All About Hurricanes

What are tropical depressions, tropical storms and hurricanes?

• A Tropical Depression is the beginning stages of a tropical cyclone with maximum sustained winds less than 39 mph.
• A Tropical Storm is an organized system of thunderstorms that forms over tropical waters with a defined circulation and maximum sustained winds of 39 to 73 mph (34-63 knots).
• A Hurricane has many of the same characteristics of a tropical storm but is more intense with maximum sustained winds of 74 mph (64 knots) or higher. In the western Pacific, hurricanes are called typhoons and similar storms in the Indian Ocean are called cyclones.
• Hurricanes are further classified according to the Saffir-Simpson Scale.
  • Category 1 (Minimal Damage):
    Pressure greater than 28.94 inches, Winds 74-95 mph, Storm surge 4-5 feet
  • Category 2 (Moderate Damage):
    Pressure 28.50-28.91 inches, Winds 96-110 mph, Storm surge 6-8 feet
  • Category 3 (Extensive Damage):
    Pressure 27.91-28.47", Winds 111-130 mph, Storm surge 9-12 feet
  • Category 4 (Extreme Damage):
    Pressure 27.17-27.88", Winds 131-155 mph, Storm surge 13-18 feet
  • Category 5 (Catastrophic Damage):
    Pressure less than 27.17", Winds > 155 mph, Storm surge > 18 feet

How do hurricanes form?

• Developing hurricanes gather heat and energy through contact with warm ocean waters. They are steered by the easterly trade winds and the temperate westerlies as well as by their own energy. Around a hurricane`s core, winds grow with great velocity, generating violent seas.
• The addition of moisture by evaporation from the sea surface powers them like giant heat engines.

Where and when do hurricanes usually form?

• In the eastern Pacific, hurricanes begin forming by mid-May.
• In the Atlantic, Caribbean, and Gulf of Mexico, hurricane season runs from June to November.
• For the United States, the peak hurricane threat exists from mid-August to late October.
• Over other parts of the world, such as the western Pacific, hurricanes can occur year-round.
• Globally on average, September is the most active month and May is the least active for hurricanes.
• Each year on average, eleven tropical storms (of which six become hurricanes) develop over the Atlantic Ocean, Caribbean Sea or Gulf of Mexico. About five hurricanes strike the United States coastline every three years. Of these five, two will be major hurricanes (Cat. 3 or greater on the Saffir-Simpson Hurricane Scale).

What are some of the hazards associated with hurricanes?

• Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property.
• Storm tide is a combination of the storm surge and the normal astronomical tide. If the storm surge arrives at the same time as the high tide, the water height will be even greater. Storm surge and storm tide can cause severe flooding in coastal areas in the path of the hurricane.
• Winds of 74 mph (Category 1) or more can destroy poorly constructed buildings and mobile homes. High-rise buildings are also vulnerable, particularly on higher floors where wind speeds are higher. Debris, such as signs, roofing material and siding become flying missiles in hurricanes. Hurricane force winds can reach well inland.
• Heavy rains and widespread flooding often accompany hurricanes. This is the major threat to areas well inland. More people have died from inland flooding than from storm surge since inland flooding is not related to the storm`s wind speed.
• Hurricanes also can produce tornadoes, which add to their destructive power. These tornadoes most often occur in thunderstorms embedded in rain bands well away from the center of the hurricane. However, they can also occur near the eyewall.

What areas are most prone to land-falling hurricanes?

• All Atlantic and Gulf coastal areas are subject to hurricanes or tropical storms. Although rarely suffering a direct hit by hurricanes, parts of the Southwest United States and Pacific Coast suffer heavy rain and floods each year from the remnants of hurricanes spawned off Mexico.
• Islands such as Hawaii, Guam, American Samoa, and Puerto Rico are also subject to hurricanes. Due to the limited number of evacuation routes, barrier islands are especially vulnerable to hurricanes.