A tropical wave is a trough or cyclonic curvature maximum in the trade wind easterlies. These waves tend to reach maximum amplitude in the lower to middle troposphere and may or may not be accompanied by thunderstorm clusters. Although there is still some debate on the issue, these easterly waves are thought to originate or become amplified as a result of meteorological conditions over the continent of Africa.
Each hurricane season approximately 60 of these waves cross the tropical North Atlantic. Although the majority of these waves pass through the basin without any significant tropical cyclone development, passage of these waves is often accompanied by squally weather with brief periods of higher sustained winds.
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A tropical disturbance is a discrete tropical weather system with apparently organized convection (generally 100 to 300 miles in diameter) originating in the tropics or subtropics, having a non frontal migratory character, and maintaining its identity for 24 hours or more.
Tropical cyclones in which the maximum sustained surface wind speed (1-minute mean) is 33 KT or less. Tropical depressions must have a closed surface circulation in order to be classified in this category.