What causes lightning?

What causes lightning? Lanney, Sandia Park, New Mexico

Within the maelstrom of a thunderstorm, lightning is born. Ice and supercooled water are the keys to the process, says E. Philip Krider, professor of atmospheric sciences at the University of Arizona. Violent winds buffet tiny hailstones as they form, causing them to collide. When the hailstones hit ice crystals, some negative ions transfer from one particle to another. The smaller particles lose negative ions and become positive and the larger more massive particles gain negative ions and become negative.

Lightning discharges an excess of positive and negative charge within clouds, between clouds, or between clouds and the ground. The drawing shows lightning striking between two clouds and also between clouds and the ground.  Drawing adapted from lightning article by Ron Hipschman.

"Ions (both positive and negative) have a lower mobility and a longer lifetime than free electrons," notes Krider. So, the charges on the ice particles tend to persist.

"The strong updrafts within thunderstorms carry the smaller positively charged ice particles to the upper regions of the cloud while the larger, heavier negatively charged (hail) particles collect in the lower and middle portion of the cloud,"says atmospheric physicist Steve Goodman of NASA’s Global Hydrology and Climate Center (GHCC) in Huntsville, Alabama.

This creates charge differences, like that between the terminals of a car battery. When the potential difference between the regions gets too great, lightning flashes, heating the discharge channel to sun-surface temperatures. The air expands explosively, forms a shock wave, and thunder cracks — nature’s sonic boom.

Kids' version:  what causes lightning?

Within the upheaval of a thunderstorm, lightning is born.  Ice and supercooled water are the keys.  Strong winds in the thunderstorm slam tiny hailstones together.  When the wildly careening hailstones hit ice crystals, some negative charges hop from one bit of ice to another.  As the ice bits hit each other, the little guys lose negative charge and become pluses.  The big guys gain negative charge and become minuses.  The charges on the hailstones last.

The thundercloud updrafts whisk the little, light guys (the pluses) up to the top of the cloud, and gravity tugs the big guys (the minuses) down to the middle and the bottom of the cloud. 

Aha!  Now we've got the charges separated, like the plus and the negative terminals of a car battery.  When the difference between the minuses on the bottom of the cloud and the pluses on the ground gets too big for the air to withstand, the charge surges from the pluses to the minuses.  Lightning flashes, and heats the air to as hot as the Sun.  The air explodes out, and thunder cracks.

Further Reading

Lightning by Ron Hipschman

Benjamin Franklin and the first lightning conductors by E. Philip Krider

(Answered Dec. 13, 2005; updated Aug. 20, 2007)

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