These wideband VLF spectrograms from our lightning sensors show radio atmospherics ('sferics') generated by lightning (vertical lines) and narrowband signals from VLF transmitters (horizontal lines). Each spectrogram spans a few seconds in time (horizontal axis) and up to either 24 or 48 kHz in frequency (vertical axis). All start at precisely the same time at exactly on the minute min past each hour and include data for the next few seconds as indicated.