What Causes Hurricanes?

Hurricanes like Katrina are the most powerful storms in nature. They begin when a group of storms comes together over warm waters in the ocean along the equator. All summer long, these areas absorb heat from the sun. By late summer, the higher water temperatures allow storms to strengthen and grow. This signals the start of “hurricane season.”

Warmer water means more energy for a storm’s development. Storm systems create areas of low pressure, which cause large amounts of water to evaporate. This makes the air very humid. As the warm air rises, cooler air rushes in to replace it. As this cycle intensifies, these rushing winds pick up speed.

At first, these winds blow in toward the center of the storm system. As the storm grows, however, the effect of Earth's rotation begins to spin the storm around. The center rotates in a counterclockwise direction. The winds now whip around the center and create the “eye” of the storm. Swirling bands of clouds and rain curve outward from the center and give the forming hurricane its familiar shape.

Once the winds reach 75 miles per hour (or 120 kilometers per hour), the storm is classified as a true hurricane. Scientists further categorize hurricanes by the speed of their winds near the center. Information can be found at

A Category 5 hurricane is the most powerful of all. When Katrina was churning in the Gulf of Mexico and heading toward the coast, the storm’s winds raged at 165 miles per hour (or nearly 265 kilometers per hour). By the time the hurricane reached land, the winds had slowed enough to make it a Category 4 storm.

Why Are Hurricanes So Dangerous?

Hurricanes like Katrina bring together many of the most destructive kinds of weather. Heavy rains and high winds mix with thunder, lightning, tornadoes, and surging floodwaters. Some hurricanes creep along despite their strong winds. These slow-moving storms continue to dump buckets and buckets of rain for many hours.

The most dangerous part of a hurricane is called the “eye wall.” This area, just outside the center of the storm, usually has the strongest winds. When the storm is over the ocean, low pressure in the eye lifts up the water. As the hurricane moves toward shallow water along the shore, these powerful waves rise up even higher. This is called the “storm surge.” When Katrina slammed into the Gulf Coast region, the hurricane’s storm surge set a record at 29 feet. That was tall enough to submerge a three-story building!

Formation of a Hurricane
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As the eye of a hurricane passes over an area, the rains and winds stop and the clouds go away. This strange effect led to the phrase “the calm at the center of the storm.” Unfortunately, this calm doesn’t last long. Soon the other side of the eye wall strikes, and the storm swings back to full force.

A hurricane may last for days, but the clean-up process can take months more. Water levels may continue to rise as rivers and lakes swell with runoff, or the rain that flows into them. Powerful storms like Katrina can permanently change the geography of an area. It may take years to fully understand how much damage Katrina has done to the Gulf Coast.

STATISTICS ACTIVITY: Select a year (a different one for each student in your class) and research how many hurricanes occurred during that time. Gather statistics such as the categories of the storms, their geographical paths, and the amounts of damage they caused. Create charts and diagrams to share this information with the rest of the class. Discuss ways in which you might combine the class’s information on new charts and diagrams.



Image Credit: a. NASA; b. Macmillan/McGraw-Hill; c. NASA