The keraunic number is a system to describe lightning activity in an area based upon the audible detection of thunder. It is defined as the average number of days per year when thunder can be heard in a given area, and the likelihood thereby of a thunderstorm. An isokeraunic map plots contours of equal keraunic number. The keraunic number has been used to set standards for safe design of electrical systems in structures connected to the local power grid.
Before technology was developed to accurately detect and record lightning flashes, keraunic measurements were the standard means to assess the probability of lightning at a location. However, a keraunic number does not distinguish between forms of lightning, such as cloud-to-cloud, or cloud-to-ground, and is limited by the requirement for the thunder to be audibly detected. For these reasons, the keraunic number has been replaced by more accurate Flash Density maps, which collect data from both ground-based and satellite lightning detectors.
The Democratic Republic of the Congo has the world's highest keraunic number with upwards of 180 thunderstorm days per year.
Also appears spelled as ceraunic.
Calculation
The annual number of lightning flashes hitting one square km of ground, Ng, can for temperate regions be calculated from the following formula:
where Td is the keraunic number.