Over on YouTube user Tech Addict Attic has uploaded a video demonstrating what lightning strikes look like on the radio spectrum. To receive the pulses he uses an RTL-SDR and a simple wire antenna located on his roof. He notes that the pulses show up at HF frequencies, and continue all the way up to the broadcast FM band and above.
When lightning strikes it emits a wideband RF pulse that can be detected several miles away by radios. On a software defined radio spectrum display the pulse shows up as a quick horizontal blip. Detecting this blip is how lightning detection websites like blitzortung.org work, although they use their own radio hardware.
In the past we posted about another user who also demonstrated lightning pulses using his RTL-SDR V3.
Lightning produces fairly wideband bursts of RF energy, especially down in the VLF to HF frequencies. Detecting these bursts with custom radio hardware is how lightning detection websites such as blitzortung.org work.