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How to Offer Wild Birds Shelter in the Winter
Not all birds migrate south for the
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It doesn't take much effort or money to provide shelter for them.
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Air thins on high
![Ultra long duration balloon flies where the air is thin — above 99 % of Earth’s atmosphere. [NASA]](http://www.wonderquest.com/images/2004-02-06-balloon.jpg) Ultra
long duration balloon flies where the air is thin — above 99 % of Earth’s
atmosphere. Photo courtesy of NASA.
Q: Why does the air get thinner at high altitude? Katie, Oregon, Wisconsin
A: Air seems nebulous but is massive. Earth’s gravity pulls air molecules
down, squeezing them together.
Air languishing at Earth’s surface has 200 miles of atmosphere piled above and a
weight of 14.7 pounds (6.67 kg) on each square inch (6 square centimeters). At
18,000 feet high, the weight above halves and the air is twice as thin.
That's why the air gets thinner at higher altitude: less mass above
squeezes it less, so the air is less dense.
(Answered Feb. 6, 2004; updated Sep. 22, 2007)
Click for printer version.
- Silly question, I know... but when my 11 year old brother asked me this I
literally babbled. So why air is gradually less denser upward ?
Thanks for the answer! Sarah, Indonesia
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