In the Summer, 1941, when the thunderstorm occurred, the atmospheric potential gradient was observed at ten places using the Benndorf's self-recording electrometer, the sadden change of electric field d e to the lightning discharge at five places by the antenna earth current, the electric charge of rain-water at five places, near Maebasi, Gumma Prefecture. Each observing places were separated with the distance from several to ten kilometers each other. Moreover at Maebasi the electric cond ctivity of the air was observed every day d_??_ring the Summer.
The line of eq_??_al potial gradient was drawn on the map and it was found that the area of large negative val e concided with that of the th_??_nderstorm, and in this area there was small local positive area which was accompanied by the strong rain.
By the simultaneo_??_s observations of the s_??_dden change of the electric field d_??_e to the lightning discharge, it was fo_??_nd that the s_??_dden change was small and occurred on both sides of the zero-line when the discharge was far away from the observing places, and the s_??_dden change was large and only on positive side when the discharge was in the neighbourhood.
From the relation between the distance from the discharge and the amount of s_??_dden change of the electric field, it was conclnded that the most lightning discharge occurred between the lower negative charge distributed over the circular cylinder of 2km radi_??_s, extending from 4km to 8km from the earth's s_??_rface, and the upper positive charge over the cylinder extending from 8km to 12m. By projecting the respective discharge on the map it was _??_o_??_nd that the discharging spots shifted from the original region to another region drawing the zig-zag course as if in the random walking. The dimension of the region where the lightning discharge occurs has the diameter of about ten kilometers. The distance of consecutive discharge ranges from one to five kilometers, and its average is 3.5km.
The electric conductivity of the air suddenly increases always when the thunderstorm occurs. The negative partial conductvity increases rather sharply than the positive. After the thunderstorm moved away, the conductivity recovers to its normal value exponentially. The half-value period of the exponential curve is about one hour.
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