1) Isoceraunic 
line connecting points of equal occurrence of thunderstorms
isoceraunic, isokeraunic Indicating or having equal frequency or intensity of thunderstorm activity. An isoceraunic line is a line drawn through geographical points at which some phenomenon connected with thunderstorms has the same frequency or intensity; used for lines of equal frequency of lightning discharges.isokeraunic 1. Having thunderstorm phenomena of equal frequency or severity, or simultaneous occurrence. 2. A line on a map that connects places of equal thunderstorm frequency.
isoceraunic.