Horses in Storms

I have presented this same question to several groups so I aplogize if you receive the same question more then once. I think it's important.

If there is a storm occuring with heavy rain, winds and lightening, Is a horse safer inside the barn in a stall or outside in a flat pasture with no trees?

Do you have any events that you know of or have heard about to share on this subject?

Thanks for any comments.