Thunderstorms and the Earth's general electrification

Abstract

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.

Charge Transfer in the Atmosphere by Point Discharge

“…It is assumed that all the points to give the same current as from the erected point, are spaced in rectangular arrays with a separation d. It is also assumed that current below a thundercloud consists only of point-discharge current and is equal to the total current of -0.5 amp. above the thundercloud as observed by Gish and Wait (1950).…”

The development of lightning discharges

“…It consists of a positive charge, 20 to 40 coulombs in size, centred at a height of about 6 km, and a negative charge of approximately the same magnitude centred at a height of about 3 km, that is, near the freezing level. This distribution was originally indicated by field-change measurements (Wilson 1916(Wilson , 1920, definitely established from the alti-electrograph work at Kew Scrase 1937, Simpson andRobinson 1941), and subsequently confirmed by the aeroplane observations of Gish and Wait (1950). Small regions of positive charge towards the base of a thundercloud have also often been noted, and Schonland and Malan (1951) have suggested that these are due to point discharge from the surface of the earth.…”

“…It seems unlikely that slow positive field-changes can be due to upward leaders from the positive charge centre near the summit of the thundercloud. Gish and Wait (1950) consider that the discharge from the top of a thunderstorm is in the nature of a small steady current; this would not be observed as causing a field-change with the recording technique described in I, Furthermore, in view of the general increase of conductivity with height, there appears to be little to prevent the vertical extent of any upwardly directed lightning flashes being considerable, and the resulting field-change will then be complex at most of the observing distances concerned.…”

The electrical environment of the Earth’s atmosphere: A review

“…They occur mostly in the tropics in the local afternoon and evening. Aircraft and balloon measurements above thunderstorms show that a total current of 0.1-6 A, with an average of 0.4 A, flows from the top of a thunderstorm cell to the ionosphere (Gish and Wait, 1950;Stergis et al, 1957;Vonnegut et al, 1973;Kasemir, 1979). Below a thunderstorm the transfer of charge by point discharge, lightning, precipitation, convection and displacement currents contributes to the net current flowing between the thunderstorm's base and the ground.…”

“…Figure 3 shows the electric charges carried into the cloud from below by the updraught, constituting an electric current density J M (Roble, 1991). Current (∼ 0.5 A) flowing upward (Gish and Wait, 1950;Blakeslee et al, 1989) from the top of cloud charges the ionosphere, which is a good conductor, and therefore the ionosphere behaves as an equipotential surface having a potential of ∼ 300 kV with respect to the Earth (Wilson, 1916). This happens over 10% of the Earth's surface for all of the ∼ 200 thunderclouds which are active at any time, and which are mainly concentrated over the tropical land masses during the local afternoons.…”