Tropical cyclones, e.g. hurricanes, often develop from a shallow disturbance embedded in the general easterly trade wind circulation. Such a migratory wavelike disturbance of the tropical easterlies is called an easterly wave, which is sometimes also known as tropical wave or equatorial wave.
Generally speaking an easterly wave is a weak trough of low pressure developing from an organized cluster of thunderstorms, known as meso-scale convective systems (MCS), either over equatorial West Africa or further west over the equatorial eastern Atlantic. In pressure maps and on satellite imagery this slow moving trough looks like a wave or inverted 'V', hence the name. Above sea waters easterly waves are characterized by extremely fine weather to the west of it (due to a process meteorologists call divergenc
e) and heavy rain and showers, thick and towering clouds to the east of the trough line. However, this asymmetric weather pattern may be greatly distorted by orographic and diurnal influences if the wave passes over land areas.
Easterly waves last from a couple of days to several weeks and new waves are developing at intervals of 3 to 4 days. Generally more slowly than the current in which it is embedded a typical easterly wave travels at speeds between 10 to 15 knots resulting in a wavelength of about 2,000 to 2,500 km. Although easterly waves form all year round, they are most frequent and most vigorous during late spring and late summer.
Initially easterly waves are fueled by a relatively shallow layer of moist air converging at the through line. However, over warm tropical Atlantic waters they often become intensified by latent heat. If there is enough 'fuel', i.e. sea surface water warmer then 26°C, the trough will be deepening becoming more organized forming a tropical storm and maybe a tropical cyclone eventually.
Easterly waves are especially frequent over the tropical Atlantic and the western Pacific Ocean. Satellite imagery provides the best view of an easterly wave as the wave is indicated by its line or cluster of cumulus clouds. While meteorolgists always keep a good eye on easterly waves, as they might hit the next coast to the west as a massive hurricane or typhoon, they are highly welcomed over West Africa as they are the main source of moisture for the dry Sahel region.