40 



TRANSMISSION EXPERIMENTS AT ANTIGUA, WEST INDIES 



one type (.)1' weather. This weatlicr condition is deter- 

 mined hu'gcly hy tlic position and strength of the 

 Bernuida higli, a large semipermanent high-pressure 

 area covering nnich of the Athmtic from 10 to 30 

 degrees nortli latitude. The northeast trades blow 

 around and out of the high's southern rim. With a 

 few exceptions during the period of the experiment, 

 the wind direction at Antigua was east-northeast. 

 Once, for a p)eriod of 3 days, it went around to north- 

 northeast and on two separate occasions blew from 

 the east. Average daily surface wind speed was 16 

 knots, with occasional variations lietween 8 and 27 

 knots. Representative air temperatures varied between 

 74 and 78 F, relative luimidities between 60 and 80 

 per ceiit. The sea water temperature was reasonably 

 constant at 77.5 F, with occasional variations between 

 76.5 and 78. No significant horizontal gradients of 

 sea temperature were found. Precipitation was wholly 



TEMPERATURE IN DEGREES F MIXING RATIO , G/KG 



Figure 7. Mean temperature and mixing ratio curves, 

 March 9 to 10, 1945. 



in the form of showers witli a maximum frequency of 

 occurrence around sunrise. Periods of relatively dry 

 weather followed by periods of relatively showery 

 weather and accompanying transitions were experi- 

 enced. It is felt that these variations were caused by 

 fluctuations in the intensity and position of the Ber- 

 muda high or by the trough effects ahead of dis- 

 sipating cold fronts. 



During the entire period of observations, a simple 

 surface duct was found to exist over the water. From 

 the second week in February through the third week 

 in March, and again in the first week of April, duct 

 conditions were essentially constant. This condition, 

 which is called herein the normal condition, is shown 

 in five figures. 



Figure 7 shows the average temperature and mixing 

 ratio values for a 2-day period plotted against height. 

 Curves of daytime and nighttime conditions are 

 shown. Soundings were taken every 2 hours. The 



water surface values are deri\ed from measurements 

 nradc on the ship. Considerable difficulty was found 

 in obtaining accurate soundings in the daytime due 

 to j'adiation from the warm land in the case of tlie 

 tower soundings and the warm ship in the case of 

 ship soundings. As mentioned in Section 4.1, somul- 

 ings were possilde on the ship only when running with 



MEAN SOUNDINGS 

 9,10 MARCH 



DAY 0700 - 1900 o-o — 



WIND 

 SURFACE ENE 17 KNOTS 

 100 FT ENE 23 KNOTS 



NIGHT 1900-0700 V— V 



WIND 

 SURFACE ENE IB KNOTS 

 100 FT ENE 23 KNOTS- 



WATER SURFACE 



350 360 370 



MODIFIED INDEX - 



3 



Figure S. Mean modified index curves, March 9 to 10. 



the wind. Thus, radiation effects of the ship were 

 maximized, especially in the daytime. However, valu- 

 able psychrometer measurements were made on the 

 outliound runs which showed the air to be consistently 



cooler than the water. On this basis, absolute values 



100 



350 



390 



360 370 380 

 MODIFIED INDEX ^ 



Figure 9. Mean sounding during low winds, Marcli 

 24 to 25. 



of temperature in the daytime tower soundings have 

 been arbitrarily adjusted. 



Figure 8 is the J\I curve computed fi'oni the tem- 

 perature and mixing ratio curves just given. The sur- 

 face duct and the small diurnal change in its proper- 

 ties are readily seen. An interesting point is the exist- 



fe of a rather sharp discontinuity at the 1-ft level. 



ent 



