892 
These figures would indicate that weak storms have 
longer calms than more intense storms. However, it 
does not appear that Deppermann considered rate of 
movement, or whether merely a chord near the edge 
of the calm center instead of the diameter passed over 
the station. Since the rate of movement varies from - 
one storm to another, the duration of the calm is not 
necessarily a measure of the diameter of the calm 
center. In the United States the diameter appears to 
be less in immature storms than in the larger mature 
storms. In the latter the calm center seems to mcrease 
in about the same proportion as the storm area. The 
smallest eye reported has been about 4 miles in diam- 
eter in a storm in the formative stage; diameters of 
20-25 miles in large mature storms are not unusual. 
An average value appears to be 12-15 miles. Upon 
recurvature the eye may assume a very elongated 
shape in the direction in which the storm is moving. 
On occasion a double eye has been observed, usually 
when the cyclone is decaying. 
Frequency of Thunderstorms and. Tornadoes. 
Thunderstorms are frequently observed in the early 
and decaying stages of tropical cyclones and also around 
the periphery of the storm. They are not usually ob- 
served in the area of winds over 60 mph in immature 
and mature storms. Tornadoes have been reported in a 
few instances in connection with tropical storms in the 
Bahamas and in Cuba and fairly frequently in Florida. 
The tornado paths are very short (only a mile or two 
in length) and several appear to have formed over the 
ocean as waterspouts. Indeed, from the standpoint of 
intensity, they seem to resemble waterspouts more 
closely than inland tornadoes. Information is not avail- 
able as to whether any particular quadrant is preferred. 
Inundations. The hurricane wave, sometimes errone- 
ously called a tidal wave, is the lifting up of the level 
of the sea at and near the center of intense hurricanes. 
It is usually not a series of individual waves but a 
rapid uplift of water and is most noticeable in the 
calm center. It has been noted in all areas where 
tropical storms occur. 
Hurricane waves occur on the shores of small islands 
as readily as on continental coast lines. They have been 
responsible for heavy losses of life and precautions 
should always be taken against their possible occur- 
rence, especially at and for a short distance to the 
right of the point where the center will cross the coast 
line. The hurricane wave is superimposed on the gravi- 
tational and hurricane tides prevailing at the time but 
will usually subside quickly within an hour or two 
after the center passes. 
The wave has been ascribed to the decrease in at- 
mospheric pressure, but even in the most severe hur- 
ricanes the maximum possible rise from decreased 
pressure has been calculated to be less than 4 ft and 
any such rise should be gradual. It is thought that 
the wave is caused by the damming effect of the wind 
on the forward side of the calm center against the 
current set up by the violent winds from the opposite 
direction to the rear of the calm center. The average 
height of the hurricane wave is 10-20 ft but higher 
TROPICAL METEOROLOGY 
waves have been reported, especially where the shore 
line permits a funneling effect. : 
The Hurricane Tide. There is little or no actual trans- 
fer of water in waves and swells, but sustained winds 
of only moderate force will set up a current flowing 
with the wind. The rise in the water level on the shore 
line may begin when the tropical cyclone is 500 miles 
or more distant and will continue until the storm passes 
inland or beyond the area. This tidal rise is called 
the hurricane tide and is superimposed on the normal 
gravitational tide. It is most pronounced in a partially 
enclosed body of water such as the Gulf of Mexico, 
where the concave coast line does not readily permit 
the escape of water which is thus piled up on the 
shore line. The strongest winds and the greatest fetch 
is in the right rear quadrant; therefore, the strongest 
current is directed with and to the right of the line 
along which the storm is moving. 
Heights of storm tides on concave coast lines vary 
from 3 to 10 ft above normal. On straight shore lines 
the average is less and in very small island groups, 
around which the current may easily flow, there is 
little or no tidal effect of this type. 
VERTICAL STRUCTURE OF TROPICAL 
CYCLONES? 
Theory. Observations necessary for proof of the vari- 
ous theories in regard to the vertical structure of tropi- 
cal cyclones are extremely sparse. Nevertheless, 
through contributions from Shaw, Durst and Sutcliffe, 
Willett, Haurwitz, Simpson, Riehl, and many others, a 
fairly coherent and logical theory has been developed 
which has found general acceptance in a broad sense 
but in many details is subject to further verification 
and, no doubt, to revision on the basis of future ob- 
servational data. 
The tropical cyclone is definitely a warm-core phe- 
nomenon, in contrast to all other tropical disturbances 
which are cold-core phenomena. The circulation is 
strongest near the surface, but as the storm intensifies 
it extends rapidly to great heights. With the approach 
of the vortex, temperatures and specific humidities 
increase at almost all levels at least up to 15-20 km. It 
further appears that convergence near tropical-cyclone 
centers is limited to a fairly shallow layer near the 
surface. 
The problem of how evacuation of air from tropical 
cyclones takes place is the most controversial portion 
of the theory of tropical-cyclone structure. The theory 
of many earlier meteorologists, and even of some who 
are currently engaged in tropical research (see [15)), 
that the pressure gradient reverses its sign at some 
level above 10-20 km, is still inconclusive. Vines’ ob- 
servations in the 1890’s of cirrus radiating outward 
in all directions from a point over the storm center 
have not been reaffirmed for many years. Riehl [12], 
while believing that a reversal of pressure gradient is 
2. This phase of tropical cyclones is discussed in greater 
detail in ‘‘Aerology of Tropical Storms” by H. Riehl, pp. 
902-913 in this volume. 
