Who, what, why: What happens when lightning hits the sea?

A man died and several other people were injured in a thunderstorm off the coast of California. What happens when lightning hits the sea, asks Justin Parkinson.

If you are in the sea and a thunderstorm looks likely in the area, there are two ways to cut the risk of getting hit - get out and find some shelter, or swim deeper.

A typical lightning flash measures about 300 million volts and 30,000 amps, according to the US National Weather Service - enough to kill.

Most of the electrical discharge spreads horizontally rather than vertically. This is bad news for people, who tend to float or swim on or near the surface.

The lightning current is likely to radiate across the surface. Various different estimates have been given for the distance over which it would dissipate to the point where it would not be a harmful to a person.

"I wouldn't recommend betting your life on that kind of calculation," says Giles Sparrow, author of Physics in Minutes. "If you get out of the water and can't find shelter, it's best to crouch into a ball, rather than lay flat on the floor, as this also raises risks. If you stay in the water, you could try to go deep, but it's unlikely you can hold your breath for long enough to avoid the danger."

Fish, which usually move around at greater depths, are safer than human swimmers. Protruding heads or even entire bodies, such as those presented by surfers or paddle boarders, could put people in greater danger.

"If you are in the open sea, rather like standing in an open field, you might become a target during a storm," says Jon Shonk, a meteorologist at the University of Reading. "Lightning takes the path of least resistance."

Boats can be fitted with lightning conductors, which direct the charge into the sea, while avoiding their most vulnerable parts, such as passenger areas or equipment rooms. It is recommended that these are fitted.

Research by Nasa shows lightning is more likely to hit land than sea and that it is rare for strikes to occur in deep ocean areas. Waters just off coasts are more often affected.

Risks also vary according to seasons. "You expect more strikes nearer to land because that's where the most heat and updrafts and storms build up, especially in the summer," says Shonk. "That can change in winter, but that's obviously a time when there are fewer people in the sea."

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