isobront

noun

1. An isoline drawn through geographical points at which a given phase of thunderstorm activity occurred simultaneously.

   “The lines joining the points where the first thunder is heard at the same instant of time are called "isobronts", or lines of equal front. Sometimes the isobront is taken as the mean of the time when first and last thunder is heard at a place. This is the custom in France.”

2. An isoline drawn through geographical points which, on average, experience the same number of thundery days per year.

   “On these charts isobronts, or lines of equal frequency of thunder, were drawn. These are shown in Figs. 1, 2 and 3, the data being expressed as percentages of days on which thunder was heard.”