Unit 4.13, Exercise 5, Page 128.0:01
The clouds you see in the sky are by no means just pretty decorations.0:25
They play a vital part in maintaining life on Earth and are responsible for some of the most terrifying acts of natural destruction.0:30
Clouds form for the same reason that mist appears on a cold metal spoon.0:38
When you breathe on it, tiny droplets of water form where the air cools.0:43
Because cold air cannot hold as much water vapor as warm air in the Earth's water cycle, the heat from the sun beating down on the oceans creates currents of warm air loaded with invisible water vapor.0:47
As the air rises, it also cools, and so the vapor forms millions of droplets around pieces of atmospheric dust.1:00
When droplets join together, they eventually become heavy enough to fall back to the ground as life giving rain.1:08
Clouds may look fragile, but they are actually one of nature's giants.1:15
A typical cloud holds millions of tons of water.1:20
Disastrous flooding can result when huge clouds release their load.1:23
One of their most dangerous capabilities is the thunderstorm.1:28
If you have ever rubbed your bare feet across a carpet and then touched a metal door handle, you may have felt an electric shock.1:31
Lightning occurs in much the same way, but on a gigantic scale.1:38
As water particles rub against each other, a negative charge builds up at the base of the storm cloud.1:42
When enough charge builds up, it is released in a dazzling flash of electricity connecting the cloud to the ground.1:49
The energy released in a thunderstorm would be enough to power every home in the USA for a whole day.1:56
Attempts have recently been made to bring these giants under our control using a technology called cloud seeding.2:03
Aeroplanes spray clouds with a substance that encourages the droplets to combine and fall as rain.2:10
Although the results are still under debate, countries including Australia, China and the USA are all investing in research.2:16
So maybe one day we will be able to have a party and order not only the food and drink, but clear skies as well.2:24
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