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What Causes Hurricanes?

Warm Air and Warm Water Combine to Create Destructive Storms


Flooded streets in Hoboken, NJ  after Hurricane Sandy

Flooded streets in Hoboken, NJ after Hurricane Sandy

Dennis K. Johnson/Lonely Planet Images/Getty Images

The two essential ingredients in every hurricane are warm water and moist warm air. That’s why hurricanes begin in the tropics.

Most Atlantic hurricanes start to take shape when thunderstorms along the west coast of Africa drift out over warm ocean waters that are at least 80 degrees Fahrenheit (27 degrees Celsius), where they encounter converging winds from around the equator.

Warm Air, Warm Water Make Conditions Right for Hurricanes
Hurricanes start when warm, moist air from the ocean surface begins to rise rapidly, where it encounters cooler air that causes the warm water vapor to condense and to form storm clouds and drops of rain. The condensation also releases latent heat, which warms the cool air above, causing it to rise and make way for more warm humid air from the ocean below.

As this cycle continues, more warm moist air is drawn into the developing storm and more heat is transferred from the surface of the ocean to the atmosphere. This continuing heat exchange creates a wind pattern that spirals around a relatively calm center, or eye, like water swirling down a drain.

Converging Winds Create Hurricanes
Converging winds near the surface of the water collide, pushing more water vapor upward, increasing the circulation of warm air, and accelerating the speed of the wind. At the same time, strong winds blowing steadily at higher altitudes pull the rising warm air away from the storm’s center and send it swirling into the hurricane’s classic cyclone pattern.

High-pressure air at high altitudes, usually above 30,000 feet (9,000 meters), also pull heat away from the storm’s center and cool the rising air. As high-pressure air is drawn into the low-pressure center of the storm, the speed of the wind continues to increase.

As the storm builds from thunderstorm to hurricane, it passes through three distinct stages based on wind speed:

  • Tropical depression—wind speeds of less than 38 miles per hour (61.15 kilometers per hour)
  • Tropical storm—wind speeds of 39 mph to 73 mph (62.76 kph to 117.48 kph)
  • Hurricane—wind speeds greater than 74 mph (119.09 kph)

Scientists Debate Cause of Temperature Changes that Create Hurricanes
While scientists agree on the mechanics of hurricane formation, and they agree that hurricanes are becoming more frequent and severe, that’s where consensus ends.

Some scientists believe that human activity already has contributed significantly to global warming, which is increasing air and water temperatures worldwide and making it easier for hurricanes to form and gain destructive force.

Other scientists believe that the increase in severe hurricanes over the past decade is due to natural salinity and temperature changes deep in the Atlantic—part of a natural environmental cycle that shifts back and forth every 40-60 years.

Frequency and Severity of Hurricanes Likely to Increase
While the scientific community debates the root cause of the temperature changes that are contributing to the current increase in destructive hurricanes, three things are apparent:

  • Air and water temperatures are rising worldwide.
  • Human activities such as deforestation and greenhouse gas emissions from a wide range of industrial and agricultural processes are contributing to those temperature changes at a greater rate today than in the past.
  • Failure to take action now to lower atmospheric levels of greenhouse gases is likely to lead to more frequent and severe hurricanes in the future.

Learn more about the greenhouse effect and what you can do personally to help reduce global warming.

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