Thunderstorms and the Earth's general electrification

In a joint project of the Department of Terrestrial Magnetism, Carnegie Institution of Washington, and the United States Air Force, acceptable electric surveys were made over the tops of 21 thunderstorms in the central United States during July to October, 1948. The purpose of the project was to ascertain whether thunderstorms supply negative electricity to the earth at the rate required to maintain the general negative electrification (negative surface charge) which is almost invariably observed during fair weather. A pressurized aircraft (B-29), equipped with instruments for registering the data required to determine the electrical conductivity of air and the vertical component of electrical field strength, as well as auxiliary data, was used. Each traverse of a survey was aimed over the center of the region which returned strong radar echoes. A “profile” of vertical current density could be constructed from the data obtained on each traverse. The distribution of current density obtained in this way is the basis for estimating the total average current during a survey of a given storm. For one storm a current of 6.5 amp was estimated, but all other values are much smaller, ranging from 0 to 1.4 amp. The mean of all values is 0.8 amp; omitting the extremely large value it is 0.5 amp. The direction of the current, in the conventional sense, is upward in all cases, thus indicating that the net charge transported from thunderstorms to earth is negative. This result supports the view that thunderstorms do maintain the general negative charge of the earth. The average current (I) derived from these surveys is of the right magnitude if the world population of thunderstorms (N) is such that IN is equal to the total electric conduction current for all fair-weather areas, namely, about 1800 amp—a value derived from numerous observations made in representative areas of the earth. For I equal to 0.8 or 0.5 amp, N should equal 2200 or 3600, respectively. C. E. P. Brooks estimated 1800 for the average thunderstorm population, but this estimate doubtless tends to be too small for the comparison in hand here because, being based on reports of thundery days noted at the meteorological stations of the world, the occurrence of more than one thunderstorm on any one day is not taken into account and no distinction is made between a simple thunderstorm with only one center (or “cell”) of electric activity and a complex storm which has two or more such centers. It, therefore, seems likely that an average value of I as small as 0.5 amp, or perhaps smaller, is adequate. The conclusion is that the results of this project corroborate other evidence which indicates that thunderstorms do maintain the universal negative electrification of the earth.