AUDIO FREQUENCY ATMOSPHERICS 509 



observed to occur at rates exceeding 50 per minute while during the 

 winter as few as one or two in five-minute periods is not unusual. 

 A night completely free from tweeks has not been observed at any of 

 our experimental locations. Fig. 7 shows results of a summer tweek 

 count when the rate was high. This curve illustrates well the rapid 

 variations which may occur during the morning twilight period. 



Swish and Related Musical Atmospherics 

 Swishes observed in Newfoundland have been described as, "Musical 

 sounds, such as made by thin whips when lashed through the air." ^ 

 They are ordinarily distinctly musical in character, the frequency 

 varying sometimes downward and at other times upward. At times 

 upward and downward progressions are observed simultaneously. 

 During the Newfoundland observations, the frequencies lay usually 

 between 700 and 2000 c.p.s., but the individual tones in most cases did 

 not exceed an octave in variation. The duration of these earlier ob- 

 served swishes varied from approximately 1/4 second to more than a 

 second. In Ireland swishes of the same nature were observed, but a 

 more usual type was longer and much clearer in tone. These swishes 

 were audible from 1/2 second to possibly 4 seconds and covered a 

 frequency range from well below 800 to above 4000 c.p.s. To the ear 

 the frequency appeared to progress steadily with perhaps a slight 

 lingering near the termination of the descending variety. 



While in the earlier Newfoundland observations the swish usually 

 appeared to be accompanied by a rushing sound, later work disclosed 

 many nearly clear whistling tones which may be identified as the 

 "long whistlers" reported by other observers. These sometimes 

 swept upward or downward through the entire voice range and at 

 other times varied only through the range between approximately 

 3000 and 4000 c.p.s. On a few occasions the whistles have been ob- 

 served to hesitate and warble slightly before disappearing. Series of 

 swishes have been observed following each other with almost per- 

 fectly regular spacing of a few seconds, the train persisting on occasion 

 for as long as a few minutes. Some of these trains have successively 

 increased in intensity, terminating abruptly while other trains have 

 reduced gradually until submerged in the usual static. In addition 

 to the distinctly musical tones, swishes have been heard in which the 

 rushing or hissing sound is prominent while the tone may be nearly or 

 entirely absent. Our observations have shown these often to appear 

 during periods when the whistling tones are frequent, to correspond 

 approximately to the length of the whistles and at times to appear in 

 regularly spaced trains. 



