Atmospheric Electricity - Ultra and Extremely Low Frequency Electromagnetic Fields - page 70

Fig. 3.11 NOAA satellite data of worldwide average annual lightning flashes per square kilometer.
Taken from the site http://www.boqueteweather.com/lightning_year.htm

frequently occur in the regions covered with cyclones, typhoons, frontal zone of
temperature inversion, climatological fronts and etc. (Watt 1967 ; Bhartendu 1969 ;
Uman and Krider 1982 ; Uman 1987 ; Nickolaenko and Hayakawa 2002 ;Rakov
and Uman 2003 ). The most favorable areas for the thunderstorm formation are
usually associated with such regions as mountain ridges, which may affect the
monsoon circulation, the river basins and bottom-lands, where the humid climate is
predominant. Additionally, the group of islands in an ocean may influence the wind
system in such a way that the wind system in this region is capable of sustaining
the generation of thunderstorm clouds (Watt 1967 ). Based on observational data
gathered for a long period one can characterize the lightning activity over the
Earth surface through the mean number of flashes per unit of square in a year
or month. An example of annual pattern of lightning activity over the globe as
observed from space by the OTD is displayed in Fig. 3.11 . The number of lightning
per square kilometer in a year is shown with different colors. There is a small
wonder that the main centers of lightning activity, termed global thunderstorm
centers, are concentrated in tropics and around the equator. As is seen from Fig. 3.11 ,
such centers cover three broad continental tropical regions, (1) the sub-Saharan
Africa; (2) Central America and the Amazon basin in South America; and (3) the
Malaysian Archipelago/Maritime continent extending from Southeast Asia across
the Philippines, Indonesia, and Borneo into Northern Australia.
The number, , of lightning discharges per one km squared in a year inside these
global thunderstorm centers is greater than 25 km 2

year 1 (Bliokh et al. 1980 ).

The value D 2:5-7:5 km 2 year 1 is typically for the regions with an enhanced

lightning activity, while the regions with a moderate thunderstorm activity can be
characterized by the value D 1:5-2:5 km 2 year 1 .

The global thunderstorm centers play a crucial role in the formation of global
electric circuit of the Earth. It is commonly accepted that the total electric current