KVIMO METEOROLOGY 



215 



that in spite of the long passage over the sea no 

 diffusion equilibrium lias been established between 



the sea surface and the moisture in the lower atmos- 

 phere, t >n the other hand, there is little difference 

 between the air and sea temperature, the latter 

 being rat her constant at 25 C and the former varying 

 between 23 and 26 C. The air is, therefore, nearly 

 in convective thermal equilibrium with the sea sur- 

 face, and no appreciably "dry" duct can develop. 

 The duct is caused by the moisture variation in the 

 lowest layers. 



200 



WATER SURFACE 



360 

 M MODIFIED INDEX 



370 



380 



\J 



330 



Figure 31. M curve over West Indian Ocean. 



A typical M curve is shown in Figure 31. It may 

 be seen that at as small a height as 0.5 m above the 

 sea M has a value much lower than at the surface 

 itself. As the surface value of M is obtained by the 

 assumption that the air in immediate contact with 

 the water is saturated with moisture, this indicates 

 that 0.5 m above the water the moisture content of 

 the air is still appreciably below saturation. The 

 moisture in the lowest levels is subject to consider- 

 able variations caused partly by turbulence, partly 

 by the waviness of the sea surface. M curves, such 

 as Figure 31, are obtained by averaging over several 

 measurements. 



These ducts are much lower than the advective 

 ducts discussed in the previous section; their height 

 is about 12 to 15 m (around 40 ft). The effective 

 decrease of M in the duct (apart from the sharp 

 decrease in the lowest half meter) is of the order 

 of 4 to 8 MU. 



The latter figure depends somewhat on the wind 

 speed. There is a maximum decrease of 8 MU at a 

 wind speed of about 8 m per sec (13 miles per hour) 

 and lower values for both lower and higher wind 

 speed. The duct height in turn shows a very slight 

 dependence on wind speed, increasing somewhat with 

 increasing speed. 



These ducts are so low that they are not very 

 effective for trapping of waves even as short as S 

 band, presumably on account of strong leakage (see 

 Section 17.2.6), and signal strength is not increased 

 when an S-band transmitter or receiver is placed 

 inside the duct. For K band, on the other hand, the 

 trapping effect is marked; on raising the transmitter 

 or receiver from the ground a maximum of signal 

 strength is observed at about 9 m, but from there on 

 the signal begins to decrease up to about 20 m (overall 

 decrease 5 db) ; at greater heights the signal gradually 

 rises again. 



These ducts appear to be a permanent feature at 

 Antigua, at least during the season these observa- 

 tions were carried on. This is probably true also for 

 many locations in the trade wind belt. The daily 

 variation of weather phenomena and of duct charac- 

 teristics at such purely maritime locations seems to 

 be insignificant. 



17.3.5 ]\f oc turnal Cooling — Daily Variations 



A daily variation of surface temperature occurs 

 only over land. During the day the heating is caused 

 by the sun's rays, and the cooling of the ground 

 surface during the night is produced by radiation 

 from the ground. The diurnal temperature variation 

 of the sea is extremely small. However, shallow 

 bodies of water sometimes have an appreciable 

 diurnal variation. 



The radiation which causes nocturnal cooling of 

 the ground is temperature or heat radiation which is 

 composed of waves in the infrared portion of the 

 spectrum. It is the same kind of radiation that is 

 given off by a hot stove or electric heater, but since 

 the temperature of the earth is less than that of a 

 stove the earth emits comparatively less heat radia- 

 tion. Nevertheless, radiation is a very powerful agent 

 in cooling the ground. From about sunrise until the 

 late afternoon, the surface of the earth gains more 

 heat from the sun and atmosphere than it loses by 

 radiation to space; in the late afternoon and during 

 the night, the surface loses more heat than it gains. 

 The amount of heat radiated is very nearly inde- 



