The single event most likely to cause a hurricane in the Atlantic is an African easterly wave, a cluster of thunderstorms that rolls off the west coast of Africa and travels across the ocean. Over the last 50 years, these waves have directly or indirectly triggered 72% of all Atlantic tropical cyclones. But a wave alone isn’t enough. It needs to cross an ocean primed with the right temperature, moisture, and wind conditions to grow from a disorganized storm cluster into a rotating hurricane.
African Easterly Waves: The Starting Event
During the summer and early fall, the temperature contrast between the Sahara Desert and the cooler, wetter regions of central Africa creates a jet stream in the mid-levels of the atmosphere. This jet becomes unstable and sends ripples westward, each one a trough of low pressure surrounded by rising air and thunderstorms. These ripples are African easterly waves, and they leave the African coast roughly every few days during peak hurricane season.
About 61% of Atlantic tropical cyclones form directly from these waves, with another 11% forming indirectly when a wave interacts with other weather features. Waves that track south tend to spark storms earlier, farther east in the open Atlantic, where conditions for development are more favorable. Waves that track north often travel much farther west before anything happens, sometimes not organizing until they reach the Caribbean or the Gulf of Mexico.
Not every wave becomes a hurricane. In a typical season, dozens of easterly waves cross the Atlantic, but only a handful develop into named storms. The difference comes down to whether the wave encounters the right set of ocean and atmospheric conditions along the way.
Warm Water as Fuel
A hurricane is essentially a heat engine, and it draws its energy from warm ocean water. Sea surface temperatures of at least 26°C (about 79°F) are generally considered the minimum threshold for development, and the warm water needs to extend to a meaningful depth, roughly 50 meters or more. If the warm layer is too shallow, the storm’s own churning will pull up cooler water from below and cut off its fuel supply.
As warm, moist air rises off the ocean surface, it condenses into clouds and releases heat. That heat warms the surrounding air, which rises faster, pulling even more moist air in from the surface. This self-reinforcing cycle is what allows a loose cluster of thunderstorms to tighten into a organized system with a defined center of circulation.
Wind Shear: The Make-or-Break Factor
Vertical wind shear, the difference in wind speed and direction between the upper and lower atmosphere, is one of the most reliable predictors of whether a tropical disturbance will develop or fall apart. When upper-level winds blow in a very different direction or at a much faster speed than winds near the surface, they essentially tilt the storm, tearing the top away from the bottom and preventing the organized circulation a hurricane needs.
Research shows that shear above roughly 10 meters per second (about 22 mph) generally prevents new hurricanes from forming. For existing storms, shear in the 8 to 10 meters per second range causes weakening, and shear of 15 meters per second (34 mph) can tear apart even an intense hurricane within a single day. The most powerful storms in simulations consistently form in environments with little to no shear.
The Role of Earth’s Spin
A hurricane needs rotation, and it gets that from the Coriolis effect, the deflection caused by Earth spinning on its axis. This effect is strongest near the poles and weakest near the equator. Tropical cyclones essentially never form within 5 degrees of latitude of the equator because the Coriolis force there is too weak to organize rising air into a spinning vortex. Most Atlantic hurricanes form between about 10 and 20 degrees north latitude, where the ocean is warm and the Coriolis force is strong enough to get a storm spinning.
What Stops a Storm From Forming
Even when an easterly wave crosses warm water with low wind shear, other factors can shut down development. One of the most important in the Atlantic is the Saharan Air Layer, a mass of hot, dry, dusty air that forms over the Sahara and drifts westward over the ocean. This layer typically sits about a mile above the sea surface and extends 2 to 2.5 miles thick. Its warmth creates a temperature inversion that caps rising air, its dryness chokes the moisture a storm needs, and the strong winds embedded within it increase shear. During years when the Saharan Air Layer is particularly active, hurricane formation drops noticeably.
Dry air in the middle levels of the atmosphere, even without Saharan dust, can also kill a developing storm. When dry air gets pulled into a thunderstorm cluster, it causes evaporative cooling that produces downdrafts instead of the sustained updrafts a hurricane requires. A storm needs consistently moist air through the full depth of the atmosphere to keep its convective engine running.
How a Tropical Disturbance Becomes a Hurricane
The path from easterly wave to hurricane follows a progression defined by wind speed. When a disturbance first organizes into a system with a closed circulation and sustained winds below 39 mph, it’s classified as a tropical depression. Once winds reach 39 mph, it becomes a named tropical storm. At 74 mph, it officially becomes a hurricane.
From there, the scale continues upward. A Category 1 hurricane has winds of 74 to 95 mph, enough to snap large tree branches and knock out power for days. Category 2 (96 to 110 mph) causes major roof and siding damage. Category 3 begins the “major hurricane” designation at 111 mph, where well-built homes can lose their roofs and utilities may be out for weeks. Category 4 storms (130 to 156 mph) cause severe structural damage and can leave areas uninhabitable for months. Category 5, at 157 mph or higher, destroys a high percentage of homes outright.
This intensification can happen gradually over days or with alarming speed. Rapid intensification, when a storm’s winds increase by at least 35 mph in 24 hours, tends to occur when a system crosses a pocket of unusually deep warm water with very low shear. Some of the most destructive hurricanes in recent decades underwent rapid intensification shortly before landfall, giving communities little time to prepare.
How Warming Oceans Are Changing the Pattern
Rising ocean temperatures are not clearly increasing the total number of hurricanes that form each year. Current models project little change, or even a slight decrease, in overall tropical cyclone frequency worldwide. What is changing is intensity. The proportion of storms that reach Category 4 and 5 strength is projected to increase over this century, and rapid intensification events are expected to become more common.
Hurricanes that do form in a warmer climate are also expected to produce significantly heavier rainfall. Climate scientists assess with high confidence that human-caused warming has already increased extreme rainfall during tropical cyclones. There is also medium confidence that storms are slowing down in their forward speed, a trend that means more rain falling on the same area for a longer period. The overall picture is fewer but fiercer storms, with the strongest hurricanes of today potentially surpassed by even more powerful ones in the decades ahead.