This claim, that you should keep your legs together in a thunderstorm, isn't merely sage advice aimed at the inhabitants of Essex. I think it stems from farmers finding livestock dead in fields following storms. The animals chiefly affected tended to have had longer bodies, such as cows and horses.

The theory goes that when lightning strikes the ground the current flows away from the strike site across the surface down an electrical gradient. You can think of this as rather like a ball rolling down a hill where the ball is the current and the hill is the electrical gradient or "potential difference".

If you are standing on that metaphorical hill, with one leg higher up the hill than the other, then one leg is at a higher electrical potential than the other. This potential difference causes a current to flow through your body. If it's sufficiently large, it can kill you.

But if you stand with your legs together, both legs are at the same electrical potential and so very little current will flow. Cows don't know this, and also find it anatomically very challenging, so they turn into toast...or should that be roast beef?

(Do note, however, that if the lightning hits you directly, unleashing the full force of its 1-10 billion joule strike at a current of about 30,000 amps, the position your legs are in will be the least of your worries! This situation is usually universally fatal).

Chris