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You’re at home, sitting on the couch. Outside, there is thunder and lightning. You notice something at the window: a strange, glowing ball of light. As you watch, it appears to pass through the glass. It wanders through the air before abruptly disappearing.

What you’ve seen isn’t magic, it’s a puzzling phenomenon called ball lightning. Rare and mysterious, witnesses often describe it as a ball of light, about the size of a grapefruit that moves slowly through the air before disappearing. It is often seen during thunderstorms, but not always.

Ball lightning is so rare, scientists don’t know what it actually is, and some people doubt if it exists at all. Possible causes proposed by scientists include microwave radiation, nuclear energy or reactions caused by more typical lightning. Now researchers from CSIRO and the Australian National University have used maths to come up a new explanation.

They propose that certain conditions in the atmosphere cause charged particles (or ‘ions’) to clump together on a surface like the outside of a glass window. These negatively charged ions attract positive ions, which group together on the other side of the glass.

Having too much of the same charge in one place is unstable. A more stable situation is to have the charges balanced, which can happen by moving charges. This is called a ‘discharge’, and often takes the form of a spark.

In this case, it is proposed that the positive charge on the inside of the glass discharges by removing electrons from gas molecules, creating a mixture of electrons and ionised gas molecules. Under certain conditions, the discharge would give off light. Instead of a short, sharp spark, the discharge could form a glowing, moving ball: ball lightning.

This is the first time that a simple mathematical formula has been used to explain ball lightning. However, it doesn’t necessarily mean that the mystery is solved. Scientists will continue to debate this subject in the future. So even though the numbers might add up, ball lightning may remain a mystery for a little bit longer.