Tropical waves are troughs of low pressure which occur in tropical areas. According to the National Oceanic and Atmospheric Administration (NOAA) a trough is “an elongated area of low pressure, usually not associated with a closed circulation”. Tropical waves develop from atmospheric disturbances as far away as eastern Africa. They then move toward the west travelling across the African continent and over the Atlantic Ocean. They are carried along by the prevailing easterly winds known as the trade winds.
Tropical waves are very large systems. Travelling at about 20 – 30 km per hour, the entire system may take up to four days to cross a particular location. To the west of a tropical wave, air is descending and the weather is fair. Wind blows from the north-east. As the trough of the wave passes over an area, the atmosphere becomes unstable and there are strong updraughts (rising currents of air). Cumulonimbus clouds form, resulting in heavy rain and thunderstorms. To the east of the wave, the wind blows from the south-east.
There are about 60 – 65 tropical waves in a typical year. They are especially common from April to November, when a new wave leaves the African coast every 2 – 4 days. Under certain conditions, a tropical wave may develop into a hurricane.