2007-09-18, Lightning: 探密-闪电

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 fully¹ agree on what actually causes the electrical charge to be dispersed². But it is generally thought that lightning often occurs within the downdrafts and updrafts of thunderstorms. Lighter³ particles moving toward the top of clouds become positively⁴ 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 volts⁵ 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 Fahrenheit⁶. 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 tornados⁷. During electrical storms, experts warn that people should seek shelter inside a building or hardtop automobiles⁸ and if caught in the open, avoid high ground and isolated⁹ trees. Lightning is a powerful force of nature and one to be cautious of.

1 fully   

adv.完全地，全部地，彻底地；充分地

参考例句：

• The doctor asked me to breathe in,then to breathe out fully.医生让我先吸气，然后全部呼出。
• They soon became fully integrated into the local community.他们很快就完全融入了当地人的圈子。

2 dispersed   

adj. 被驱散的, 被分散的, 散布的

参考例句：

• The clouds dispersed themselves. 云散了。
• After school the children dispersed to their homes. 放学后，孩子们四散回家了。

3 lighter   

n.打火机，点火器；驳船；v.用驳船运送；light的比较级

参考例句：

• The portrait was touched up so as to make it lighter.这张画经过润色，色调明朗了一些。
• The lighter works off the car battery.引燃器利用汽车蓄电池打火。

4 positively   

adv.明确地，断然，坚决地；实在，确实

参考例句：

• She was positively glowing with happiness.她满脸幸福。
• The weather was positively poisonous.这天气着实讨厌。

5 volts   

n.（电压单位）伏特( volt的名词复数 )

参考例句：

• The floating potential, Vf is usually only a few volts below ground. 浮置电势Vf通常只低于接地电位几伏。 来自辞典例句
• If gamma particles are present, potential differences of several thousand volts can be generated. 如果存在γ粒子，可能产生几千伏的电位差。 来自辞典例句

6 Fahrenheit   

n./adj.华氏温度；华氏温度计(的)

参考例句：

• He was asked for the boiling point of water in Fahrenheit.他被问到水的沸点是华氏多少度。
• The thermometer reads 80 degrees Fahrenheit.寒暑表指出华氏80度。

7 tornados   

n.龙卷风，旋风( tornado的名词复数 )

参考例句：

• And the national weather service reports several tornados touch down. 国家气象中心报告预测龙卷风将来袭。 来自互联网
• They had stock footage of lightning, tornados, and hurricanes. 他们存有关于闪电、龙卷风和飓风的电影胶片。 来自互联网

8 automobiles   

n.汽车( automobile的名词复数 )

参考例句：

• When automobiles become popular,the use of the horse and buggy passed away. 汽车普及后，就不再使用马和马车了。 来自《简明英汉词典》
• Automobiles speed in an endless stream along the boulevard. 宽阔的林荫道上，汽车川流不息。 来自《现代汉英综合大词典》

9 isolated   

adj.与世隔绝的

参考例句：

• His bad behaviour was just an isolated incident. 他的不良行为只是个别事件。
• Patients with the disease should be isolated. 这种病的患者应予以隔离。