seeinglightning

To Do and Notice: Seeing the Lightning

by Paul Doherty 26 June 90 version 5

In an experiment that has become legendary, Benjamin Franklin flew a kite into a thunderstorm. Franklin drew sparks from a key that he had suspended from the kite string and noticed that these sparks were identical to sparks that came from electrostatic experiments that he had performed in his laboratory. He concluded that lightning was a form of electrical discharge&emdash;a giant spark.

You shouldn't try to repeat Franklin's experiments. (The next two men who did were killed by the lightning they attracted.) But you can still experiment with lightning. In this To Do and Notice, we will concentrate on noticing lightning, rather than doing anything to it.

Observers who followed Franklin found out that lightning is a flash of light produced by a spark of moving electric charges. The moving charges rip electrons off air molecules, ionizing the molecules. At the same time, they collide with the air molecules, heating them to 30,000 Kelvins, a temperature five times hotter than the surface of the sun. The hot, ionized air emits light and produces sound &emdash; thunder. The heated air expands as a supersonic shock wave for a couple of meters before it turns into a sound wave which is heard as thunder.

In the spirit of Franklin's research, brave physicists have flown gliders into thunderstorms to measure the distribution of electric charge in the storm. (They wear parachutes in case the storm rips the wings off of their gliders.) These intrepid researchers find that towering cumulonimbus clouds usually have a large negatively charged region near the bottom of the cloud and a positively charged region at the top. Scientists still do not know exactly how the high winds, water vapor, water drops, and ice in a thunderstorm actually produce this charge separation. But they do know that negative charges at the bottom of the cloud push away negative charges on the ground so that a positive charge region follows the cloud like a shadow. Lightning occurs between positive and negative charge regions, either between a cloud and the ground, or between clouds, or within a single cloud.

A lightning bolt is too fast for the human eye to analyze in detail. In 1933, however, B.F.J. Schonland recorded the motion of cloud to ground lightning using a Boys camera. This camera has a lens that moves rapidly around in a circle. The moving lens spreads the images of lightning across the film. (Sir C.V. Boys, who invented this type of camera in 1903, tried to photograph lightning with his camera for thirty years, and never succeeded.)

The Boys camera gave us our first glimpse at what really goes on in the average lightning bolt. It showed that a single flash lasts only a few thousandths of a second. The average stroke of cloud to ground lightning begins as negative charge accumulates near the bottom of a cloud. The charge on the cloud is kept from the opposite charge on the ground below by thousands of feet of insulating air. However, even uncharged air molecules are made of positive and negative charges. The negative electrons of air molecules are repelled by the negative charge region on the cloud while the positive nuclei are attracted. When the charge region at the bottom of the cloud grows large enough, it pushes electrons off air molecules, ionizing those molecules. Schonland's photos show that the air near the cloud ionizes first , along a cylindrical region half as long as a football field and about as thick as your forearm. As negative charge from the cloud moves along the ionized, therefore conducting, tube of air, the air gives off a dim flash of light. After fifty microseconds or so, the ionized channel fills with enough charge to trigger the breakdown of air along a new channel extending from the end of the first channel but with a slightly different direction.

This process repeats itself over and over, creating the zigs and zags of a lightning bolt. Sometimes two channels are created at the same time, and the lightning bolt forks. Each time a new section of air is ionized, it and the entire remaining column emit a flash of light. This step-by-step propagation of ionized air is called a stepped leader. The channel heads toward positive charges, perhaps on the ground. When the channel nears the ground it triggers a breakdown of the air that carries positive charges upward to meet the descending stepped leader about 50 meters above the ground. When these two strokes meet, there is a complete electrical connection, a short circuit, between the cloud and the ground. The stage is set for what we on earth see as the lightning bolt, the bright flash that physicists call the return stroke.

The electrons nearest the positive charges of the ground flow out of the lightning channel first. These flowing electrons cause the air to heat and ionize and so to emit a bright light. As the electrons nearest the ground move toward the ground, the electrons in the channel above also begin to move down. The motion of these higher electrons is also accompanied by emission of a bright light. So, the bright flash of lightning moves from the ground to the cloud, travelling upward at one-tenth the speed of light. The charges however move downward, from the cloud to the ground. (If you are having trouble visualizing this, picture a tube full of dry sand. If you open the bottom of the tube, the sand at the bottom falls down and out, and is followed by the sand above. The place where the sand starts to move progresses up the tube even though the sand is moving down the tube.)

