The keraunic level indicates the number of thunderstorm days per year in a specific area or location. The keraunic level is subject to strong regional fluctuations and ranges from less than 1 in the Arctic and Antarctic to up to 180 near the equator. If the local keraunic levels are plotted on a map, lines of equal frequency, so-called isokerauns, can be drawn around areas of constant levels. Depending on this and other criteria of the risk analysis, the use of surge protection devices may be recommended or even mandatory.