How dangerous is lightning to swimmers?
The other day I was swimming at Shepperton lake when a storm blew in. I saw a flash on the horizon, turned to a nearby swimmer and asked: “Was that lightning?”
“Well, I don’t think you got caught by a speed camera,” he replied.
As most of you will know, lightning is supposed to be bad for swimmers. Like any responsible venue operator would, the staff at Shepperton evacuated the lake, but it still took me a few anxious minutes to get out, during which time I saw several more lightning strikes. I started trying to figure out the probability of lightning hitting me or the lake, and what the consequences might be if it did.
Later, when I got home, I did a bit of research on the subject on the ever-reliable internet and combined that with what I know (i.e. vaguely remember from many years ago) about electricity and water, to try to answer the question: should we really be wary of lightning when we’re swimming?
The short answer is ‘yes’, but there are other considerations.
The biggest fear is a direct strike by lightning. When lightning strikes, it often (but not always) hits the tallest thing in the vicinity. If your head is above water level, it’s potentially the highest point around and therefore a nice target for lightning. Not good.
But think twice before scrambling out of the water onto the nearest boat as that will lift you higher and potentially put you at greater risk. Statistics from the US show that between 2006 and 2015, there were 71 water related deaths from lightning strikes of which 20% were of people in boats and 8% were swimming. This doesn’t prove that you’re safer in the water than on a boat as we don’t know the relative number of people involved in each activity but the bottom line is that you can definitely be killed by lightning while on or in the water.
A direct strike is not the only way lightning can kill you. When lightning strikes land, it dissipates through the surrounding ground. If you are standing nearby, the current may find an easier path through your body – these indirect strikes are a more common cause of death, particularly in livestock. We know that water is a good conductor of electricity, so if lightning strikes water it will generate a current. The question is, if you’re in the water near the lightning strike, will this harm you? I don’t know the answer, but I suspect it will depend on how well the water conducts electricity relative to your body. If your body conducts it less well, it could be bi-passed by the current and you will escape unharmed. However, I wouldn’t want to test this!
This does prompt another question: are fish killed by lightning strikes? I couldn’t find out much about this but what I did see suggests the answer is usually not. The idea is that any current generated by a lightning strike is more likely to spread out across the surface than to travel downwards, so as long as the fish are sufficiently far below the surface, they should be OK.
The next point is that while being in the water might be bad, being next to it could be worse! Going back to the US statistics, of the water-related lightning deaths, 46% were fishing while another 25% were involved in beach activities. Being near water is clearly dangerous. When you leave the water, you are taller and dripping wet, which sounds like a potentially fatal combination. It’s no use running to the nearest tree for shelter either.
Your best bet is to get inside a building as quickly as possible or, failing that, a car, even if you do risk spoiling the seats.
Finally, if there is no suitable shelter on land and your choice is between being in the water or exposed on the beach when lightning strikes, there’s a case to be made for staying in but I hope you never find yourselves having to take such a decision. It could end badly either way.
Image: Lightning strikes Minorca by Markus Trienke