Introduction

More than 75 years ago Brooks (1925) estimated that the global lightning flash rate, the total of both cloud and ground flashes, was about 100 s−1. He did so by combining the following information: (i) an estimated average of 16 storms per year over the assumed observation area, 512 km2, of an individual weather station, which was extrapolated to the entire Earth' surface, about 5.1 × 108 km2 (both land and sea included); (ii) an assumed average storm duration of 1 hr; and (iii) the flash rate of 3.5 min−1 observed by Marriott (1908) during a 28 min thunderstorm period in England. More recent estimates of the global flash rate, based on satellite measurements and many assumptions, are within a factor 2 or 3 of this value but are not necessarily more accurate (subsection 2.5.3). The number of flashes per unit area per unit time is called the flash density, and the number of ground flashes per unit area per unit time (usually per square kilometer per year: Section 2.5) is called the ground flash density. A global flash rate of 100 s−1 corresponds to a total flash density over the entire Earth's surface of about 6 km−2 yr−1 and a ground (land or sea) flash density of about 1.5 km−2 yr−1 if we assume a 3 : 1 global ratio of cloud to cloud-to-ground flashes (Section 2.7).