What is atmospheric pressure? Why do we care about it? Part 1.

 A few days ago, a blog reader emailed about atmospheric pressure.  

He asked:  Since can't feel it or sense it, why should we care?

I was surprised by the question since atmospheric pressure is so important in meteorology and weather prediction.     So let me give you a lowdown on pressure--by the time I finish,  you will run out and buy a barometer!

Atmospheric pressure at the surface of our planet is not negligible.  

On average, pressure at sea level is about 14.7 pounds per square inch!   That is like a bowling ball pushing against every square inch of your body!

Since the human body has about 2600 square inches of surface area, the is equivalent to the force of several thousand bowling balls.  Or 38,000 pounds of force or 19 tons!

Why aren't we CRUSHED???     

Because the pressure inside our body balances the external force from atmospheric pressure.    But you can tell that this massive pressure is there by lowering the pressure inside an object---it is summarily crushed!

If you want proof, check out this youtube video in which the pressure was reduced inside a tanker car.  Let's say this tanker will not be used again.

Atmospheric pressure is related to the weight of the air above you.    Yes, air has weight!  

A volume of a cubic meter near the surface weighs about 2 pounds.  Now air gets less dense as one moves upwards, but there is still a LOT of weight from all the air above you.

Pressure is given in a lot of confusing units.

Pounds per square inch (typically around 14.7 as noted above).

In the metric system (preferred in science) we use hectopascals (hPa).  

And, another approach is to give pressure in terms of the height of a mercury column supported by atmospheric pressure (see illustration of a mercury barometer below).  At sea level, that height is typically about 29.92 inches.    Atmospheric pressure pushes down on a bowl of mercury and the pressure is communicated to the vertical tube of mercury.  The downward weight of the mercury is supported by the upward force resulting from atmospheric pressure.

Higher pressure results in a taller column of mercury.  And  vice versa.

What if we replaced the mercury with water, which is much less dense (and thus weighs less)?  In that case, atmospheric pressure could support a column about 30 feet high!

Finally, here is your amazing factoid for today.   How do straws work?  What is the longest possible straw in the world?  Turns out the answers have to do with atmospheric pressure.

When you sip a liquid with a straw, why does the liquid move upward towards your mouth?  Are you somehow pulling up the liquid like a pump?  

NO!

Atmospheric pressure is pushing the liquid up the straw!     When you are sucking air at the top of the straw, you are reducing pressure on the upper portion of the straw, creating a zone of lower atmospheric pressure.   The liquid in your glass or soda can is at atmospheric pressure, and thus at higher pressure than at the top of the straw.  As a result, there is a force from high to low pressure, which pushes the beverage into your mouth.

One final thing....what is the longest a straw can be to work?   

30 feet!  That is the maximum column of water that can be supported by atmospheric pressure.  So if someone tries to sell you a 40 ft straw, turn down the offer.