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When you are frolicking at the shore and a thunderstorm is heard in the distance most sane people pay heed an leave the scene for safer quarters. Most people know that lightning is more likely to strike a person standing in water that if they were in a secure building. Because they are well grounded by the water and present a "high" object compared to the flat surface of the water, the lightning strike potential increases.
Divers do not always get the same warning. They may enter the water with the sun blazing and no hint of a thunderstorm in the area. After 45 minutes of diving the conditions may change without the diver being aware of it. So, what should you do if you are underwater and a thunderstorm rolls in? Should you come to the surface? And, what would happen if lightning struck the lake with divers in the water? Would they experience the "hot squat"?
When a diver is suspended in water they are not a target. They do not present a direct link to the Earth as they would if they stuck their head out of the water and lightning struck that. If there was a danger of electrocution then many fish would be killed each time lightning strikes a body of water. Now, if the diver was close to the surface, let's say no more than 5' underwater, and lightning struck immediately above, then there is the possibility the diver (and the surrounding area) would become instantly charged and then immediately discharged which could become lethal. The chance of that happening would be less than having lightning travel down a bathroom pipe and hit someone sitting on the toilet.
Once while diving in a lake a thunderstorm arrived overhead. When lightning occurred the light was noticeable underwater to the divers that were 20' down. Ascending toward the surface the bullet-like raindrops pounding into the lake's surface were quite visable. Needless to say we did not surface. Instead we used our compasses to go to the nearest shore and then traveled along to the exit point. If you have the air to remain underwater it would be better than swimming on the surface. But, swimming along the shore is judged to be safer than swimming on the surface in the middle of the lake.
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