GULF OF MEXICO 



95 



400 



SWAN ISLAND, W.I. 



NEW ORLEANS 



BROWNSVILLE 



600 



800 



1000 



Mb 



-10 10 30 



Figure 21. — Average radiosonde observations from selected stations in the Gulf of Mexico area. 



(1951) demonstrated the modifying effects of the 

 sea surface. He was able to support the usually 

 assumed logarithmic distribution of water vapor 

 with height, the variation of the evaporation 

 coefficient with wind speed and the essential 

 similarity of the heat and moisture transfer 

 processes. 



A discussion of the extent to which the sea 

 surface affects the overlying atmosphere is given 

 by Jacobs (1951). He computes the average 

 winter evaporation in the Gulf to be of the order 

 of 0.40 gms. per cm^/day and compares this with 

 the other ocean areas of the world. Similar com- 

 parisons are made for the other energy exchange 

 processes operating between the sea and the 

 atmosphere. 



NORTHERS 



A period of north winds lasting from 1 to 3 or 

 4 days and having speeds greater than approxi- 

 mately 20 knots is known to seamen as a 

 "norther." Each year 15 or 20 northers are 

 observed in the Gulf, usually between November 

 and March. Many of ihem reach as far south as 

 Key West, Florida, and Tehuantepec, Mexico. 

 The low temperatures of the polar air masses 



which move in and the large waves built up in the 

 open Gulf have considerable effect upon shipping 

 and work at sea. Also, the water level is often 

 lowered in the shipping canals and in harbors 

 along the north coast markedly affecting ship 

 transportation in the area. According to the 

 Coast Pilot a veering of the north winds into the 

 east and southeast brings a resumption of fair 

 weather, while a backing toward northwest means 

 more foul weather. 



METEOROLOGICAL TIDES 



Water levels along the coasts of the Gulf change 

 noticeably with changes in wind speed and direc- 

 tion. Two extreme conditions are the change 

 associated with a norther, mentioned above, and 

 the storm tide associated with a hurricane and 

 discussed later. However, the stress of the wind 

 acting upon the sea surface at times other than 

 times of northers or hurricanes may also be suf- 

 ficient to bring about a water level change of the 

 same order of magnitude as that resulting from the 

 periodic tide-producing forces. This leads to 

 considerable deviation of the observed water 

 levels from those published in the tide prediction 



