The two essential ingredients in every hurricane are warm water and moist, warm air. That's why hurricanes begin in the tropics.
Many 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. Other hurricanes originate from unstable air pockets popping out in the Gulf of Mexico.
Warm Air and Warm Water Make Conditions Right
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, like water swirling down a drain.
Where Does a Hurricane's Energy Come From?
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 pulls heat away from the storm's center and cools 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 a thunderstorm to a 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)
Climate Change and Hurricanes
Scientists agree on the mechanics of hurricane formation, and they agree that hurricane activity can surge in an area over a few years and die off elsewhere. That, however, is where the consensus ends.
Some scientists believe that the contribution of human activity to global warming (increasing air and water temperatures worldwide) is making it easier for hurricanes to form and gain destructive force. Other scientists believe that any increase in severe hurricanes over the past few decades would be 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.
For now, climatologists are busy examining the interactions between these facts:
- Air and water temperatures are rising worldwide. Average global temperatures reached a record high in 2016.
- 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.
- At the same time, hurricane activity in the Atlantic basin has been in a relative lull for many years now. Pacific typhoons (hurricanes in the Pacific basin), on the other hand, have been increasing in frequency and severity.