Tuesday, May 3, 2011

Weather: How Much Air is Pushing on You?

The air all around you is filled with molecules, all of which exert pressure on you. 

Picture this...
You're standing upright.  Rising straight up from your head, into the furthest reaches of the atmosphere, is a column.  This column is filled with air molecules.  While the effect of each individual molecule is miniscule, their combined effect is a force with which to be reckoned.  How much atmospheric weight do you think your head has to support?  Go ahead, take a guess...

First we need to find out how large your head is.  For the purposes of this activity, we're going use inches so we can get an answer in pounds.  It's rather un-scientific of us, but it will provide us (in the U.S.) with the greatest understanding.

Back to your head.... find the circumference of your head, using either a fabric measuring tape or a length of string that you then lay against a meter stick.  I come up with 22 inches.

Now you'll need to do some math to find the radius.  Circumference is equal to 2 x pi x radius.  So, to get the radius, you'll need to divide the circumference by pi and then divide that number by 2.  For me, it's 3.5 inches.

Now you'll use the radius to find the area of the top of your head.  Area is equal to pi x radius x radius.  For me it's 38.47 square inches. 

Atmospheric pressure at sea level is 14.7 psi (that's pounds per square inch), and while I don't live exactly at sea level, that number will work well enough for our purposes.  So, the area of my head multiplied by atmospheric pressure gives me the weight of air pushing on my head.  In my case, it's 38.47 square inches x 14.7 psi = 565 pounds. 

Pretty unbelievable, isn't it?  But it's true.  We aren't aware of it because we're used to it, we've never known anything different.  And we aren't crushed by that force because there are fluids inside our body exerting pressure that keeps things balanced.  Those air molecules are pushing on all sides of your body, not just on top of your head, which also helps keep things balanced.

If you're interested, atmospheric pressure in Denver, with an approximate altitude of 1 mile, is 12.2 psi.  You might want to have your students determine how much the atmospheric weight changes as they go from sea level to 1 mile.

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