line connecting points of simultaneous storm development
• (n.) An imaginary line, or a line on a chart, marking the simultaneous development of a thunderstorm, as noted by observing the time when the thunder is heard at different places.
I'so·bront (ī'so*brŏnt) noun [ Iso- + Greek 
isobront A line drawn through geographical points at which a given phase of thunderstorm activity ooccurred simultaneously.
a line on a weather map or chart connecting various points where a given phase of thunderstorm activity occurred simultaneously.