How St. Elmo's fire works

These are the lights that can be seen on sharp peaks — ship masts, towers, high rocks. In the Middle Ages they were often seen on the spire of St. Elmo’s cathedral in Germany — providing the name. In this new lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what St. Elmo's fire is and how it works.

Let’s take it as an example and see how the glow appears

The lights are electric charges between the spire and dust particles in the air. The spires have to be positively charged and the air around them — negatively, or vice versa.

Let’s recall that:

If an atom lacks electrons it’s positively charged (+) and ready to take electrons ‘upon itself’.
If an atom has at least one extra electron it’s negatively charged (-) and ready to give electrons ‘from itself’.

Why are spires charged?

Before thunderstorm strong air currents cause the collision of dust particles with spires. Upon collision, electrons are passed from the particles to the spires and vice versa.

As a result, a spire can get extra electrons — then it’s negatively charged. If electrons ‘jump’ from the spire to the dust particles, the spire is positively charged.

One more condition is necessary for the electric charges — the air has to carry current. It can happen in two cases:

When air is highly ionized, i.e. rich in ions. That’s what happens due to strong wind when dust collides with air molecules causing friction.
Very humid air. Water has a high conductivity.

The glow

How does the electric charge pass? The electron that broke away from the spire collides with another object’s atom. The atom’s energy changes — it drops. The glow comes from the energy difference. The more collisions, the brighter it is.