Chapter 7 

 METEOROLOGICAL EQUIPMENT FOR SHORT WAVE 



- 1 METEOROLOGICAL EQUIPMENT FOR 

 PROPAGATION STUDIES » 



^•^•^ Outline of Problem 



EXPERIENCE GAINED during the last 3 years with 

 radar, especially microwave radar, and with ex- 

 perimental microwave communication equipment has 

 shown that the electromagnetic radiation field pro- 

 duced by a transmitter is subject to large variations 

 depending on the weather. These variations are caused 

 by refraction and so are related to variations of dielec- 

 tric constant in the atmosphere. Pressure, temperature, 

 and humidity determine the dielectric constant (re- 

 fractive index) . 



It has been found that above a certain height vari- 

 able with season and geographical location but rarely 

 exceeding 1,500 m above ground, atmospheric refrac- 

 tion is reasonably constant. In the lower levels and 

 especially in the lowest hundred meters of the atmos- 

 sphere, temperature and moisture conditions strongly 

 affect the radiation field and thereby influence the 

 operation of radar and other short and microwave 

 equipment. In order to evaluate this effect in quanti- 

 tative terms, the temperature and moisture distribu- 

 tion in the lowest layers must be determined with as 

 high a degree of accuracy as is compatible with speed, 

 _ ea^se of operation, and other practical limitations. 



A number of methods have been tried during the 

 past 3 years which range from measurements with 

 ordinary radiosonde equipment to the use of a psy- 

 chronieter on the steps of a fire ladder. Two facts have 

 appeared rather clearly : First, hairs are not suitable 

 for moisture measurements of this type on account of 

 their great sluggishness (except perhaps for station- 

 ary use on towers), since the time of adaptation of a 

 hair to appreciable changes in humidity is of the 

 order of 3 to 5 minutes. Secondly, it has been found 

 that ordinary radiosondes are not usually appropriate 

 because the readings obtained from them normally are 

 taken about 100 m apart in vertical distance and for 

 this particular problem a more detailed knowledge of 

 the temperature and moisture distribution is necessary. 

 \Yith a clock-driven radiosonde this can be remedied 



'By W. M. Elsasser, Columbia University Wave Propaga- 

 tion Group. 



by loading the sonde down Ijy means of a ballast 

 (water or sand) which slows down the ascent of the 

 instrument in the lower levels. If the ballast is made 

 to run out gradually, the full lift of the balloon may 

 be restored at any given level. This method cannot be 

 applied to the U. S. Weather Bureau radiosonde in 

 which temperature and moisture data are sent out by 

 a mechanism in which electric contacts are closed by 

 a pressure cell at predetermined levels (see, however, 

 Section 7.1.8, below). 



On the whole it has been found more advisable to 

 develop new or improved instruments or to adapt spe- 

 cial instruments for a low-level sounding technique 

 rather than to rely on the existing facilities for aero- 

 logical measurements. The methods developed so far 

 involve the use of planes and dirigibles as well as cap- 

 tive balloons and kites. For the lowest strata, specially 

 built towers and ship installations have come into use. 



'■^•=' Wet and Dry Bulb Methods 



The use of humidity data for radio propagation 

 problems involves new features in instrumental tech- 

 nique because the main effects of strong refraction are 

 found under approximately calm weather conditions. 

 Therefore, when wet and dry bulb methods for humid- 

 ity measurements are used, particular care must be 

 taken to insure satisfactory aeration of the wet bulb. 

 As a rule, an air speed of about 3 m per second ( about 

 6.5 mph) is considered adequate ventilation for the 

 wet bulb. In a plane, dirigible, or kite the necessary 

 aeration is automatically provided. But on a tower 

 or when carried by a captive balloon, artificial aeration 

 will frequently be necessary. It has been claimed,'' 

 however, that if a wet bulb electrical resistor is used in 

 conjunction with a captive balloon adequate aeration 

 can be provided by giving the balloon cable a few 

 violent jerks of about 5-ft amplitude. 



x\n ordinary sling psychrometer held out of the 

 window of a flying plane and aerated by the slip stream 

 has been found to give fairly reliable results, provided 

 the wet bulb is kept properly moistened. This method 

 has been used with good success for preliminary re- 

 search work. It may be presumed that the use of a 



''Data, courtesy of U.S. Navy Radio and Sound Laboratory, 

 unpublished. 



97 



