Multiple cloud-to-cloud and cloud-to-ground lightning strokes in a night thunderstorm, with time-lapse photography.
Courtesy NOAA

A red sprite on July 4, 1994, reached an altitude of more than 85 kilometers. The bright area below the sprite is normal lightning high in a thunderstorm in the Texas panhandle. Sprites were discovered several years ago; their cause is not understood. A cool 939K slo-mo movie of a sprite.
Courtesy: NASA

The four were not injured seriously, but the event did get us Why Filers to thinking about lightning. Aside from flooding, lightning is the deadliest storm-related hazard in the United States. In the average year, it is even more deadly than tornadoes.

Lightning is certainly a lot more common. New satellite data says there are 1.2 billion lightning flashes per year -- although not all reach Earth.

What is lightning? How does it injure and kill? And what has been learned in the past few years from the millions of dollars spent studying nature's electricity?

Boom-boom room
Thunder -- the cracking or rumbling you often hear -- is caused by thermal expansion and contraction. Lightning bolts can get far hotter than the sun's surface -- up to 20,000 ° Celsius. That heats the air, causing it to expand, and starting a shock wave that departs the area as sound waves -- thunder.

If you're close to the lightning bolt, you'll hear a cracking; but further away, you'll hear rumbling because you are hearing sound that's coming from several parts of the bolt, and reflected from buildings and hills.

And yes, if you start counting "one Mississippi," when you see the flash, you can estimate the distance to the bolt: Light essentially reaches you instantly, but sound takes about five seconds to travel one mile. Divide the number of seconds by five to find miles, or by three to find kilometers.

Silence is -- mysterious
One of the many lightning mysteries is this: Sometimes you hear the thunder, and sometimes you don't. For example, "heat lightning" is an eerily silent flash that often lights clouds in thunderstorms.

What happened to the sound? Credit an audio version of the visual mirages that cause desert travelers to see water in the driest desert. These visual mirages are caused by heat that bends light waves. You look straight ahead, but you actually see the sky, shimmering like a tempting lake.

Anyone for a swim?
Similarly, in a thunderstorm, the sharp boundaries between warm and cool air can channel sound waves away from the observer, as you can see from the nifty applet, below.

Want to see how sound waves get channeled by temperature differences in a storm? See the Java applet in action.

Much the same phenomenon was noticed during the Civil War, when artillery visible in the distance was audible in some parts of the battlefield but not in others.

How does nature make lightning?