Climate of Hawaii - Cloud Formation

Cloud Formation

Under trade wind conditions, there is very often a pronounced moisture discontinuity between 4,000 and 8,000 feet (1,200 and 2,400 m). Below these heights, the air is moist; above, it is dry. The break (a large-scale feature of the Pacific High) is caused by a temperature inversion embedded in the moving trade wind air. The inversion tends to suppress the vertical movement of air and so restricts cloud development to the zone just below the inversion. The inversion is present 50 to 70 percent of the time; its height fluctuates from day to day, but it is usually between 5,000 and 7,000 feet (1,500 and 2,100 m). On trade wind days when the inversion is well defined, the clouds develop below these heights with only an occasional cloud top breaking through the inversion.

These towering clouds form along the mountains where the incoming trade wind air converges as it moves up a valley and is forced up and over the mountains to heights of several thousand feet. On days without an inversion, the sky is almost cloudless (completely cloudless skies are extremely rare). In leeward areas well screened from the trade winds (such as the west coast of Maui), skies are clear 30 to 60 percent of the time.

Windward areas tend to be cloudier during the summer, when the trade winds and associated clouds are more prevalent, while leeward areas, which are less affected by cloudy conditions associated with trade wind cloudiness, tend to be cloudier during the winter, when storm fronts pass through more frequently. On Maui, the cloudiest zones are at and just below the summits of the mountains, and at elevations of 2,000 to 4,000 ft (600 to 1,200 m) on the windward sides of Haleakala. In these locations the sky is cloudy more than 70 percent of the time. The usual clarity of the air in the high mountains is associated with the low moisture content of the air.