322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



rough terrain, the storm will lose intensity ; and if the terrain is very 

 mountainous the lower levels of the hurricane's circulation may be 

 completely disrupted. 



The fuel that supplies the energy of a hurricane is the warm moist 

 air that is found in tropical oceans. This explains why introduction 

 of cold dry air to the inner core of the hurricane's circulation can 

 cause the hurricane to weaken. It does not explain, however, why 

 some hurricanes will barely reach hurricane intensity even though 

 both types remain over the warm tropical oceans for long periods. 



Two types of air circulation are required to keep a hurricane going : 

 (1) Inward-directed flow in the lower levels, and (2) outward-directed 

 flow in the upper levels that evacuates the air from the hurricane. 

 To produce intensification of a storm and to provide lower pressure 

 in the center it is necessary that more air be evacuated from the top 

 than is brought into the hurricane circulation at lower levels. It is 

 quite probable, therefore, that a key to the intensification of a hurri- 

 cane is in the upper levels of the troposphere. Most of our ideas as 

 to what will cause a hurricane to intensify or to weaken are based on 

 theoretical rather than empirical deductions because of lack of suf- 

 ficient data to prepare accurate weather maps at the higher levels. 

 Dr. Herbert Riehl, University of Chicago, in private discussions, has 

 expressed the idea that a hurricane is more likely to intensify if the 

 circulation at high levels is such that the air coming out of the hurri- 

 cane can be rapidly removed from the tropical area. Further intensifi- 

 cation is much less likely and, if it does occur, will proceed much more 

 slowly in cases of sluggish circulation at the higher levels. 



After a hurricane moves into middle latitudes the warm air of the 

 storm is frequently brought in contact with a colder air mass. This 

 will usually change the character of the hurricane, and it sometimes 

 occurs in such a manner that the storm reintensifies with middle lati- 

 tude characteristics where the energy is derived from the potential 

 energy made available by the juxtaposition of two air masses of dif- 

 ferent densities. This was the case when hurricane Hazel moved into 

 Pennsylvania in 1954. 



4. RAINFALL 



Rainfall associated with hurricanes causes more damage and kills 

 more people than any other hurricane force except the storm surge. 

 Hurricane Diane became the first billion-dollar hurricane in the his- 

 tory of the United States, largely because of the floods following the 

 rains associated with it. One hundred eighty-four people were killed — 

 mostly drowned. The floods following the passage of hurricane Hazel 

 of 1954 over Haiti were estimated to have drowned several hundred 

 people. Likewise, several hurricanes entering Central America and 

 Mexico during the past decade have each drowned over a hundred 



