Hurricanes are large, swirling storms. They produce winds of 119 kilometers per hour (74 mph) or higher. That’s faster than a cheetah, the fastest animal on land. Winds from a hurricane can damage buildings and trees.
Hurricanes form over warm ocean waters. Sometimes they strike land. When a hurricane reaches land, it pushes a wall of ocean water ashore. This wall of water is called a storm surge. Heavy rain and storm surge from a hurricane can cause flooding.
Once a hurricane forms, weather forecasters predict its path. They also predict how strong it will get. This information helps people get ready for the storm.
There are five types, or categories, of hurricanes. The scale of categories is called the Saffir-Simpson Hurricane Scale. The categories are based on wind speed.
- Category 1: Winds 119-153 km/hr (74-95 mph) - faster than a cheetah
- Category 2: Winds 154-177 km/hr (96-110 mph) - as fast or faster than a baseball pitcher’s fastball
- Category 3: Winds 178-208 km/hr (111-129 mph) - similar, or close, to the serving speed of many professional tennis players
- Category 4: Winds 209-251 km/hr (130-156 mph) - faster than the world’s fastest rollercoaster
- Category 5: Winds more than 252 km/hr (157 mph) - similar, or close, to the speed of some high-speed trains
- Eye: The eye is the “hole” at the center of the storm. Winds are light in this area. Skies are partly cloudy, and sometimes even clear.
- Eye wall: The eye wall is a ring of thunderstorms. These storms swirl around the eye. The wall is where winds are strongest and rain is heaviest.
- Rain bands: Bands of clouds and rain go far out from a hurricane’s eye wall. These bands stretch for hundreds of miles. They contain thunderstorms and sometimes tornadoes.
A hurricane starts out as a tropical disturbance. This is an area over warm ocean waters where rain clouds are building.
A tropical disturbance sometimes grows into a tropical depression. This is an area of rotating thunderstorms with winds of 62 km/hr (38 mph) or less.
A tropical depression becomes a tropical storm if its winds reach 63 km/hr (39 mph).
A tropical storm becomes a hurricane if its winds reach 119 km/hr (74 mph).
Scientists don’t know exactly why or how a hurricane forms. But they do know that two main ingredients are needed.
One ingredient is warm water. Warm ocean waters provide the energy a storm needs to become a hurricane. Usually, the surface water temperature must be 26 degrees Celsius (79 degrees Fahrenheit) or higher for a hurricane to form.
The other ingredient is winds that don’t change much in speed or direction as they go up in the sky. Winds that change a lot with height can rip storms apart.
An hurricane or tropical storm is a tropical cyclone that forms in the Ocean.
“Tropical” refers to the geographical origin of these systems, which form almost exclusively over tropical seas. “Cyclone” refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor’easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter. Every year tropical cyclones impact various regions of the globe including the Gulf Coast of North America, Australia, India and Bangladesh.
The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth’s rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are very rare in the South Atlantic (although occasional examples do occur) due to consistently strong wind shear and a weak Intertropical Convergence Zone. Conversely, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea, while cyclones near Australia owe their genesis to the Asian monsoon and Western Pacific Warm Pool.
The primary energy source for these storms is warm ocean waters. These storms are therefore typically strongest when over or near water, and weaken quite rapidly over land. This causes coastal regions to be particularly vulnerable to tropical cyclones, compared to inland regions. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to wind and severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones draw in air from a large area and concentrate the water content of that air (from atmospheric moisture and moisture evaporated from water) into precipitation over a much smaller area. This replenishing of moisture-bearing air after rain may cause multi-hour or multi-day extremely heavy rain up to 40 km (25 mi) from the coastline, far beyond the amount of water that the local atmosphere holds at any one time. This in turn can lead to river flooding, overland flooding, and a general overwhelming of local water control structures across a large area. Although their effects on human populations can be devastating, tropical cyclones may play a role in relieving drought conditions, though this claim is disputed. They also carry heat and energy away from the tropics and transport it towards temperate latitudes, which plays an important role in regulating global climate.