200 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



standard condition. This corresponds to a water 

 vapor pressure of approximately 10 mb at sea level 

 and a rate of decrease of water vapor pressure in the 

 lower levels of about 1 mb per 1,000 ft. At higher 

 levels the rate of decrease of the water vapor pres- 

 sure is less rapid. These conditions are represented in 

 Table 1 for the atmosphere up to 1,500 m. 



Both the dry and the moist standard atmosphere 

 exhibit a very nearly linear increase of M with 

 height. According to equation (12), 



M 



Mo = 



ka 



10 6 = 0.157 



h 



k ' 



h in meters . 



By using this formula in conjunction with Table 1 it 

 is easily shown that k = % for the dry standard 

 atmosphere, and k = % for the standard atmos- 

 phere with a 60 per cent relative humidity. This 

 value of k is the one commonly adopted in coverage 

 diagrams corrected for standard refraction. 



Because of the great variability of the moisture 

 content of the atmosphere with season, geographical 

 location, etc., a moist standard atmosphere has a 

 limited physical significance. The standard should 

 rather be defined in terms of a fixed linear slope of 

 the refractive index, and for this purpose the value 

 k =ji has been chosen. 



17.2.2 



The Measurement of Refractive Index 



The lower atmosphere frequently is stratified by 

 nonstandard distributions of temperature and hu- 

 midity which vary rapidly and irregularly as func- 

 tions of the height. The refractive index is then no 

 longer linear but has a more complicated dependence 

 on height, determined from equation (16). The strati- 

 fication which is of particular importance in tropo- 

 spheric propagation is found in the lower part of the 

 atmosphere, that is, below about 4,000 to 5,000 ft 

 and frequently in the lowest few hundred feet above 

 ground. 



Since the variation in the atmospheric pressure 

 gradient is small, interest is mainly centered in the 

 dependence of the modified refractive index M on the 

 temperature and humidity distributions. Methods, 

 useful in the field, have been developed for obtaining 

 rapid determinations of temperature and humidity in 

 the lowest levels of the atmosphere. The ordinary 

 radiosonde (radiometeorograph) is not well adapted 

 for this purpose since it is usually designed to give 

 data at levels about 100 m apart, which often is not 



close enough to reveal the significant details of the 

 M curve. Consequently it has proved to be necessary 

 to develop new instruments for this purpose. 



Several types of instruments have been designed 

 which can be placed on towers, or carried by slow- 

 flying airplanes or dirigibles or carried aloft by captive 

 balloons or kites with wires connecting the tempera- 

 ture and humidity elements to measuring or record- 

 ing equipment located on ground or aboard ship. 



Some such measurements have been made with 

 instruments using electrical methods in which dry 

 and wet electrical resistance elements are connected 

 into a circuit to give "dry bulb" and "wet bulb" 

 temperatures. Another electrical method uses the 

 same "dry" temperature element but, in place of 

 the wet bulb, obtains a relative humidity measure- 

 ment by using an electrolytic humidity element of 

 the type employed in the U. S. Weather Bureau 

 radiosonde. Hair hygrometers are definitely not 

 suitable for this type of work on account of their 

 lag in adjusting themselves to changes in relative 

 humidity (of the order of 3 to 5 min for appreciable 

 changes in humidity). 



Measurements made from airplanes have the 

 advantage that it is possible to survey a compara- 

 tively large area within a short time. This can be of 

 great importance along coasts where conditions in 

 the lowest levels of the atmosphere sometimes change 

 rather rapidly with increasing distance from the 

 shore. In the absence of suitable special equipment 

 an ordinary psychrometer held out of the window of 

 a plane will give quite satisfactory results in slow- 

 flying planes, providing care is taken to keep the 

 wet bulb sufficiently moist. When measurements are 

 made from an airplane the height above the ground 

 is determined for each measurement by means of 

 the plane's altimeter. Unless carefully done this 

 introduces the possibility of considerable error. 



In another method captive balloons, kites, ordi- 

 nary radiosonde balloons, and, occasionally, barrage 

 balloons have been used to carry the measuring 

 elements aloft. Ordinary captive balloons will work 

 in wind speeds up to about 8 miles per hour; in 

 higher winds kites or, occasionally, barrage balloons 

 are used. Kites can be flown from boats even at low 

 wind speeds or in calm weather. With this type of 

 equipment the electrical measuring elements aloft 

 are connected to an indicating or recording instru- 

 ment at the ground or aboard ship by means of fine 

 insulated wires that are wound around the cable 

 holding the balloon. 



