380 
PACIFIC SCIENCE, Vol. XVII, October 1963 
Fig. 1. Location map. 
basic circulation in Hawaii; all other weather 
types are perturbations in this basic current. The 
eastern portion of a subtropical anticyclone is 
characterized by divergence and subsidence and 
consequent adiabatic heating and pronounced 
dryness aloft. There is, however, a mixing layer 
near the surface where the air in contact with 
the cold ocean currents becomes cold and humid. 
These two layers of air are separated by a zone 
of temperature inversion which, along the Cali- 
fornia coast, occurs at an altitude of about 1,500 
ft. Downstream, toward the west, as water tem- 
perature rises and subsidence weakens, the trade- 
wind inversion reaches a height of about 6,000 
ft in Hawaii. 
Figure 2 shows the vertical structure of trade 
wind in Hawaii. The air below the inversion is 
moist, with a mixing ratio of 12-15 gm/kg at 
the ground surface. In the absence of orographic 
uplift this moist air is a poor rain-producer be- 
cause the trade inversion acts as a lid to oppose 
the development of convective clouds; therefore, 
in lowland areas, the trade wind is atmospheri- 
cally moist in terms of humidity but ecologically 
dry in terms of rainfall. 
The air below the trade inversion has a lapse 
rate close to dry adiabatic. It requires only a 
little uplifting to produce rainfall. Baer ( 1956) 
has demonstrated theoretically that if the rela- 
tive humidity is approximately constant below 
the inversion layer, rainfall would increase with 
height exponentially up to the base of inversion. 
The logarithmic distribution of rainfall with 
height is a unique climatic feature in Hawaii. 
The prevalence of trade-wind weather in Ha- 
waii is dependent upon the location and strength 
of the Pacific subtropical anticyclone. In July 
and August, when the subtropical ridgeline in 
the high troposphere is located to the north of 
the islands, the trades prevail during 97% of 
