Chapter 14 — AIR MASSES AND FRONTS 



The appearance of heavy swell waves with a 

 period of 9 to 15 seconds during the hurricane 

 season in those waters is an indication of the 

 existence of a tropical storm in the direction 

 from which the swell waves come; the longer 

 swell wave period, 12 to 15 seconds, are almost 

 certain signs of the hurricane. 



One of the most severe effects of hurricane 

 damage occurs along coastal areas by large 

 ocean waves. The most severe waves can be 

 expected where land partially surrounds bodies 

 of water such as the Bay of Bengal and the 

 Gulf of Mexico. Strong sustained winds in the 

 right-hand semicircle cause a piling up of water 

 along coastal areas as much as 10 feet above 

 normal. Sometimes these tides are referred 

 to as storm tides. An even greater threat is 

 the so-called hurricane wave. The term 

 "hurricane wave" has been applied to the 

 marked rise in the level of the sea near the 

 center of intense tropical cyclones. This rise 

 sometimes amounts to 20 feet or more and 

 affects small isolated islands as readily as 

 continental shores. In partially enclosed seas 

 they may be superimposed on the hurricane 

 and gravitational tides. The hurricane wave 

 may occur as a series of waves, but is usually 

 one huge wave. These waves have produced 

 many of the major hurricane disasters of 

 history. There is usually little warning of their 

 approach. However, they should be anticipated 

 near, and to the right of, the center of intense 

 tropical cyclones. 



8. Vertical Characteristics. The vertical 

 structure of a tropical cyclone also differs 

 considerably from the extratropical cyclone. 

 The first difference is that the tropical cyclone 

 is always a warm core low. The storm may 

 build from the top down as well as from the 

 surface upward, and it is for this reason that 

 only the mature model should be considered 

 for comparison. Subsidence occurs in the eye 

 of the storm below about 30,000 feet extending 

 to about 3,000 feet above the surface, accounting 



for the lack of or sparsity of low and middle 

 clouds. 



There is present in a mature storm, con- 

 siderable horizontal and vertical mixing, 

 extending from near the surface to between 

 10,000 and 20,000 feet. There is a net hori- 

 zontal inflow of air at all levels to about 3,000 

 feet or higher, above which there is a net 

 horizontal outflow of air. This net outflow of 

 air is usually very pronounced in the vicinity 

 of the 200-mb level, and it is for this reason 

 that the 200-mb level is one of the primary 

 analysis tools in the Tropics. The outflow at 

 the 200-mb level is manifested by an anti- 

 cyclonic flow, unless the storm is unusually 

 severe and penetrates even that level, in which 

 case the flow is cyclonic. 



Seasons and Regions 

 of Occurrence 



Tropical cyclones may occur during any 

 month of the year with a maximum occurrence 

 from May to November. Frequency, however, 

 varies from ocean to ocean. 



There are nine principal regions of tropical 

 cyclone formation. Six regions are in the 

 Northern Hemisphere, three, in the Southern 

 Hemisphere. The South Atlantic Ocean is 

 entirely free from tropical cyclones. The 

 southwestern North Pacific, on the other hand, 

 has by far the greatest number of tropical 

 cyclones developing in it. A detailed break- 

 down of formation areas, with main tracks 

 and months of most frequent occurrence, is 

 found in table 14-3. 



Not all tropical cyclones reach hurricane 

 intensity. Tropical cyclones that reach hurri- 

 cane intensity are more prevalent in the 

 respective late summer and early fall in both 

 the Northern and Southern Hemispheres. This 

 does not preclude the formation or the intensifi- 

 cation of tropical cyclones of any intensity 

 during the other seasons. Tropical cyclones 

 are also much less frequent than the extra- 

 tropical ones. 



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