TROPICAL CYCLONES 
By GORDON E. DUNN 
U. S. Weather Bureau, Chicago, Illinois 
INTRODUCTION 
Tropical cyclones of certain degrees of intensity are 
known as hurricanes in the Atlantic Ocean, the Carib- 
bean Sea, the Gulf of Mexico, and the eastern North 
Pacific Ocean (off the coast of Mexico); as typhoons 
in the western North Pacific and over most of the 
South Pacific Ocean; and as cyclones in the Indian 
Ocean. Locally in the Philippines these storms are called 
baguios and in Australia willy-willies. All have essen- 
tially the same origin, structure, and behavior. 
Interest in tropical meteorology and tropical storms 
received considerable impetus in the 1930’s with the 
extension of airline routes through the tropics and the 
consequent need of more accurate weather observations 
and forecasts in these areas. However, the meteorologi- 
cal requirements of the various military forces engaged 
in the tropics durmg World War II resulted in the 
collection of more observational data and in more rapid 
progress toward the solution of several of the more ur- 
gent and vexing problems of tropical meteorology than 
ever before. 
A tropical cyclone is essentially a maritime phe- 
nomenon and, therefore, as a rule, aerological data are 
relatively sparse over large areas traversed by tropical 
storms. Even in the Antilles and along the South Atlan- 
tic and Gulf coasts of the United States, where raob and 
pilot-balloon stations are located, it has been extremely 
difficult to obtain upper-air observations within the 
storm area itself. Therefore, many of the characteristics 
and mechanics of the tropical storm model are still not 
definitely known. 
SURFACE CHARACTERISTICS OF 
TROPICAL CYCLONES 
Sources of Information. Knowledge of the surface 
characteristics of tropical cyclones has gradually ac- 
cumulated from a half century of observations. 
Mitchell’s work [11] on areas of origin and normal rate 
and direction of movement of hurricanes in the North 
Atlantic Ocean, together with forecasting precepts, was 
one of the earliest and most important studies. Several 
years later Cline [1] published the results of many 
years’ study of hurricanes in the Gulf of Mexico region 
with considerable emphasis on their characteristics. 
Probably the most systematic analysis and compilation 
of the characteristics of tropical cyclones were those 
of Deppermann [2, 3, 4] for the Philippine area. Tanne- 
hill [18] has compiled a large amount of statistical in- 
formation on North Atlantic hurricanes for as far back 
as 1494. Information on the surface characteristics of 
tropical cyclones from these and many other sources 
such as G. Norton, E. Gherzi, and others, has been 
summarized by Dunn [5]. These and similar sources are 
887 
incomplete and in many respects not entirely satisfac- 
tory otherwise. All weather services in the tropical 
cyclone areas should set up procedures for a systematic 
tabulation of all surface and upper-air observational 
data to establish more definitely many of the character- 
istics of tropical cyclones. 
Apparently very small changes in lapse rates, specific 
humidities, and the field of motion will result in marked 
deviation from the so-called ‘‘normal” features of trop- 
ical cyclones, in addition to the normal changes as they 
move from low to higher latitudes. In general, only 
those characteristics observed before recurvature are 
considered here. 
Classification of Tropical Cyclones According to In- 
tensity. Tropical storms are variously described as cy- 
clones, hurricanes, storms, depressions, lows, ete. It is 
desirable that these terms be defined; the following 
classification of tropical storms has received general 
acceptance in the United States: 
1. Tropical Disturbance. Circulation slight on the 
surface, possibly more marked aloft, one or no closed 
isobars. Common throughout the tropics and subtropics. 
2. Tropical Depression. One or more closed isobars, 
wind force equal to or less than Beaufort 6. Frequently 
observed in the intertropical trough, much less fre- 
quently in the trades. 
3. Tropical Storm. Closed isobars, wind force more 
than Beaufort 6 and less than Beaufort 12. 
4. Hurricane or Typhoon. Wind force Beaufort 12 
(75 mph or more). 
All, of course, must originate in the tropics, but many 
tropical disturbances and depressions never reach hurri- 
cane intensity. An International Meteorological Con- 
ference in Manila in June 1949 adopted slightly different 
definitions as follows: tropical depression—winds up to 
34 knots; tropical storm—winds 35-64 knots; typhoon 
—winds 65+ knots. 
Classification of Tropical Cyclones According to Stage 
of Development. Like extratropical disturbances, trop- 
ical cyclones undergo constant metamorphosis from 
birth through maturity to decay. The general char- 
acteristics of a tropical cyclone vary considerably both 
as regards surface and upper-air structure as the storm 
progresses from one phase of development to another. 
Thus tropical cyclones will rather consistently exhibit 
different characteristics at latitude 30° than at latitude 
20°. The life history of tropical cyclones may be divided 
into four stages: 
1. The formative or incipient stage, which begins 
when a tropical disturbance first develops surface cir- 
culation and ends when it reaches hurricane intensity. 
2. The stage of immaturity or deepening, during 
which the cyclone continues to deepen until the lowest 
central pressure and the maximum intensity are 
