National Academies Press: OpenBook

The Earth's Electrical Environment (1986)

Chapter: Current Above Thunderstorms

« Previous: Global Thunderstorms
Suggested Citation:"Current Above Thunderstorms." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
Page 212

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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 212 Figure 15.3 (a) Annual curve of the diurnal variation of the atmospheric electric field on the oceans (volts per meter) as measured by the Carnegie and Maud expeditions (Parkinson and Torrenson, 1931) and (b) annual curve of the diurnal variations of global thunderstorm activity according to Whipple and Scrase (1936). are the generators in the global electrical circuit, there is still considerable uncertainty concerning the details, as discussed by Dolezalek (1972) and Kasemir (1979). Moreover, the data on which the thunderstorm activity is based are only of a qualitative nature: ''a thunderstorm day is a day when thunder has been heard." The commonly quoted UT diurnal patterns, shown in Figure 15.3, are averages over a long time that were made to reduce the influence of various disturbing factors. When shorter time averages are used—and even single diurnal variations—the correlations show great departures from the average curves. The variability of thunderstorm frequency can be large, with significant departures from the average; for example, whole continents may be cloudless for a long time. The measured electric field on the ground is also highly variable as a result of local influences, and it generally takes a week's worth of averaging or more to bring out the diurnal UT pattern. Most measurements are made on continents, where the electric field displays variations with local time, and these measurements do not fit into a daily worldwide pattern but must be averaged to determine worldwide characteristics. Paramanov (1950) suggested that local influences might cancel out if the electric fields measured on many continents were synchronized to universal time and averaged. Dolezalek (1972), in examining the data available at the time, concluded that a globally controlled current does flow vertically through the atmosphere but that its connection to thunderstorm activity is tenuous and, in fact, is often contradicted by the proper interpretation of available measurements. More recently, Orville and Spencer (1979) examined lightning flashes recorded in photographs by two satellites in the Defense Meteorological Satellite Program (DMSP) and found that most of the lightning is confined to land areas and that the ratio of global lightning frequency during northern summer to that of southern summer is about 1.4 for both the dusk and midnight satellite data. They pointed out that this summer-winter difference in global lightning frequency is opposite to the electric-field measurements. The hypothesized relation of the global atmospheric electric current to thunderstorms is still an unsettled question and clearly needs to be resolved to make further progress in understanding the Earth's global atmospheric electric circuit. Current Above Thunderstorms A few measurements have been made that give the magnitude of the current flowing upward over the whole area of a thundercloud (Gish and Wait, 1950; Stergis et al., 1957a; Vonnegut et al., 1966; Imyanitov et al., 1969; Kasemir, 1979). The currents range from 0.1 up to 6 A, with an average between about 0.5 and 1 A per thunderstorm cell. Gish and Wait (1950) flew an aircraft over a thunderstorm in the central United States and found an average upward current of 0.8 A at an altitude of about 12 km. They measured electric fields of up to 70 kV/m. Stergis et al. (1957b), in a series of balloon flights at altitudes of about 25 km in central Florida, measured an average upward current of 1.3 A. The electric fields that they measured at this altitude were on the order of a few hundred volts per meter. Holzworth (1981), with a balloon near 20 km over a large thunderstorm at Fort Simpson, NWT, Canada, on August 15, 1977, measured a vertical upward electric field of more than 6.7 V/m (the instrumentation threshold) for longer than 2 hours. These few data indicate that a positive current flows toward the ionosphere above thunderstorm regions that is of sufficient magnitude to account for fair-weather conduction current. Is it possible to relate current output to such factors as frequency of cloud-to-ground strokes, charge structure and separation distances, and cloud-top height? Both Pierce (1970) and Prentice and Macherras (1977) presented relationships for a latitudinal variation in the ratio between cloud-to-ground and cloud-to-cloud flashes. This ratio is about 0.1 in the equatorial region, increasing to about 0.4 near

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This latest addition to the Studies in Geophysics series explores in scientific detail the phenomenon of lightning, cloud, and thunderstorm electricity, and global and regional electrical processes. Consisting of 16 papers by outstanding experts in a number of fields, this volume compiles and reviews many recent advances in such research areas as meteorology, chemistry, electrical engineering, and physics and projects how new knowledge could be applied to benefit mankind.

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