This results in a much weaker tradewind flow and on occasion no tradewinds flow over the islands. The tradewind flow produces marked, sometimes dramatic day to day weather variations throughout the islands. It is the major cause of the variable cloudy conditions and rainfall experienced on Oahu. During normal tradewind flow, rainfall is almost totally derived from orographic lifting of the tradewind air mass up slope on the windward side of the mountains causing the formation of cumulus, which sometimes builds to considerable heights. Showers usually begin on the windward and upper slopes and continue to the leeward side of the mountains.
Rainfall associated with the undisturbed tradewinds falls from cumuliform type cloudiness.
The amount of rainfall expected for any given locale is related to the stability of the air, amount of vertical wind shear, distance from the mountain summits in both leeward and windward areas, time of day and the strength and depth of the tradewind flow. Heaviest rainfall occurs during sunrise and sunset, with a maximum around sunrise in an area just leeward of the Koolau mountain range summit. This is directly related to the diurnal tropical cloud maximum. The Tradewind Inversion . It exists throughout most of the year over the Hawaiian Islands.