Depending on propagation an AM radio can hear lightning crashes many hundreds, even thousands of miles away which is why DXing the band is best done in the winter months. If you want to use one as a lightning detector it's best to use some cheap POS with poor sensitivity and having an internal antenna tuned between stations on the low end of the band where it has lower sensitivity and lightning travels best.
Many years ago I built a simple detector using a 50-0-50 DC microammeter connected between my long wire antenna and ground. It reacted to currents flowing both up and down, you'd be surprised at all the electrical activity on a clear day and how it gets intense when a storm is in the area. Later I built an AC version that only reacted to EMP created by discharges when a storm was nearby, both can be quite interesting.
To I have to remind you to disconnect a lightning detector when the storm gets close before it goes POOF? (;->)
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73 de Warren
Amateur Radio KB2VXA
Station powered by atomic energy, operator powered by natural gas.
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