Back before lightning was being detected and located by satellites and ground based networks, thunderday and thunderhour statistics were used.   A day on which thunder is heard at a weather station is recorded as a thunderstorm day in the station's log.  A world thunderstorm day map is shown below.  The peak value about 200 days is found in South America.  There are several locations in Africa where thunder was heard on 180 days/year.


adapted from: World Distribution of Thunderstorm Days, Part 2, World Meteorological Organization Publ. 21, Geneva, 1956 (reprinted 1969).


Attempts were then made to use thunderstorm day data to estimate the ground flash density (flashes/ (km2  year)).  The relations were generally of the form:
Ng = a (Td)b
The constants a and b were determined in localized areas where some other means of determining lightning flash density was available (often networks of lightning counters).  In a study in South Africa, for example, the relation

Ng = 0.023 (Td)1.3
provided the best fit between independent measurements of Td and  Ng  (Td ranged from 3 to about 100 and  Ng  from about 0.1 to 12 flashes/ (km2  year) ) .