On July 24, 2006, I had the honor of almost being struck by lightning. That was not the interesting part, though.
The truly interesting thing was the metallic click I heard just before. I was doing measurements on lightning for my PhD thesis in Finland at a meteorological observatory. The equipment was in a detached outbuilding to minimize radio disturbances. The storm was overhead, and I was having as much fun as a nerd with an oscilloscope can have. Excellent data.
Taking a small break, I went to the door to enjoy the sights. Immediately, lightning struck an electric pole on the other side of the road.
I have never been more frightened. The shock wave from so close is truly mind-boggling, in the sense of boggling your brain. The hairs on your body rise up and the skin goes into goosebumps. For some reason, you feel nauseous for a long time afterwards, as well as a metallic taste in the mouth. I suspect the latter is due to an adrenaline rush. All of this is standard operating procedure (SOP) for near lightning strikes.
What was not SOP was something I did not expect. And could not immediately explain. At the same exact time the lightning hit the pole, and significantly BEFORE the shock wave, I heard a loud metallic click. Very much like the crack an electrified fence makes when touched (try it).
One thing to understand is that a lightning stroke does not in fact travel all the way to the ground. Rather, it travels down from the cloud as a stepped leader. Just before it hits the ground, there is actually an upward stepped leader which travels from the ground up and connects with the downward leader. The real current only starts to flow and the big kabuum is heard when the connection occurs.
Figure 1: Downward and upward leaders.
The best hypothesis was that an upward attempted leader was the cause of the click. An attempted leader is after all a spark that can meters to tens of meters in height, and could create a sonic boom of it own. Such a noise simply had not been recorded before.
Figure 2 shows a map of the area. There were two possibile sources: a 20-meter metal observation tower about 100 meters away, or the hut itself. There were also some grounded buildings nearby. Looking at the time dfferences, the most logical choice was that the upward leader was driven from the hut. In other words, if the downward leader had happened to connect with “my” upward leader, the flash would have hit the hut and, with any luck, me.
The hut was well grounded as it had been used for storage of hydrogen for weather sounding balloons. It probably would not have been damaged badly. Nevertheless, the idea was not attractive. I was standing at the door of the hut, so the lightning could have taken any path. Almost certainly, there would have been hearing loss.
Figure 2: Map of the situation. The instrumented tower and the observation point are almost equidistant from the strike, so the tower cannot have been the source of the click. The source almost certainly was the hut itself.
The anecdote remained just that: an interesting occurrence that no one seemed to have replicated. Some amateur storm chasers did report various kinds of crackles before close flashes. (Professional lightning researchers, at least those who want long careers, try to avoid close flashes). My hunch was plausible — but unverifiable.
However, in 2009 I noticed a paper which suggested my explanation was correct. The phenomenon had been observed accidentally by Lu and Walden-Newman (2009). An attempted leader occurred near their equipment, and they got both (partial) video, electric field, and audio signals from it, ensuring that the interpretation is correct. Figure 3 shows the audible “click” quite clearly about half a second before the krakabuuuum of the actual lightning flash.
Figure 3: The first known actual recording of such a “click”, by Lu and Walden-Newman (2009).
There was not enough data for me to even consider publishing the event (no video, no audio, only a badly distorted electric field recording) so that the authors are the first ones to have measured this effect in any real scientific sense. It is nevertheless a satisfying feeling to have observed something, come up with a hypothesis, and then later found proof that suggests the hypothesis was correct.
Moral of the story? Perhaps it is always worthwhile to stay observant for anomalies, whatever the situation? And take notes. I collected what information I had, and wrote it up rather quickly. There is an old science saying: “If you didn’t write it down, it didn’t happen”. It’s a good motto for life in general.
Writing this post, I feel nostalgic for my research days. Science is cool, especially lightning science.