Usually, after the first stroke of lightning, there is a pause of a thirtieth of a second or so, and then another stroke re-ionizes the main channel and triggers a second return stroke. Lightning may be composed of as many as twenty five of these repeated strokes within a two second period.

In the following To Do and Notices you can perform your own observations of lightning.

To Do and Notice: Photographing lightning

Driving across the grasslands north of Lake Okeechobee (sp) in Florida, my wife Ellen and I found ourselves in the middle of a night-time lightning storm and stopped to watch nature put on a show. After watching the amazing shapes of the lightning bolts for half an hour, I decided to photograph them.

To photograph lightning at night you can follow the same steps that I did. First I made sure that I was safe from lightning strikes by remaining inside my car. (Inside a building is also a safe place.) I attached my camera to the half-rolled down car window with a clamp, opened the camera to its widest aperture (f/2.8), set the exposure to B, focussed on infinity, and opened the shutter. (I was using ASA 64 film, but this same recipe works for most films.) I held the shutter open until I saw a lightning bolt, and then closed the shutter and advanced the film. I shot a roll of film and obtained some good shots of lightning. The photos show the twisted forked nature of the lightning channel.

If your camera is an automatic exposure camera without a B setting, you can still use it. Just point it at the storm, push the shutter release, then, both you and the camera will wait for the flash of light to expose the film. Some automatic cameras work better than others, but give yours a try.

Sometimes, you can photograph lightning during the day. Driving across the New Mexico desert, Ellen and I encountered a lightning storm in the afternoon. I could see the lightning flicker so I knew that this storm was producing lightning with multiple strokes.

To photograph multiple stroke lightning during the day, use your camera exactly as you would to take an ordinary picture of the scenery. Aim it at the storm and wait for a lightning bolt. By the time your brain sees the lightning and you shoot a picture, the first lightning bolt will be over and gone. But you may catch one of the later bolts in your camera. These secondary bolts follow only the main channel of the first lightning bolt and so they show very little forking.

At night you can spread the bolts of multiple stroke lightning across your film. Set your camera exposure to B, open the aperture to its widest, focus at infinity, and then slowly pan back and forth, taking at least ten seconds to swing your camera across the storm. After each flash of lightning, close the shutter. The multiple flashes will each be imaged at a different place on the film.

To Do and Notice: Black Lightning

It was on a dark and stormy night in Nebraska that I first saw black lightning. The air was clear; above me, stars were visible. But all around, huge cumulonimbus clouds spit lightning bolts. Bright, jagged bolts hit the ground, and hidden bolts flashed within clouds lighting them up with an internal glow. Shortly after watching a particularly nice forked bolt, I spotted a jagged streak of black against the clouds. I was stunned. I watched closely and sure enough I saw a second bolt of black lightning. As soon as I saw the second bolt of black lightning, I knew the secret of its formation. The second bolt was identical to the first! I watched more carefully and saw it a third time: the black lighting appeared against a bright background flash of light provided by a cloud lit internally by regular lightning.

The black lightning was, in fact, an illusion created inside my eye. Where light from the bolt created an image on my eye's retina, the light-sensitive rods and cones fired nerve impulses. Then, when the cloud flashed with internal light, that light also made an image on my retina, firing the rods and cones. But the rods and cones that had recently fired along the path of the image of the bolt fired less rapidly this second time, creating a dark shape on my retina matching the shape of the original white bolt &emdash; black lightning. The black lightning was a negative after-image of the initial bright flash of a lightning bolt. I spent the next hour watching for, and finding more black lightning.

If you can find a lightning storm that has both bolts and flashing clouds, you too can see black lightning. Just watch the sky until you see a bright bolt of lightning. Then watch for the flash of cloud.

If you can't wait for a lightning storm, draw a white lightning bolt on a black background. Under bright lights, stare at the bolt for 15 seconds. Then look at a white surface, and you should see the afterimage of a black lightning bolt.