TABLE OF CONTENTS The number of lightning flashes occurring in a given region is of interest to both power engineers and lightning protection engineers. Historically, the lightning activity was measured in terms of the number of thunder days for each region. A thunder day is a day in which thunder is heard by meteorological observers. On the basis of this data, the World Meteorological Organisation has published a thunder-day map covering the whole world (see Chapter 1). More recently, information on lightning activity has been obtained by counting the number of lightning flashes occurring over a given region by (i) using satellites to capture the optical signals generated by lightning flashes [178,179], (ii) using lightning flash counters [180], (iii) employing magnetic direction finding systems [181] and (iv) VHF lightning mapping techniques [1,5] (see section 4.11 for a detailed description of some of these techniques). Some attempts have also been made to evaluate the number of lightning flashes occurring in the atmosphere by measuring the radio noise with an ionospheric sounding satellite [182]. Recently, advances have been made to relate the global lightning activity to the Shumann resonance [183,184]. Shumann resonances are caused by the lightning- induced excitation of the earth ionospheric waveguide. The resonance frequencies are located at 8 Hz, 16 Hz, 32 Hz etc. Various estimates making use of these different techniques indicate that at a given time there are about 1000 2000 thunderstorms active around the globe. An isolated thundercloud may produce lightning at a rate of a...
