Lightning Returns, with a Flash and a Bang

As I watched a movie with friends recently, three dogs in the room suddenly began to act out their roles as house guardians by running to the windows and barking at a distant rumbling. We humans were puzzled until flashes in the cloudy night sky soon identified the intruder, and we walked out on the deck to enjoy a rare May thunderstorm.

As often happens when natural phenomena occur, questions were thrown at the nearest science writer: "what causes lightning and thunder?" and, "doesn't lightning strike from the ground up?"

Scrambling for the textbook Meteorology Today, I relearned that lightning is an electrical discharge identical in everything but size to the spark that jumps from your hand to doorknob in a carpeted room.

The lightning that most impresses us---that which strikes the ground---represents only about 20 percent of the lightning generated within a cloud. Most lightning strikes occur within a single cloud, while others reach from one cloud to another or from a cloud to the surrounding air.

Alaska thunderstorm clouds, which form most frequently in the Interior, are born of the sun, which heats the ground to cause an updraft of moist air. Water vapor carried on this elevator of warm air condenses to form clouds as the temperature cools with elevation. If the air at about 10,000 feet also is moist, and temperatures decrease rapidly in the air above, the tiny cumulus cloud we see forming as a flat-bottomed head of cauliflower can grow to heights of 40,000 feet. At that level, where the stratosphere begins, stable air causes the ice crystals within the cloud to flatten out on top like an anvil.

How lightning is generated is still one of the wonderful mysteries of science. Separate regions within the columnar, anvil-topped cloud, now called a cumulonimbus, contain opposite electrical charges. Scientists theorize these charges may be formed when hailstones collide with colder ice crystals, leaving the hailstones negatively charged and the ice crystals positively charged. Hailstones fall to the bottom of the cloud, which becomes negatively charged.

The cloud causes the ground surface below it to be positively charged. Protruding objects such as trees, buildings and humans in open fields often build up a more dense positive charge than flat ground, setting up an electric potential for current to flow through.

The lightning strike we see is actually a series of steps too quick for our eyes to distinguish. Charged electrons jump from the bottom of a cloud in an invisible line about 150 to 300 feet long. After a pause of about 50 millionths of a second, the electrical charge continues on a crooked path toward the ground, at which time a current of positive charge starts upward (usually from tall objects) to meet it.

When the path is complete, large amounts of electrons flow to the ground. This pathway triggers a luminous current flow from the ground to the cloud, which we see as a sharp vein of lightning. If our brains processed information fast enough to detail that one ten-thousandth of a second, we could see lightning strike upward.

Lightning heats the air through which it passes to 54,000 degrees F, which causes a dramatic expansion of air. This produces a shock wave, which becomes a sound wave we hear as thunder.

Thunder's noise lags behind lightning because light travels much faster than sound. Because of this, you can determine how far away lightning struck by counting the number of seconds it takes for the thunder to arrive. It takes about five seconds for the sound of thunder to travel a mile, so if you count to fifteen between sighting lightning and the sound of thunder, the lightning strike occurred at a safe distance, about three miles away. But the dogs still won't believe you when you say it's OK.