Climatology and Sugar Cane — CHANG 
383 
trough extending to low latitudes persisted along 
the west coast of North America throughout the 
summer and fall season. This trough, through 
an energy dispersion mechanism, sharpens the 
next downstream subtropical trough, thus pro- 
moting cyclogenesis in the latter area ( Ramage, 
1959). The increase in radiation and tempera- 
ture in Hawaii during the last decade, which 
will be discussed in a later section, was probably 
also an important cause for the increased hurri- 
cane activity. 
RAINFALL 
If the islands of Hawaii did not exist, the 
annual rainfall over the ocean would be in the 
neighborhood of 30 inches; the actual average 
rainfall over the islands is about 70 inches. This 
increase of 40 inches of rainfall is the result of 
orographic uplifting of trade wind and its per- 
turbation-easterly waves. The effect of easterly 
waves is strongest near Hilo and diminishes 
northward toward the Hamakua coast. The me- 
dian annual rainfall at Hilo is 139 inches, con- 
siderably higher than any other coastal area in 
the state (Fig. 4). 
The trade-wind rainfall increases with alti- 
tude exponentially up to a certain point and 
then decreases again. The belt of maximum 
rainfall varies from 3,000 to 4,000 ft, depending 
upon the effect of local topography on the flow 
patterns. This effect has been discussed in detail 
by Leopold (1949). The steep isohyetal gradient 
in a trade-wind climate is best illustrated in 
Kauai, where Mt. Waialeale, with a mean annual 
rainfall of 465 inches, is only 15 miles away 
from the semiarid west coast. Most of the sugar 
plantations are located in lowlands with a me- 
dian rainfall of less than 50 inches. A few cane- 
growing areas on Hawaii, however, receive as 
much as 200 inches of rainfall in a year. 
Except along the Kona coast of Hawaii, where 
the upslope sea breeze produces frequent sum- 
mer afternoon showers, winter is the wet season 
throughout the state. The contrast between sum- 
mer and winter rainfall is most accentuated in 
the dry lowlands, where the monthly rainfall 
in summer is often less than 0.5 inch. Summer 
rainfall minima are found only in a very few 
places in the tropics and are designated as "As” 
climate in the Koppen classification. 
In spite of the large areal and seasonal varia- 
tions, the rainfall distribution for any particular 
month bears a close resemblance to that of the 
annual. Thus Stidd and Leopold (1951) were 
able to express the monthly rainfall as a function 
of annual rainfall in the following manner: 
y = a (x — 30) -f b 
where y is monthly rainfall; x, annual rainfall; a, 
the gradient factor of observed orographic in- 
crease of rainfall through increment of average 
annual rainfall; b, a geographic constant quan- 
tity derived from a rainfall blanket of uniform 
thickness over the islands and adjacent ocean. 
The constant 30 is inserted because the mini- 
mum annual rainfall in the dry lowland is about 
30 inches. 
This same concept was later expanded to de- 
scribe daily rainfall distribution during trade- 
wind weather. For the island of Oahu, for ex- 
ample, a dry index of zero is assigned to a 
theoretical station having a zero mean annual 
rainfall, and a wet index of 100 to a station 
having 250 inches mean annual rainfall. A daily 
forecast chart is constructed by plotting the in- 
dices as the abscissa and the daily rainfall 
amounts as the ordinate. The forecaster fore- 
casts the daily rainfall for the dry and wet index 
stations. From the straight line connecting these 
two reference points, daily rainfall for any actual 
station may be read off as the ordinate corre- 
sponding to the abscissa of the station’s mean 
annual rainfall. 
The trade-wind rainfall is more likely to occur 
during the night or in early morning than during 
the day. Loveridge (1924) attributed the noc- 
turnal rainfall to the radiative cooling at the 
top of the clouds. Leopold (1948), however, 
added that cooling at night would lower the 
condensation level. Nocturnal rainfall, in distinct 
contrast to afternoon showers in many tropical 
countries, is in many ways beneficial to agricul- 
ture. 
Trade-wind showers are very light, with drop 
size less than 2 mm in diameter (Blanchard, 
1953), and with intensities usually much less 
than 0.2-0.3 inch per day. Only kona storms 
and hurricanes are capable of causing severe 
crop damage. On January 24, 1956, a kona storm 
deposited 38 inches of rain at Kilauea, Kauai. 
Such severe storms are rare, however, averaging 
