4. Hurricanes are powerful windstorms. A hurricane begins in a low pressure area over warm water. Air is pulled inward by the low pressure. Wind begins to rotate counterclockwise (逆时针旋转). As pressure drops near the center, wind speed increases. It pulls wet air from the ocean's surface. The wet air moves upward in the center. The air cools and rainstorms form. This pattern continues as the storm feeds on itself, growing stronger and higher. When the wind speed is greater than 73 mph, it is a hurricane. A column of calm, warm, cloudless air forms in the center. It is the eye of the storm.
A hurricane can become 10 miles high and 200 miles wide. It travels across the ocean at several different speeds. It might change direction several times. That is why it is hard to know when and where it will hit land. If it hits land, it can cause more destruction than any other type of storm on Earth. It weakens fast as it passes over land.
Hurricanes are different, depending on their wind speeds. The winds of Class I are 74 — 95 miles per hour (mph). The highest class is 5. This means the winds are moving faster than 155 mph. The higher the class, the more damage it causes.
Hurricanes are given names so weatherman can tell them apart when there are more than one over the ocean. Names were first given to possible hurricanes by the United States Weather Bureau in 1953 with women's names such as Carol, Sarah, Maria, or Helene. On May 12, 1978, it was announced that hurricanes would also have men's names like Hugo, David, Gilbert, or Alberto. Now, alphabetical (按字母顺序的) lists of male and female names are used. By putting the names in alphabetical order, it is easy to know if the hurricane is the first, third or fifth one in that year. Each list includes up to 24 names, but they are seldom all used.