then, just as quickly, the winds and rain begin again, but this time from the opposite direction.
2.6.2. The Eyewall. The dense wall of thunderstorms surrounding the eye has the strongest winds within the storm. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm's intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form.
2.6.3. The Spiral Rainbands. The storm's outer rainbands (often with hurricane or tropical storm-force winds) can extend a few hundred miles from the center. These dense bands of thunderstorms, which spiral slowly counterclockwise, range in width from a few miles to tens of miles and are 50 to 300 miles long.
2.6.4. Hurricane Size : Hurricane Size. . Typical hurricanes are about 300 miles wide although they can vary considerably (Figure 2.5.). Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), the most devastating hurricane of this century, was a relatively small hurricane. Do not focus on the location and track of the center, because the hurricane's destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward to about 25 miles from the storm center of a small hurricane and to more than 150 miles for a large one. The area over which tropical storm-force winds occur is even greater, ranging as far out as almost 300 miles from the eye of a large hurricane.
2.6.5. Hurricane Circulation and MovementFigure 2.6. Circulation and Movement. . In the Northern Hemisphere, hurricane winds circulate around the center in a counter-clockwise fashion. This means that the wind direction at your location depends on where the hurricane's eye is. A hurricane’s speed and path depends on complex interactions between the storm with its own internal circulation’s and the earth’s atmosphere. The air in which the hurricane is embedded is a constantly moving and changing "river" of air. Other features in that flow, such as high and low pressure systems, can greatly alter the speed and the path of the hurricane. In turn, it can modify the environment around the storm. Typically, a hurricane's forward speed averages around 15-20 mph. However, some hurricanes stall, often causing devastatingly heavy rain. Others can accelerate to more than 60 mph. Some hurricanes follow a fairly straight course, while others loop and wobble along the path (Figure 2.6.). These seemingly erratic changes are difficult to forecast.
2.6.6. The Right Side of the StormFigure 2.7. The Right Side. Figure 2.8. Storm Surge. . As a general rule of thumb, the hurricane's right side (relative to the direction it is traveling) is the most dangerous part of the storm because of the additive effect of the hurricane wind speed and speed of the larger atmospheric flow (the steering winds) (Figure 2.7.). The increased winds on the right side increase the storm surge. Tornadoes are also more common here