Thunderstorm weather is not

A, thunderstorm weather can't shelter from the rain under the big tree, because the tree is relatively high, there is water on the tree, and the tree becomes a conductor, which is easy to lead the lightning down. People who shelter from the rain under the tree may get an electric shock. This option is incorrect;

B, playing football on the playground in the rain, the playground is flat, people are easy to fall, this option is incorrect;

C, mobile phone electromagnetic wave is a good conductor of lightning, electromagnetic waves in the humid atmosphere will form a conductive magnetic field, which is very easy to attract new lightning strikes. This option is correct;

D, metal bar is a conductor, lightning will be introduced into the human body through the metal bar, causing electric shock. This option is incorrect.

So choose C.