One of my colleages sent round a circular warning people not to carry umbrellas in thunderstorms. I suspected that, especially in the part of Central London where we work (tallish buildings everywhere), it was unlikely to make any difference. I was right:
http://wvlightning.com/lmwn3.html
The rest of the site is a very interesting read, but it's a bit disturbing to learn that there's pretty much nothing you can do to reduce your chances of getting struck if you're outdoors during a thunderstorm!
http://wvlightning.com/lfaq.html
Rich :¬)