258 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 11 



the wave-length, the sunrise and sunset effects are greater for the 

 longer wave-lengths. 



Effects of time of year.— The increase of signal strength at night 

 and the scarcity of strays in winter indicate that the Heaviside surface 

 is more nearly level and encumbered with fewer masses of ionized 

 air which are emitting disturbances. This more quiescent condition 

 is probably a result of the sun's acting on the atmosphere for fewer 



hours each day. 



Effects of atmospheric electricity, terrestrial magnetism, solar activ- 

 ity. — There is slight connection between radio phenomena and the 

 usual variations of atmospheric conductivity and similar quantities 

 at the surface of the earth. During intense strays and fading, the 

 conductivity and potential gradient have been known to fluctuate 

 more than usual. This indicates that the masses of ionized air that 

 cause the radio phenomena have a slight effect extending down to 

 the earth's surface. 



The visible aurora seems to involve a lowering of the Heaviside 

 surface far down into the stratosphere or even lower. It is known 

 to extend down to about 60 kilometers above the earth's surface. 

 This is done with much turmoil so that it usually stops short-wave 

 radio altogether, or, if signals can be heard, the strays are violent. 

 It completely upsets all usual radio conditions, in fact, in special 

 cases it improves short-wave signal intensity over a certain territory, 

 probably because in this case the lowering of the Heaviside surface 

 envelops the usual source of disturbances. 



Discontinuities. — While often difficult to explain, discontinuities 

 are probably caused by some local ionization which acts like a barrier 

 for that particular region. The fact that transmission is good for 

 points beyond the silent zone substantiates the general theory of trans- 

 mission of waves at night by the upper part of the stratosphere. In 

 such a case they probably go around the barrier. 



The origin of this local ionization is unknown. Apparently some 

 local condition on the earth's surface may cause it. It may be direct, 

 as by radioactive emission, or indirect, as by extreme temperature 

 dift'erences or vertical convection currents in the air. Such indirect 

 causes seem probable since there are many cases of discontinuities 

 on or near the coast or in mountainous regions. 



The cause of discontinuities is doubtless related to some sort of 

 ionization change such as that accompanying sunrise and sunset. 



