Three main features make up a hurricane:
- Eye wall
- Rain bands
The eye is the center of the storm. It is the most calm place in a hurricane since conditions are clear with light winds usually less than 15 mph. The eye is typically 20 to 40 miles across; however, they have been know to be a small as a few miles and much greater than 40 miles. It does not usually develop until winds have reached hurricane strength - exceeding 74 mph - and thus is a good indicator of a strong, organized storm.
2. Eye Wall
The eye wall surrounds the eye. The strongest winds and heaviest rains are found in the eye wall, making it the most dangerous part of the storm. It is made up of deep convective thunderstorms, which form a complete ring around the eye. Contraction or expansion of the eye wall can cause changes in wind speed and storm strength. As a tropical cyclone grows and changes, it can build concentric eye walls that replace the original eye wall. Changes in eye wall size and eye wall replacement pose the greatest challenges in forecasting the strength of the storm.
3. Rain Bands
Rain bands are long, arching bands of clouds and thunderstorms that spiral out from the eye wall. Heavy bursts of rain and wind are usually associated with rain bands. These structures form the outer most fringes of the tropical cyclone structure, and the winds contained within the bands decrease outward from the eye wall. With landfalling hurricanes, tornadoes are a common threat associated with rain bands coming onshore. Also, gaps between the bands are often calm with no wind or rain.