On the western sides of oceans in the trade wind belt, surface winds vary little and heavy rainfall occurs mostly as periodic rather than continuous events. This rainfall is most frequently associated with a wavelike disturbance of the normal isobaric pressure pattern, a disturbance called an easterly wave.

In the North Atlantic Ocean, between 5 and 15 degrees north, the waves are first seen in April or May and continue until October or November. With a wavelength of about 2000–2500 km, and a period of 3–4 days, they move at approximately 18–36 km/h. Passing from the Africa onto the cool Eastern Atlantic Ocean, the waves generally decay, but some survive to regenerate over the Western Atlantic and Caribbean. About 10% of all easterly waves survive to develop into gale-force tropical storms or hurricanes.