Climatology and Sugar Cane — CHANG 
381 
the time (Yeh et al., 1951^) . In winter, when 
the subtropical anticyclone moves southward, 
the frequency of trade-wind weather decreases 
to 40-50% (Fig. 3). 
OTHER WEATHER TYPES 
During one-third of the year, when the trades 
are not prevalent, weather in Hawaii falls into 
two general types: mixed and cyclonic. The 
mixed type is characterized by the presence of 
an essentially east-west polar front north of the 
islands. This type is a very poor rain-producer 
as the front is located at a distance. The mixed 
type often ushers in cyclonic weather. 
Cyclonic weather occurs in Hawaii during 
22% of the year. The cyclonic weather may be 
further divided into four subtypes: (1) extra- 
tropical cyclones, ( 2 ) kona storms, ( 3 ) easterly 
waves, and (4) tropical cyclones or hurricanes. 
The first two subtypes occur most frequently in 
winter while the latter two subtypes are pri- 
marily summer disturbances. 
Uncomplicated extratropical cyclones occur 
only when the zonal westerlies move southward 
into the tropics. They are, however, weakened 
on their journey southward and produce only 
TYPICAL LAPSE RATE CURVE SHOWING THE 
STRUCTURE OF TRADE WIND IN HAWAII 
I4°F 23 32 41 50 59 68“F 
FIG. 2. Structure of the trades in Hawaii. 
Fig. 3. Monthly variation of frequencies of three 
weather types. 
small rains. Some of these cyclones reach only 
the northern island of Kauai. 
The kona storm is essentially a cold-core low 
developed in the tropics. Simpson (1952) has 
described two main processes in its formation: 
( 1 ) the transformation of mid-latitude cyclones 
which have been trapped at low latitudes by 
blocking action of a warm high; (2) the cy- 
clogenesis in the easterlies triggered by the 
building down of a pre-existing cold upper low 
usually at the southern extremity of a polar 
trough. Once formed, all kona storms seem to 
assume the same characteristics. The term kona 
indicates that the wind during such a synoptic- 
period is southerly. Kona storms occur about 
three or four times a year, bringing heavy rain- 
fall throughout the islands. In the dry lowland 
areas they may account for more than half of 
the annual rainfall. 
The occurrence of winter cyclones and kona 
storms in Hawaii is closely related to the general 
circulation. Yeh et al. (1951 b) have shown that 
winter rainfall in Hawaii increases as the lati- 
tude of the jet stream increases, unless the latter 
penetrates to the latitude of the islands them- 
selves. They explained that when the jet stream 
is weak and far to the north, Hawaii will ex- 
perience a period of low zonal index. Low index 
