The usual clarity of the air in the high mountains is associated with the low moisture content of the air. Except for occasional periods of a few hours duration, which seem to accompany the arrival of more moist air from below, the relative humidity is generally below 40 percent, and it often falls to 10 or even five percent. Such low humidities are characteristic of the zone above the inversion that separates the lower and upper air.
Below the inversion, the lowlands and along the lower mountains, the relative humidity commonly averages 70 to 80 percent in windward areas and 60 to 70 percent in leeward ones. Winter relative humidities are somewhat higher than summer ones; and in terms of daily variations the maximum values occur with the minimum temperatures, during the late night and very maximum values occur with the minimum temperatures, during the late night and very early morning. Nighttime values in the cooler and rainier areas often exceed 90 percent, but are more frequently between 70 to 80 percent in the leeward lowlands and other drier localities. Afternoon values are commonly between 60 to 70 percent in the windward and uplands regions, not infrequently between 50 to 55 percent in the drier, warmer coastal zones, and seldom fall below 40 percent anywhere at elevations below the trade wind inversion. Thus, except on the high mountains, the general regime in Hawaii is one of high humidities as compared with conditions in most other states.