This is an animated map illustrating how the daily probability of lightning (the light blue glow) varies with the seasons.
The highest lightning probabilty corresponds to the warmest locations at any given time (a higher probabilty of thunderstorms) and also with the seasons. The warmest locations tend to be even warmer in summer so lightning probability is even higher in those locations.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 126.96.36.199.0