2007-09-18, Lightning: 探密-闪电（在线收听） |
| The image is unmistakable. Lightning is one of the most incredible natural phenomenons and one that scientists are still learning about. It's a common occurrence during summer when the heat of the day is broken by strokes of lightning. Worldwide, it's estimated lightning occurs 50 to 100 times a second. The greatest concentrations of lightning strikes are in Central Africa, the Himalayas and South America. Lightning is often seen flashing between storm clouds and the earth. The bursts of light are pure electricity. Scientists don't fully agree on what actually causes the electrical charge to be dispersed. But it is generally thought that lightning often occurs within the downdrafts and updrafts of thunderstorms. Lighter particles moving toward the top of clouds become positively charged, while heavier particles heading toward the bottom become negatively charged. When the positive and negative charges grow large enough, lightning is released between these regions. Most of the lightning takes place within the cloud but some strikes the earth in bold flashes. In these cases, the charge escapes the cloud, making a branching path that reaches for the ground. The energy of the lightning strike contains hundreds of millions of volts and lasts only a fraction of a second. What seems to be a single flash is actually a series of return strokes of electrical energy, reaching back up into the clouds. The path reaches temperatures of around 50,000 degrees Fahrenheit. This extreme heat creates the booming thunderclap as excessive pressure within the lightning path expands at supersonic rates on return strokes. In the US, lightning occurs most often in Florida. Its hot moist climate is perfect for creating thunderclouds which produce lightning. But lightning is a deadly natural phenomenon, taking nearly 100 lives a year on average in the US, more than hurricanes or tornados. During electrical storms, experts warn that people should seek shelter inside a building or hardtop automobiles and if caught in the open, avoid high ground and isolated trees. Lightning is a powerful force of nature and one to be cautious of. |
| 原文地址：http://www.tingroom.com/lesson/guojiadili/57595.html |