ih:i\s.iTi..i.\ric N.inio ir.i.i-riioxr. i h-.ixsMissiox 4<.q 



tlost- a (U-ritlfd st'asoii.il iiitliifnce. The etTt'ct i>f lliis seasonal iii- 

 thifiue ill shiflinjj the diurnal transmissiim rharailerislic is ln-tter 

 shown in l"\^. 10. This tij^ure consists of the same monthly average 

 diurnal ciirNes ,is an- .issemhled in Fig. 8, arranged one aliove the 

 citlur instead of side h\ side. 



Kig. 9 — Signal l"ii-lil Jiiiic — .Night conditions showiiii; proximity of transmission 

 path to sunlit heniisphcrc 



In particular, there should be noted: 



1. The time at which the sunset dip occurs changes with the change 

 in time of sunset. 



2. Similarly, the time at which liie morning drop in field streiigtli 

 occurs changes with the time "of sunrise. 



3. The period of high night-time values, boinided between the 

 time of sunset in the I'nited States anil the time of sunrise in ICiiglaiid, 

 is much longer in the winter than in the summer months. 



It is also to be observed that, as a rule, full night-time values of 

 signal field strength are not attained until some time after sunset at 

 the western terminal and that they begin to decrease before sunrise 

 at the eastern terminal. In other words, the daylight effects appear 

 to extend into the period in which the transmission path along the 

 earth's surface is unexposed to direct rays of the sun. The effect 

 of this is that with the advance of the season from winter to summer 

 the time at which the high night-time value is fully attained occurs 

 later and later whereas the time at w'hich it begins to fall off occurs 

 earlier and earlier, until the latter part of April when these two times 

 coincide. .At this time, then, the transmission path no sooner comes 

 into the full night-time conditions than it again emerges. As the 

 season further advances into summer, the day conditions begin to 

 set in while the night-time field strength is still rising. The proximity 

 to the daylight hemisphere, which the transatlantic path reaches at 

 night during this season of the year is illustrated in Fig. 9. 



