TROPICAL CYCLONES 
heavy or steady rain is reported from several nearby 
points, a developing or approaching tropical storm 
may be suspected. Solid cirrostratus or altostratus also 
indicates a disturbed condition. 
In the North Atlantic Ocean, tropical storms do 
not as a rule develop in the equatorial convergence 
zone. The only exceptions may be those developing 
off the coast of Africa (the Cape Verde hurricanes) 
and in the southwestern Caribbean, north of Panama. 
In the Pacific Ocean it appears that some do develop 
along the equatorial front. Along this front a low- 
pressure envelope with a longitudinal extent of 5-25 
degrees may appear. It will contain several depressions 
which drift irregularly westward until a polar trough 
aloft or major ridge aloft (high troposphere) is en- 
countered, when deepening of one of the depressions 
may result. 
Trade winds of relatively constant direction and 
speed set up swells which may travel for several thou- 
sand miles. The normal frequency of swells in the 
trade region is 8 per minute over the Atlantic and 14 
per minute in the Gulf of Mexico. Hurricane winds 
set up swells with a greater wave length and periodicity, 
and in severe storms the frequency decreases to 4 per 
minute. Swells move outward in all directions from 
the storm center and will approach the observer, in 
areas where the swells remain unmodified by large 
islands and irregular coast lines, from the approximate 
location of the storm center. Therefore, abnormally 
high swells or an abnormal direction of movement will 
indicate the existence of a tropical storm. The height 
and periodicity of the swells indicate the intensity of 
the storm. The usefulness of swells in forecasting the 
movement of tropical cyclones is discussed in the next 
section. 
Modern technique in the detection of tropical storms 
involves the dispatch of reconnaissance planes into 
the area where any observations may have indicated 
the possibility of the existence of a tropical storm. 
Upper-Air Indications of a Tropical Storm. In the 
very early stages of development, tropical storms are 
occasionally better developed between 10,000 ft and 
20,000 ft than at the surface. Therefore, streamline 
analysis is reeommended for pibal charts at these levels, 
or a very careful analysis of the 700-mb and 500-mb 
constant-pressure charts if sufficient data are available 
for their preparation. Perturbations in the normal 
easterly flow should be carefully watched. 
FORECASTING MOVEMENT OF 
TROPICAL CYCLONES 
Monthly Variation in Normal Storm Track. Tropical 
storm development is a seasonal phenomenon wherever 
it occurs, embracing principally the summer and fall 
months in each hemisphere. In most, if not all, of the 
tropical-cyclone-susceptible areas the mean storm track, 
and to a lesser extent the place of most frequent origin, 
changes from month to month during the storm season. 
In Fig. 8 the change from month to month in the 
mean storm track is evident. However, from year to 
year the individual storm tracks scatter considerably. 
897 
May is an unimportant hurricane month in the 
tropical North Atlantic, with a tropical storm occurring 
once in only about every ten years. It is not included 
in Fig. 8. The official record includes no tropical storm 
of full hurricane intensity in May although at least 
ba 1 iy 
JUNE 
AUGUS 
% | FROM CAPE 
ERDE ISLANDS 
Fic. 8.—Mean track of tropical cyclones, southwestern North 
Atlantic Ocean, June-October. (Continued on p. 898.) 
one was attended by winds of more than 60 mph for a 
day or more. All storms developed in the Caribbean or 
near Florida with one exception, the most severe, which 
developed to the east of Trinidad. 
June storms are rather small in area but may be 
quite intense. A tropical storm will develop on the 
average every other year but one will reach hurricane 
intensity only once every five years. Such storms 
usually develop in the extreme western Caribbean or 
