GULF OF MEXICO 



93 



Coast Pilot shows rain in from 13 to 21 percent 

 of the hourly observations available for the entire 

 year between 22^2° to 27 K° N. latitude and 80° 

 to 90° W. longitude. Rainfall was most fre- 

 quently reported in the mid afternoon — 19 percent 

 versus 15 to 17 percent at other times of the day. 



The average depression of the wet bulb is 3° F. 

 for the fall quarter — September, October, and 

 November. During the remainder of the year 

 it is 2° for the western Gulf and 3° for the eastern. 



In the summertime the air and the sea surface 

 differ in temperature by less than 1°, the sea 

 temperature being higher according to the Atlas 

 of Climatic Charts of the Oceans. In the fall 

 and winter differences increase, with the sea 

 temperature being as much as 5° higher than the 

 air temperature in the area just west of the 

 Florida Peninsula. This leads to heating from 

 below on the average and explains the high fre- 

 quency of cumulus-type clouds over the Gulf. 

 Monthly average sea surface and air temperatures 

 are tabulated in the chapter on physical ocean- 

 ography (p. 119). 



WEATHER OBSERVING STATIONS 



It is the purpose of this summary to discuss 

 weather over the water in the Gulf. Since the 

 observations here are sparse, some of the conclu- 

 sions are drawn from observations made on the 

 surrounding land areas. The weather observing 

 stations in these areas are shown in figure 20. 

 Those on the coast are listed in table 1. A par- 

 ticularly interesting feature of the Gulf is that 



Table 1. — Weather observing stations along the coast of the 

 Gulf of Mexico 



I 



although it covers some 700,000 square miles, it 

 is more than 90 percent surrounded by land. The 

 rather complete coverage of weather information 

 around its perimeter makes it an unusual natural 

 laboratory in which to study changes in the 

 character of air masses as they pass across the 

 large body of water. 



TYPICAL UPPER AIR SOUNDINGS 



A comparison of the upper air soundings from 

 Swan Island, south of the Gulf, with those of 

 New Orleans and Brownsville, on the north and 

 northwestern coasts, illustrates the modifying 

 effects of the water surface. Monthly average 

 radiosonde observations for a summer month 

 and a winter month at each of these stations are 

 shown in figure 21. It will be noted that the an- 

 nual change in structure at Swan Island, which is 

 almost completely controlled by oceanic factors, 

 is very small, the most noticeable change being 

 the higher relative humidities in the summer. 

 At Brownsville and New Orleans the sea surface 

 is cooler in winter, but, also, the continental 

 influence tends to make winter temperatures 

 definitely lower than those in summer. 



A rather complete discussion of the tropical 

 Gulf air mass is given by Willett (1943). He 

 states that the uniformity of the water tempera- 

 ture in the source regions of tropical maritime air 

 masses has proved to be of more importance in 

 fixing the properties of the masses at all levels 

 than has the previous life history of the individual 

 air masses. Evidence is given demonstrating that 

 the structure of the lower stratum of the air mass 

 results from the turbulent mixing of saturated air. 

 The air masses are characterized by marked 

 potential instability, implying that all convective 

 or mechanical turbulence up to at least 5 kilo- 

 meters elevation must effect an upward transport 

 of latent heat. The high relative humidities 

 indicate that active convection extending above 

 this level can be initiated by very little vertical 

 displacement. Summer thunderstorms are more 

 likely along the eastern part of the northern 

 Gulf coast than along the western. There are 

 higher relative humidities in the eastern area. 



Using a series of atmospheric temperature and 

 moisture soundings to 45 feet elevation made at 

 4-hour intervals in March 1949 from an oil 

 platform in the northwestern Gulf, Gerhardt 



