382 
flow pattern is characterized by large meridional 
flow and frequent interaction between tropical 
perturbations and troughs in the westerlies. 
On a hemispherical scale the mid-tropospheric 
westerlies are composed of roughly sinusoidal 
waves. In winter an upper air trough is climato- 
logically anchored off the east coast of Asia. 
The location of the next downstream trough 
depends upon the zonal velocity according to 
the well-known Rossby formula. The zonal ve- 
locity in the western Pacific is such that the 
trough is usually located to the west of the is- 
lands in January and to the east in February 
(Namias and Mordy, 1952). Since the trough 
is characterized by convergence ahead and di- 
vergence behind, cyclones in Hawaii are more 
numerous in January than in February. The low 
rainfall in February is especially evident in the 
PACIFIC SCIENCE, Vol. XVII, October 1963 
northern island of Kauai (Leopold and Stidd, 
1949). 
The easterly waves are trade-wind perturba- 
tions that may form during any part of the year 
but with a maximum during summer. In August 
the easterly waves occur along the east coast of 
Hawaii every 3 or 4 days, bringing with them 
decks of high clouds and rainfall; however, they 
dissipate readily over the rugged land and sel- 
dom reach Oahu and Kauai. 
Since 1904 there have been only four tropical 
storms with wind of 74 rnph or more passing 
through the islands, all in the 1950’s. The for- 
mation of a hurricane in the central Pacific is 
somewhat of a climatic anomaly and requires a 
combination of favorable conditions. In 1957, 
when two hurricanes passed through Hawaii, 
Frazier (1957) noted that a well-developed 
Fig. 4. Median annual rainfall (inches) in Hawaii. 
