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| Hurricane Basics |
Hurricane Structure
EYE
EYE WALL
- Located at the hurricane's center and can measure 20 to 30 miles wide (10 to 65 km).
- An eye will usually develop when the maximum sustained wind speeds go above 78 mph (130 kph).
- A cloud free area of sinking air and light winds that usually doesn't exceed 15 mph (24 kph).
- The calmest part of the storm.
- Generally shrinks in size when the storm strengthens.
RAIN BANDS
- Completely or partially (at least 50%) surrounds the eye of a mature hurricane and is an area where winds may gust to more than 200 mph (320 kph).
- Consists of a ring of tall thunderstorms that produce heavy rains and very strong winds.
- Has the most destructive section of the storm on the side where the wind blows in the same direction as the storm's forward motion.
- Curved bands of clouds that trail away from the eye wall in a spiral fashion.
- Capable of producing heavy bursts of rain and wind, perhaps one-half or two-thirds the strength of those associated with the eye wall.
- May cause a hurricane's diameter to extend outward up to 340 miles (548 km).
Hurricane Formation
The hurricane season in the Atlantic, Caribbean, Gulf of Mexico, and Central Pacific is between June 1 and November 30. August and September are typically the peak months of the season.
The hurricane season in the Eastern Pacific is between May 15 and November 30.
Hurricane development often begins over tropical areas of the ocean near the equator where the water is at least 80º F (27º C). As the heat and moisture rise due to convection, cluster thunderstorms are formed, creating a tropical disturbance.
These cluster thunderstorms may then begin to rotate due to the Coriolis force, resulting in an organized mass of thunderstorms that move in a particular circular direction (counterclockwise in the northern hemisphere and clockwise in the southern hemisphere).
As the swirling clouds and rain become more organized and begin to circulate around a center with sustained wind speeds of less than 38 mph (61 kph/33 kt), the storm is classified as a tropical depression. Once the winds gain speed and become constant at 39 mph, the tropical storm is given a name to identify and track it.
The tropical storm may then gain energy as it is fueled by water vapor that is pulled up from the warm ocean surface by upper level winds. With an increase in energy, the storm winds may reach a constant speed of 74 mph (119 kph/64 kt) and develop into a hurricane.
Hurricanes can last for more than two weeks over open waters and can run a path across the entire length of the eastern coast. When a hurricane crosses land or cooler waters, it loses its source of power, and its winds gradually slow until they are no longer of hurricane force.
Hurricane Movement
Prevailing Wind Currents - The prevailing wind currents that surround a storm determine the movement of a hurricane. The storm is guided in the direction of the wind currents, which also determine the forward traveling speed of the storm. When the steering winds are strong, it is easier to predict where a hurricane will go. When the steering winds are weak, a storm often follows an erratic path that makes forecasting very difficult.
Circulating Winds - The movement of a hurricane affects the speed of the winds that circulate around the eye of the storm. On one side of the storm, where the circulating winds and the entire storm are moving in the same direction, the forward movement of the storm increases the wind speed. While on the opposite side of the storm, the forward motion decreases the circulating wind speed. For example, in the Northern Hemisphere a hurricane's strongest winds are usually found in its right-front side.
Tropical Trade Winds - The average hurricane moves from east to west due to the tropical trade winds that blow near the equator.
High-Pressure Zones - The clockwise rotation of air associated with high-pressure zones is the driving force that causes many hurricanes to stray from their east-to-west movement and start northward.
Coriolis Force - The Coriolis force can have an effect on the movement of some storms. For example, in the northern hemisphere, the Coriolis force may cause a tropical system to curve to the north and then move eastwards.
Wind Shear - Tropical systems weaken when their sources of heat and moisture are cut off or when they encounter strong wind shear.
