OCEANOGRAPHIC CLIMATE OF HAWAIIAN ISLANDS REGION 



387 



700 



600 



300 



T — I — I — I — I — 1 — I — I — I — I r 



, WAIPIO 19501957 



V— WAIPIO 1943 1950 



J L 



X 



_l_l I I I I I I L 



NOV JAN MAR MAY JUL SEP NOV JAN MAR 



Figure 12. — Amount of computed radiation absorbed by 

 the water (Q,) in the vicinity of Oahu, and measured 

 radiation reaching the surface at Waipio, Oahu, mean 

 1943-50, and mean 1950-57. 



Figure 12 shows the results of this computation 

 for the vicinity of the island of Oahu. The heat 

 absorbed by the water ranges from 340 cal. cm.~^ 

 day"' in December to 510 cal. cni."^ day"' in 

 Jime. Comparable values for a clear sky at 20° N. 

 (fig. 11) are 520 cal. cm.-^ day"' and 830 cal. 

 cm.~^ day"', respectively. In the vicinity of the 

 Hawaiian Islands, cloud cover, tlierefore, reduces 

 the insolation by iibout 35 to 40 percent. 



Although no direct measurements of Q^ for the 

 marine vicinity of Oahu are available, the radia- 

 tion rcaclhng the surface has been measured at 

 Waipio (near Pearl Harbor), Oahu, by the Sugar 

 Planters Association and the Pineapple Research 

 Institute of Hawaii. The mean data for the years 

 1943 to 1950 and 1950 to 1957 have been plotted 

 in figure 12.''. The mean observed solar radiation 

 from November through February for the 1943- 

 50 pei'iod is approximately tlie same as that com- 

 puted for the vicinity of Oahu. During the 

 remahider of the yenr, the computed vahies are 

 up to SO cal. cm."^ day"' (about 15 percent) too 



' Data were supplied i>y Dr. Paul C. Ekern. Pineapple Research Institute, 

 Honolulu, Hawaii. 



low. The mean observed values for the 1950-57 

 period are about 30 cal. cm.~^ day"' higher in 

 winter and up to 140 cal. cm."' day"' (about 25 

 percent) higher in summer than the computed 

 values. In other words, the computed insolation 

 values are of the right order of magnitude, in good 

 agreement witli observed values during the winter, 

 but 15 to 25 percent too low durmg the summer. 



The seasonal discrepancy between the computed 

 and observed insolation can be explained in several 

 ways. During the whiter months medium and 

 high clouds are an important part of the cloud 

 cover, whereas during the remainder of the year 

 trade wind cumuli predominate. First, then, the 

 discrepancy may be due to seasonal variations in 

 tlie difference between the cloudiness over Waipio, 

 located on the leeward side of the island, and that 

 over the ocean, since orography would affect low 

 tradewind cloudiness more than medium and high 

 cloudiness. In addition the cloud correction for- 

 mula may be sufficiently accurate for the medium 

 and high cloudiness during the winter, but not for 

 the trade wind cumuli in summer. Since cloud 

 cover is observed obliquely from a ship, there is a 

 tendency to overestimate cover of the latter type. 

 F'urthermoi-e, the formula used above does not take 

 into consideration the reflection from trade wuid 

 cumuli. 



The next term to be considered in the net heat 

 exchange across the sea surface is the back radia- 

 tion. This is a function of the absolute surface 

 temperature of the sea, radiating almost like a 

 black body, the relative himiidity, and the type 

 and amount of the cloud cover. According to 

 Sverdrup et al. (1942:112) back radiation can then 

 be expressed by 



Q, = Q,o (1 -0.0S3C). 



Tliis applies to average conditions oiil_\', since tlie 

 reduction of the effective back radiation, Qt,o, due 

 to clouds depends upon the altitude and the 

 density of the clouils. C is the cloudiness on the 

 scale from to 10. 



The necessary data to compute the back radia- 

 tion were obtained from the Atlas of Climatic 

 Charts of the Ocean (McDonald, 193S) and from 

 Sverdrup et al. (1942: fig. 25). Back radiation 

 ill tlie Hawaiian region ranged from 115 to 150 

 cal. cm."^ day"', and in the vicinity of Oahu 

 from 130 to 150 cal. cm."- day"', with the lower 

 values occurring during spring and summer. 



