134 
through all kinds of precipitation and under varying 
meteorological conditions, established the basic mech- 
anisms responsible for precipitation static. By using a 
specially instrumented aircraft, it was found that the 
two most important sources of electrification of the 
aircraft were (1) closely adjacent, highly electrified 
cloud centers, and (2) friction of snow and ice crystals 
as they slid over the wings at low temperatures. Ordi- 
nary rain or shower clouds showing little vertical con- 
vection were relatively inactive. 
The Research Team was able to demonstrate that 
the interference with communications on the aircraft 
resulted from St. Elmo’s fire or corona discharges from 
the aircraft antenna or closely adjacent structures. 
Because of the intermittent pulselike nature of the 
corona discharge, adjacent radio circuits were strongly 
shock-excited and rendered insensitive to the ordinary 
radio signals. 
Normal thunderstorm activity produces large electric 
fields in the atmosphere; when the plane is in the vicin- 
ity of these fields, corona currents frequently are pro- 
duced on the aircraft. Because an airplane traverses a 
typical electrically active cloud center in a relatively 
short interval of time, this type of disturbance (while 
especially severe) does not persist for long and there- 
fore is not a serious handicap to navigational radio 
communication. The main difficulty arises from the 
fact that lightning sometimes strikes the aircraft or 
that very intense electric fields break down the imsu- 
lating wire used on antennas. 
The second type of electrification is self-produced 
by the aircraft as a result of frictional effects of snow 
or ice crystals as they slide over metallic parts of the 
aircraft. In frontal conditions this type of electrification 
may last for hours and, because radio navigational 
facilities must be constantly employed on aircraft, it is 
evident that such continuous electrification constitutes 
a dangerous operational hazard. 
The frictional charge produced on the airplane proper 
when flying in dry snow or ice crystals is always nega- 
tive, while flakes leaving the plane after sliding along 
its surface carry a positive charge. Experimental in- 
vestigations have established the fact that the rate of 
charge production depends on the character of the 
metallic surface intercepting the precipitation. The 
charging increases with snow or ice-crystal density and 
with the cube of the air speed. As one might expect, 
the Research Team found that the charging rate was 
dependent upon the temperature, being relatively small 
near the freezing temperature and increasing as the 
temperature dropped to about —15C. 
It is impractical to review the detailed effects that 
result from flying through precipitation. Interested 
readers are urged to read the extensive technical re- 
ports of the Precipitation-Static Project [7, 18]. 
As an illustration of the severity of precipitation 
static, it seems worth while to give some numerical 
results obtained off Yakutat, Alaska, in a typical up- 
slope storm. Cloud and charging conditions were singu- 
larly uniform and serious electrification was observed 
for more than three hours. A four-engine B-17 aircraft, 
ATMOSPHERIC ELECTRICITY 
cruising at 165 mph, generated an average current of 
750 wa. This current transferred a negative charge to 
the airplane and raised its potential to more than 450, 
000 v. The electrical energy dissipated, therefore, was 
330 w. It was not surprising that corona discharge 
from the antenna was initiated causing such severe 
radio interference as to override urgently required com- 
munications. 
Precipitation static is still a serious operational haz- 
ard because the present high operating speed of air- 
craft has greatly increased the charging rates that 
were already troublesome at low speed. The modern 
trend toward still higher speeds will ultimately de- 
mand housed antennas for communication purposes. 
Such construction will greatly assist in meeting future 
requirements. 
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