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The mesosphere is the coldest layer of the earth's atmosphere.
http://www.answers.com/topic/layers-of-the-earth-s-atmosphere-1
The layer of earth's atmosphere that protects against these dangerous UV rays is ozone. It is in atmosphere.
At about 1% of Earth's atmosphere, argon is the 3rd-most plentiful of the atmosphere's elements.
The amount of water in the earth's atmosphere is 1/3000.