AUDIO FREQUENCY ATMOSPHERICS 



507 



It is possible that aural frequency observations result in erroneous 

 determinations because of the rapid reduction in frequency which 

 occurs during a tweek. If the damping is not excessive, the ear 

 distinguishes the low frequencies of the tweek and thereon establishes 

 the tonal characteristic. If the damping is great the lower frequencies 

 may be reduced below audibility while the ear may distinguish the 

 higher or intermediate frequencies as possessing tonal quality and 

 thereon may base its estimation of frequency. Judging from the 

 observations of resonance, where the sound may be almost con- 

 tinuous, it appears likely that these frequency determinations are of 

 fair accuracy. 



Observations have been made at various times to determine the 

 time of appearance of the first and last tweeks of the night-time period. 

 Fig. 6 shows the time of first tweek to be quite variable, extending from 



P 60 



m 



o 

 o 



o 



JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. 



Fig. 6 — Obser\-ations of first and last tweeks of night-time periods. 



approximately 1/2 hour before sunset to 1| hours after. The time of 

 the last tweek varies from 40 minutes before sunrise to a few minutes 

 after sunrise. The points obtained in Florida differ somewhat from 

 those obtained in Newfoundland and Ireland, possibly because of 

 the diliference of latitude. Since the Florida observation point lies 

 approximately 24° south of the latter locations, it follows that here 



