Ask a Nerd


Jason of ??? asks:

If you're struck by lightening, you may very well have your clothing and shoes thrown off. What causes that?

The Nerd responds: This is a neat question and is similar to one I asked myself sometime ago, so I can answer your question fairly easily. A couple of years back, a tree close to my house was struck by lightening. When I went to look at it the next morning, I found that the bark had exploded right off of the trunk in some places. The scattered bark was lying on the grass in a debree trail just like you would expect from some sort of explosion. All evidence pointed to an explosion, but why would there be an explosion?

As you can read in my previous entry What causes thunder and lightening, a lightening stroke is really just the closing of a connection between negatively charged clouds and the ground. The current passing through a tree being struck by lightening is, of course, extremely powerful, but that by itself doesn't explain the occurance of an explosion. So what does?

One of the effects of the enormous energy in a lightening bolt is that the electrons ionize the medium through which they pass. The medium (the air, the tree, or whatever) resists this sudden change in state, and this resistence produces friction. A large portion of the energy produced by this effect is released in the form of heat. The power of a lightening bolt is so great that this heat can be tremendous and the change in temperature can be almost instantaneous.

Now, if you have ever boiled a pan full of water with a lid on it, you know that once the water gets boiling, the lid actually starts to wobble up and down. This is because the heat from the stove is turning the water from a liquid to gas. In its gasseous form, water has more volume and thus won't continue to fix inside the pan. Eventually, it actually pushes the lid off the pan and "burps" some of the water vapor free. Using a pressure cooker (in which the lid is actually bolted down), you can actually cause the pot to explode if you do not regularly release the excess steam.

Okay, given enough time, a stove can produce enough heat to knock a lid off a pan full of boiling water. A lightening bolt has much, much more energy. Any moisture found in the ionized path of a lightening bolt will be instantly vaporized. As you can imagine, a tree has quite a bit of moisture in it (especially after standing out in a rainstorm for a few hours). When a tree gets hit by lightening, all the moisture trapped between the bark and the trunk of the tree goes instantly from droplets of water to water vapor. This sudden change of volume actually blows the bark right off the tree. This is what caused the bark debree trail I saw the next morning.

The same effect can be seen for your question. As you can imagine a person's shoes and socks can be full of moisture (sweat). When hit a person is hit by lightening, the bolt actually travels around the outside of a person towards the ground. The moisture it finds along its path, such as the sweat in shoes gets vaporized just like the water droplets in the bark of tree. The result is that the vaporized sweat actually blows the socks and shoes right off.