This is Scientific American 60 Second Science.

I'm Julia Rosen.

When lightning sparks across the sky, it sends out low frequency radio waves that

researchers can use to determine its location. Recently, scientists were looking

over a map of lightning activity when they noticed something strange.

Narrow lines of increased lightning frequency

stretching across the Indian Ocean and the South China Sea.

Straight features like these are rare on a spherical spinning planet

except where humans are up to something. In this case

sending ships across the sea. So I had pretty immediately in my mind postulated that

this was a result of the pollution from ship exhaust influencing the way the storms develop.

Joel Thornton, an atmospheric chemist at the University of Washington.

That hypothesis had already been put out generally in the field more than a decade ago.

But we had been lacking sort of clear evidence that this was happening on a sort of large scale,

long time horizon.

The new maps changed that. Thornton and his team found that on

average lightning activity almost doubled over shipping lanes compared to

adjacent areas of the ocean. The researchers ruled out natural explanations for the pattern and concluded it must be related to ship exhaust.

Exhaust increases the number of tiny particles in the air on which water vapor condenses to form cloud droplets.

Spreading the same amount of water over more particles leads to smaller droplets,

which get lifted by updrafts until they reach altitudes cold enough for them to freeze.

For reasons scientists still don't fully understand,

...