The Boy with the U. S. Weather Men Part 37

"One of the reasons that buildings and trees are struck by lightning is because they project up into the air, and according to their height, they remove a corresponding amount of the poorly conducting air. If the lower edge of a thunder-cloud is two thousand five hundred feet above the air, and the spire of a church is five hundred feet high, it follows that it is easier for a flash to jump two thousand feet than two thousand five hundred. So when the electricity-bearing cloud comes over the church spire the flash will leap to the church, five hundred feet of obstacle being removed. The highest building, therefore, is usually struck first, or the highest tree in a forest.

"A lightning-rod or conductor is the best preventive against the destruction of a building by lightning, if the rod sticks up in the air above the building, even a couple of feet. The current will more readily strike the lightning-rod. As these are made of metal--copper or iron, generally--which are extremely good conductors, the current flows through them to the ground without harming the building.

"The big lightning flashes that you see, boys, aren't always a single flash, but often a whole series of flashes, which occasionally run up as well as down. The resistance of the air being broken down, makes a path for the electrical discharge, so that the conductor does not have to stand the entire strain of the cloud at once, but only in a series of discharges. Photographs of lightning flashes show these very clearly."

"I've never done any lightning photography," said Ralph disgustedly, "I'd never thought of it."

"You try it," said the Forecaster, "and you'll find that there are no two flashes of lightning that look alike. Some of them are several miles long. One thing you will notice at once, Ralph, and that is that lightning is never zigzag, the way you see it in pictures, but runs in an irregular line, winding a little like a river-course."

"How about sheet-lightning?" asked Ralph.

"That's just the same as any other kind of lightning," was the reply, "except that it doesn't come to the earth or is so distant that the earth flash is not visible. It is generally due to discharges between upper and lower clouds, and the lower clouds are illuminated by the lightning. Heat-lightning, as it is called, is pretty much the same thing."

"Father told me once," said Fred, "that during a thunder-storm, a ball of fire came down on the chimney and rolled all around the room like a bubble of quicksilver and then struck a shovel that was standing in the corner, when it blew up with a bang. What was that, Mr. Levin?"

"That's globe, or ball lightning," was the reply. "There have been some very curious freaks done with these electric b.a.l.l.s. One of them, in a baker's shop at Paris, jumped into an open oven door and exploded, giving off so much heat that a pan of biscuits was baked in the fraction of a second. At least, so Flammarion tells the story, though it sounds a bit queer."

"But what's the cause of ball-lightning?"

"We don't know," answered the Forecaster, simply.

"A couple of days before the Galveston hurricane," put in the young observer, "I noticed two or three examples of St. Elmo's fires, and even had them from my fingers."

"What are St. Elmo's fires?" queried Fred.

"Corpse candles, they used to be called," the young observer answered, "or St. John's fires. They are brush-like discharges of electricity, being discharged from the earth towards the sky, and generally gather on elevated points, such as the masts of ships, the tips of trees or the iron railings around a roof. It was on the top of the Weather Bureau building in Galveston that I saw them, just the other day. They look like a bluish flame, and give a crackling sound. I had my hand on the rail and was reaching up with the other hand towards the anemometer when I noticed from my third and little fingers two blue flames burning. It looked exactly as if my hand were alight."

"Weren't you afraid of being killed?" the boy asked.

"No," said the observer, "that's not the way that one gets killed with lightning. The St. Elmo's fire is a very weak electric discharge. My fingers tingled a little, that was all."

"But do many people get killed with lightning?" queried Ross. "I thought that it was really quite rare."

"Not as rare as you would think," the Forecaster answered. "About five hundred people are killed by lightning every year in the United States and there is an annual property loss of eight million dollars."

"Is that high as compared with other countries?" Anton asked.

"Yes," the Forecaster replied, "more people are killed by lightning in the Western States than in any place in the world. In the Dakotas, out of every million deaths twenty-seven are due to lightning; in Missouri, twenty-one. In Hungary sixteen out of every million deaths are due to lightning; in the United States as a whole, ten; in Germany, six; in England, four; in France and Sweden, three, and in Belgium, two. The greatest number of deaths by lightning are on the plains, the fewest in the cities."

"I should think lightning would be much worse in the city," said Ross, "because if a building is struck with a lot of people in it, they'd all be killed."

The Forecaster shook his head.

"Not at all," he said. "Last year, for example, a church was struck by lightning on a Sunday morning, during a religious service. There were three hundred people in the building. It was a bolt of unusual force, which practically wrecked the church. Only six people were killed by lightning, thirty were injured from the falling timbers, seventy were made unconscious by shock, and two hundred were absolutely uninjured.

"The largest number of persons killed by lightning at any one time in America was in an amus.e.m.e.nt park in Chicago. Eleven people had huddled into a zinc-lined hut under a pier, for protection from the rain. The lightning struck the pier and jumped to the hut. If the hut had touched the wet sand, none of them would have been hurt, but the hut was on posts a couple of inches above the beach. The lightning could not escape to the ground and it spread from the zinc sides, killing every one there. A piece of wire a sixteenth of an inch thick and six inches long, running from the hut into the ground, might have saved every life."

