Lightning Dynamics and St. Elmo’s Fire

As an old physics teacher, I am interested in lightning dynamics. Most of us have seen the effects of lightning and have been impressed with its power and destructive potential.

The question is, what produces lightning and its incredible power? On Earth, lightning is caused by water. If you have a steady stream of water from a faucet and bring a charged glass or plastic rod near it, the water stream will move toward the rod. The water molecule is polarized, meaning it has a positive and negative end. The charged rod attracts the oppositely charged end of the water molecule.

When warm moist air rises to form a thunderhead, it can reach velocities of 100 mph. The air cools and forms ice crystals that can become positively charged by collisions with other ice crystals. That means the lower parts of the storm cloud will acquire a negative charge. Conversely, the very bottom of the cloud will generate a positive charge as a result of the negative sections of the cloud. When this charge imbalance becomes great enough, an inch-wide stream of electrons travels between the negative and positive areas. This can be between the two layers of the cloud, the negative midsection and positive bottom of the cloud, or the ground and the positive lower parts of the cloud. This stream of electrons typically moves at 200,000 mph.

Lightning dynamics can heat the air around the stream of electrons to 50,000 degrees Fahrenheit, turning oxygen into ozone and sending out a shock wave which we hear as thunder. A tall object like a tree, a church steeple, or a lightning rod will accumulate a charge because it is closest to the charged cloud. Sometimes charges around the tall object will cause the air to glow in what is called “St. Elmo’s Fire.” That name is credited to a monk who first recorded seeing it at the top of his church and thought he was having a vision.

Lightning dynamics are very complicated and not a threat to humans unless we are careless. However, lightning has various benefits, such as producing nitrogen fixation so that plants can grow and resupplying the ozone layer, protecting us from some of the dangers coming to Earth from outer space.

— John N. Clayton © 2022

Reference: “Seconds count when lightning is spotted” in The Herald Bulletin, Anderson, Indiana, August 13, 2022, page C3.