What causes thunder?

Theories seeking to explain the cause of thunder have been revised many times since the days of the Greek philosophers who believed that thunder was caused by clouds bumping into one another. In more recent times it was almost universally believed _by scientists that the noise of thunder is produced by the sudden inrush of air into the partial vacuum created by the. expansion of the air when heated by lightning. This theory has been largely abandoned. The United States Weather Bureau is of the opinion that thunder results directly from the explosive-like expansion of the air along the lightning path, incident to the intense heating, ionization and probably the disintegration or rupture of at least part of the air particles or molecules. According to this theory, when a streak of lightning rushes through the atmosphere it decomposes particles of the gases. Each of the parts of the shattered particles takes up about as much space as the original particle did, and the result is a sudden and terrific expansion that produces an explosion similar to that caused by dynamite. The intensity of the sound produced by lightning.depends chiefly on the [ 397 ] magnitude of the flash and-the proximity of the observer. The long dura! tion of thunder is due largely to the fact that different parts of a streak of lightning are at different distances from the person who hears it. Sound travels at the rate of about a mile in five seconds and thunder may be heard at a distance of ten miles or even twenty or thirty miles under favorable conditions. The majestic roll that often makes thunder so impressive is caused by echoes that are reflected from the surfaces of clouds or back and forth between the clouds and the earth. Sometimes the roll may be due to the twisting of the sound waves as they pass through layers of atmosphere of different temperature.-Lightning is an electrical phenomenon about which there is still much to be learned. For instance, occasionally lightning strikes from a clear sky without producing thunder, and “like lightning out of a clear sky” is a proverbial expression. There is a similarity in the cause of a peal of thunder and the report of a gun. When powder is ignited it turns into gas, which takes up more room than the powder did. Consequently the expansion of the gas drives the bullet out of the barrel of the gun. When the gas suddenly escapes it sets up a. sound wave in the air and causes the report. It is not the rushing together of the air that causes the sound. The report of a gun is not so loud wheij the gun is discharged in dry air as it is when it is discharged in moisture-laden air.