Would you feel lightning strike a house?

09 May 2010

Question

I was woken up last night by the loud sound of thunder and I swear that I felt the house shake. We live on the top floor of a condo complex. Is it possible to feel lightning strike the house?

Dave - Lightning is basically a huge spark. You get about 100,000 Amps of current flowing down through the air. This gives out about a thousand trillion Watts for about 30 millionths of a second, so the total amount of energy released is about 30 mega Joules of energy. That's very roughly similar to a 30-kilogram bomb.

Chris - It depends on what the bomb is made of, doesn't it?

Dave - Yes, but it's similar of order of magnitude. Quite a lot of that energy is going to be a long way up into the sky though. So basically, that energy is equivalent of about a kilogram of, say, TNT going off near your house. Most of that energy goes into heating up the air and it gets very, very hot. When hot air expands, that creates a shockwave of air pushing outwards and that's what you hear as a thunder. If that happens very, very close to you, then you will actually get quite a large overpressure, like a bomb going off, and if a bomb can shake your house, then a lightning strike should be able to.

Chris - I did some back of the envelope calculations. I think it's 120,000 pieces of toast you could make with the energy in one lightning bolt, assuming you could turn all of it into toast, obviously. So that's really quite a lot, isn't it?

Dave - For the time period, certainly, yes.