Chapter 14 — AIR MASSES AND FRONTS 



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 Figure 14-27.— Satellite picture of typhoon Gilda in the Pacific Ocean showing a distinct "eye". 



orographic effects produce concentrations of 

 rainfall, which often results in costly floods. 

 Hurricane force winds forcing moisture-laden 

 tropical air up a steep mountain slope often 

 result in phenomenal rainfall. A fall of 88 

 inches was recorded during one storm in the 

 Philippines. At the other extreme, as little 

 as a trace has been recorded at a station 

 in Florida, which had winds up to 120 knots 

 during the passage of a hurricane. 



7. State of the Sea. One of the first signs 

 of the tropical storm is the swell, which comes 

 in a series of waves with the time interval 

 between crests considerably longer than in 

 waves usually observed in the Tropics. Winds 

 of the storm create waves on the sea which 

 move outward from its center more rapidly 

 than the storm progresses and thus outrun the 

 storm and herald its approach. As the wave 

 moves onward, its height from crest to trough 

 diminishes, its length is reduced, and it 

 becomes a low undulating wave, known as a 

 swell. The size and speed of waves created 



by the winds depend upon the velocity of the 

 winds and the length of water surface over 

 which the winds blow. 



In a tropical cyclone the waves and swell 

 move outward from the storm center at a rate 

 which nearly always exceeds the speed of 

 progressive movement of the storm. The swell 

 waves continue to move in a straight line 

 through the storm area, whereas the winds 

 turn to the left in the Northern Hemisphere and 

 to the right in the Southern Hemisphere. 



The direction of swell waves in the open 

 sea gives some indication of the location of the 

 storm center. When considered in connection 

 with the direction of the wind, the movement 

 of the swell waves is significant. 



The period of the swell waves, that is, the 

 time in seconds between the passage of 

 successive swell wave crests, is helpful in 

 determining the intensity of the storm. In the 

 Caribbean Sea and the Gulf of Mexico long 

 period swell waves do not commonly occur 

 except in connection with a tropical storm. 



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