Question

Why it is dangerous to stay in an umbrella during thunderstorm

Answer 1

During a thunderstorm, clouds build up enormous electrical charges. If the charge in one cloud is substantially higher than that of an adjacent cloud, electrons will jump from one to the other, to equalize the charges. A significant amount of the lighting in any storm, is that which goes between clouds.

And yes, this only happens when the amount of difference in potential is very high. How high depends on the distance between the clouds, and what is in the air, and some other factors.