In the distance a flash of lightning followed by a low rumble of thunder told of the nearer approach of the storm.

The Galveston observer took his watch from his pocket and counted the seconds between the flash and the thunder.

"Fifty seconds!" he continued. "The front of the storm is still ten miles away."

"Do you reckon five seconds to a mile between the lightning and the thunder?" asked Anton.

"Yes," the observer replied, "light travels so fast that for something as near as a lightning flash, you can reckon it as instantaneous, while sound only travels at a little more than a thousand feet a second."

"But why does thunder make a noise?" asked Fred. "You told me the clouds didn't bang together."

"They don't," the Forecaster answered. "Thunder is caused by the electric discharge. You've heard Bob's big wireless outfit crackle, when he sends out a spark, haven't you?"

"Sure," said Fred, "you can hardly hear yourself talk, when Bob's got his wireless busy."

"And why does that crackle? Do you know, Bob?" he asked, turning to the wireless expert.

"No," answered the boy.

"You've often heard the crackling of a near-by thunder compared to an irregular volley of rifles, haven't you?"

"Yes."

"Naturally, because that's exactly what it is. A rifle shot is an explosion caused by the firing of a powder, which, in turn, means the expansion of the powder into gases, the force of that expansion driving forward the bullet. Sound, as you know, is a series of air vibrations.

The explosion wave sets up a series of these vibrations, by compressing the air in front of it.

"Lightning does the same thing. When a lightning flash breaks down the resistance of the air, and pa.s.ses through a channel of air, it heats the air suddenly to a temperature of two or three thousand degrees, causing a terrific expansion along the entire length of the flash and starting an explosion wave. This compresses the air on all sides and sets sound vibrations in action. As soon as the flash is discharged, the air rushes back to fill the partial vacuum that the heating by electricity has caused, adding force to the vibrations.

"That's why you hear the crackle of near-by thunder. You are near enough to hear the explosions made by all the little side-branches of the lightning flash--you can hear the same sometimes when you comb your hair or rub a cat's fur--while the big crashes are due to your hearing, all at once, the main wave of sound set in action by the flash jumping from the cloud to the earth or from one cloud to another.

"The rumble of the thunder--which used to be thought the rolling of a chariot in the sky, is due to the different distances of various portions of the discharge, to the echo of the explosions from the projecting hills and valleys of the cloud forms, and to the irregular shape of the earth, when the sound waves strike the ground."

"Hail is electric, too, isn't it?" said Anton. "In a hail-storm the other day I noticed that the hail jumped up a lot higher from an old piece of iron that lay on the ground than from a stone right beside it.

I tried the iron and the stone with a marble, after the storm was over, and the marble bounced higher from the stone. I figured that there must be some kind of electric repulsion and that the hail must be electrified."

"It is, very often," the Forecaster answered. "In some very violent electric storms, you'll see hail jump up as if it were alive, when it strikes the earth. Of course, boys, there's some slight elasticity in a hail-stone, too, because a good many of them are made like an onion or a pearl, with a number of layers round each other."

"But why in the world should a hail-stone be made like an onion?" said Fred, with a puzzled stare. "Isn't hail just frozen rain?"

"No," answered the Forecaster, "frozen rain is sleet, which is never seen in summer. It is caused by the rain in the upper air falling through a cold layer of surface air and becoming frozen on the way.

Sleet is ice, and transparent.

"Hail never falls in winter, only in summer, and almost always in connection with a thunderstorm. It is made by drops of moisture, like very fine rain, being carried by the strong upward currents of a thunderstorm to alt.i.tudes where the air is very cold, there becoming coated with a layer of snow, and becoming heavier, falling through the less active upward currents on the edge of a storm. As these snow-covered frozen raindrops fall through the clouds, they grow bigger, because on their cold snow surfaces the moisture condenses and is frozen to a skin of ice. At the base of the cloud, they are often sucked in by the upward current and carried up again for another layer of snow, falling again through the clouds and being covered with another skin of ice. This may happen a dozen or a hundred times, the hailstones growing in size with every successive layer of snow and ice, until at last they become so heavy that they can no longer be carried up by the ascending currents, and fall to the ground."

"No wonder hailstones sometimes get so big!" exclaimed Fred. "I've seen them as big as pigeon's eggs. I never could understand it."

"I've seen hailstones that weighed more than half a pound," the Forecaster answered. "Not so very long ago, two ranchers and six hundred head of cattle were killed by hail in one Texas storm. Not a single animal was left alive. The loss from hail in our Western states is so large that most of the progressive farmers pay heavy hail insurance.

Jagged bits of hail the size of a child's fist are not at all uncommon.

If I'm not mistaken," he continued, "we may have some hail this afternoon, but nothing like that. This county isn't in the regular hail-belt."