ULTRA-SHORT-WAVE TRANSMISSION 515 



ture which indicated weak boundaries at higher altitudes up to, roughly, 

 5.5 km.; most of the patterns, however, were characteristic of layers 

 below two kilometers. The path difference corresponding to two kilo- 

 meters is 85 meters. The theoretical limit of resolution of the amplifier 

 band for a maximum to minimum frequency spacing is Al = 2{C/Af) 

 where Al = path difference, C = velocity of light and A/ = frequency 

 band. For A/ = 2 X 10® cycles this gives 75 meters, and hence 

 boundary heights at and below 1900 meters are unresolvable by our 

 receiver. It is a remarkable result that the bulk of the disturbing 

 boundaries should lie so low. 



It was mentioned earlier that several observations referable to air- 

 mass boundaries have been published. In addition there have been 

 reports, for three consecutive years, of long distance ultra-short-wave 

 reception by American amateurs ^ during the month of May. We 

 have copies of the U. S. Weather Bureau atmosphere cross-sections 

 for several of these days and have been curious enough to examine 

 them. On May 9, 1936, during the long distance amateur reception, 

 there was an extensive boundary at 4 km. between an upper Superior 

 air mass and a lower Tropical Gulf air mass. On May 15, 1937, a 

 similar boundary at 4-5 km. had a Superior air mass above a wedge of 

 Transitional Polar to Tropical Atlantic air. Below this at 3-4 km. lay 

 a Transitional Polar Continental air mass. 



On June 11, 1936, when Colwell and Friend'^ report an extra strong 

 0-2 km. " C" reflection, a subsiding Transitional Polar Pacific air mass 

 lay above a Transitional Polar Continental air mass with the boundary 

 at about 1.5 km. On June 29, 1936, when they reported a very strong 

 3.5 km. "C" reflection, there existed four wedge-shaped air masses 

 with a Superior air mass over a Transitional Polar air mass at 3-4 

 kilometers. The wave-lengths used were 186, 125 and 86 meters 

 approximately. 



These coincidences may or may not be significant but it is very 

 questionable that any boundaries at such altitudes are due to either 

 electron or gas ion distributions. 



The characteristic properties of North American air masses have 

 been published, ^^ as average summer and winter values, and show some 

 marked seasonal differences. The greater dielectric constants for 

 summer conditions are due chiefly to greater water content. 



For a single air-mass distribution, horizontal stratifications are at a 

 minimum and the radio transmission is via the "B" component. 

 This component can be calculated from the corresponding effective 

 earth radius. The table below gives this radius for three important 

 air mass types. 



