1. Born in West Africa When hot, dry air from the Sahara desert meets cooler, moist air from the Sahel region to the south, small storms form.

2. Across the Atlantic Some storms remain small. But a few gather warm ocean moisture, speeding up as they travel west. When bands of these thunderstorms form a swirling pattern, the system is called a tropical depression.

3. Tropical storms and hurricanes If the storm holds together, a column-shaped "eye" forms at its center. Winds spin around it, faster and faster. When winds reach 40 miles per hour, the system is called a tropical storm. When winds reach 74 miles per hour, it's a hurricane.

4. After the storm Hurricanes weaken after they reach land. Some never hit land, but instead turn northeast and die out over the Atlantic Ocean.

Other winds help build the storm Upper-and lower-level winds blowing nearby in the same direction help hurricanes gain strength. Winds blowing in different directions or at different speeds can tear the storm apart.