Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 58

H , km

10

1

8

2

j g

3

j f

6

v t

4

4

2

5

Fig. 3.2 A sketch of thundercloud structure. 1 and 2 —centers of positive and negative charge
distributions. 3 —a lightning return stroke. 4 —air flow lines. 5 —a downpour. Horizontal arrow v t
shows the direction of the thundercloud motion

thunderstorm is characterized by the local mean flash frequency about 0:05 s 1 .
Whence we can estimate the mean interval between lightning flashes as 20 s. The
charge of thunderstorm clouds is thus renewed for this short interval of time.
We now raise an interesting question, what is the basic mechanism for electric
charge formation in the thunderstorm cloud? The electric charges in the clouds are
concentrated on the small particles such as rain-drops, snowflake, pieces of ice and
aerosols. In stratus and stratocumulus clouds the charge of rain-drop reaches the
value of q 0 D .10-100/e, where e is elementary charge, while in the nimbostratus

the charges of separate drops amounts to q 0 D 10 5 -10 6 e and in the thunderstorm

clouds the separate charges amounts to a very great value of the order of 10 6 -10 7 e
(Israël 1970 , 1973 , Imyanitov et al. 1971 , Muchnik and Fishman 1982 ).
Despite that the electrical structure of a typical thundercloud is rather a strat-
iform, the most part of positive charges tend to pile up at the upper portion of
the thundercloud whereas most of negative charges predominantly accumulate at
its bottom (Coroniti 1965 ;Nelson 1967 ; Bhartendu 1969 ; Wahlin 1973 ;Winn
et al. 1974 ; Uman 1987 ; McGorman and Rust 1998 ; Rakov and Uman 2003 ). The
simplest model of the spatial charge separation in the thundercloud is shown in
Fig. 3.2 . In the middle latitudes the thundercloud top amounts to 8-12 km, while
in tropics the thundercloud top may be as high as 20 km. The thunderstorms are
mainly formed in the zone of power convective fluxes. The separation of oppositely