SOLAR RADIATION AND ATMOSPHERE — STETSON 



163 



from the elapsed time the height to which the pulse ascended before 

 it was turned back by the ionosphere. 



Soundings made of the ionosphere reveal different conditions at 

 various times, displaying marked changes in the ionic density that 

 are dependent upon the hour of the day and the season of the year. 

 Koutine radio soundings include changing the frequency at which the 

 radio pulse is emitted. If the frequency is sufficiently increased, the 

 shorter and more penetrating waves may pass completely through the 

 ionized layer and not return. When such a frequency is attained, it 

 is known as the critical frequency. 



1936 



I '3 4 



FiGUKE 4. — Critical frequencies follow suuspots. Curve smoothed by 12-montli 



moving averages. 



The close relation between the observed critical frequencies and the 

 rise and fall in solar activity marked by sunspot numbers has been 

 so apparent during the last 10 years that the National Bureau of 

 Standards now undertakes to predict 3 months in advance the best 

 usable frequencies for radio conmiunication based upon the sun- 

 spot cycle. 



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