HOW COME? The shocking noisy effects of lighting

But what causes the sound? And why does the rumble lag behind the flash, sometimes by many seconds?

Lightning heralds its arrival in crashes and booms by superheating the air it travels through. Think of a lightning bolt as a giant spark. Imagine the tiny spark you sometimes see (and, ouch, feel) when you touch a doorknob in a dry room. Just as the spark connected your finger and the knob, so can a lightning bolt spark between two clouds or a cloud and the ground.

A lightning bolt is an electric current zigzagging through the air, connecting positive and negative charges within clouds or between clouds and the Earth. A typical household current is 20 amps; the current in a lightning bolt can average 30,000 amps.

When sparks flash across a staticky, rustling blanket in winter, you may hear little crackles. Likewise, when lightning strikes down the road, a crack of thunder may rattle your windows.

How come? Temperatures in the air channel carved out by a lightning bolt can reach more than 50,000 degrees, much hotter than the sun's surface. In a fraction of a second, the air around the channel superheats, exploding in a shock wave. Traveling faster than the speed of sound, the shock wave decays into an ordinary sound wave within a few feet. When the sound wave reaches your ears, you hear the explosive sound.

Why the lag between seeing lightning and hearing thunder? Ordinary sound waves travel through air at about 1,100 feet a second. But light zips along at about 982 million feet a second. So we see the lightning before we hear and feel it. The less time between the flash and its boom, the closer the lightning strike.

We expect to hear thunder's rumble in strong summer storms. But why don't most snowstorms flash and boom? Weather scientists say "thundersnow" is rare, requiring warm, moist air near the ground rising into very cold air above.

In one scenario, a late winter/early spring thunderstorm in the southern United States moves north into colder regions, where the heavy rain changes to snow. Thundersnow researcher Patrick Market of the University of Missouri notes winter lightning is usually accompanied by heavy, fast-falling snow. Since snowflakes dampen sound and scatter light more than raindrops, the thundersnow event will probably be muted. You may see brightening clouds above the falling snow and hear muffled thunder. Market says what's going on in snow clouds to make lightning is a mystery.