502 



BELL SYSTEM TECHNICAL JOURNAL 



daytime only. During the sunlight hours, disturbances occurring in 

 the vicinity of the observation point may be received by direct trans- 

 mission without unusual attenuation. Atmospherics of distant or 

 high origin should suffer considerable attenuation in passing through 



10 



o 8 



t 6 



? 4 



> 



^2 



8 12 4 



PM. LOCAL TIME-HOURS 



Fig. 2 — Typical diurnal intensity curves, for frequency range from 150 to 3000 c.p.s. 



the damping region. Following sunset, the damping ionization may 

 be expected to gradually dissipate, resulting in a slow increase of the 

 static intensity as transmission from the upper atmosphere and from 

 horizontally distant regions is improved. 



It is probable that in the morning the damping ionization appears 

 at a given point almost immediately upon arrival of the first direct sun- 

 light, and that the transition period corresponds to the time required 

 for the earth to rotate through an angle corresponding to that section 

 of the damping region which may appreciably affect the atmospherics 

 reaching the observation point. 



Our observations have shown that the general intensity of the 

 regularly occurring types of atmospherics increases in the spring, the 

 rise beginning about March. During a period from possibly May to 

 September, the intensity is comparatively high. During September 

 and October a reduction occurs, and from the latter part of October 

 until March the intensity is low. The periods as given above are 

 appro.ximate, since they are based on fractional year observations in 

 all except one case. 



Comparison of Fig. 2 with diurnal variation curves of Potter ^ for 

 50 kilocycles and 2 megacycles, and wdth seasonal variations presented 

 by Espenschied, Anderson and Bailey ^ for 50 kilocycles shows definite 

 similarities. 



* R. K. Potter, "Frequency Distribution of Atmospheric Noise," Proc. I. R. E., 

 20, p. 1512, September, 1932. 



" Espenschied, Anderson and Bailey, "Transatlantic Radio Telephone Trans- 

 mission," Proc. I. R. E., 14, p. 7, February, 1926. 



