May 25, 2021

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.

15 comments:

  1. I was sucking on a straw while reading your very apt description of atmospheric pressure!

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  2. Love it Cliff—can’t wait for Part 2!

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  3. Thanks for this. I've struggled with a lot of the weather vocabulary. Dumb question, is the pressure at sea level 14.7 #/sq inch because gravity is pulling on the air molecules, or is there something else causing the pressure? I guess I'm asking is "pressure" the same thing as "weight" or mass?

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    1. Correct me if I'm wrong, but pressure is the amount of force pushing on something. Sometimes it is measured in pounds. So yes, it is the same thing as weight, but atmospheric pressure is just specific to the weight of the air above us.

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    2. Pressure is defined as weight per area -- pounds per square inch, newtons per square meter (a.k.a. "Pascals") A weight pressing against you, whether it's a bowling ball or the atmoshpere, applies a certain amount of force to your body. Dividing that by the surface area of your body is expressed as a pressure.

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    3. Yes, it's gravity. Same thing as the added pressure that divers feel. It works on gas mixtures like air too. Good thing, too, otherwise the air would escape into space and we'd all suffocate (actually we'd never have evolved to begin with).

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    4. So glad that I read your blog. Someone just tried to sell me 40-foot straw! 😁 Always appreciate your sense of humor!!!

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  4. Your blog reader said, "Since we can't feel it or sense it, why should we care?"

    At my age, I can feel and sense it when air pressure changes enough to aggravate my arthritis. Let's give this condition a name: Age Related Air Pressure Anomaly Response Syndrome (ARAPARS).

    OK ... Before I had surgery recently to correct an injury to my left foot which happened forty-four years ago, it was highly sensitive to any changes in the weather.

    I used to call it my 'weather foot' and would claim that I could use it to predict the probability of rain that day, which direction the wind would be coming from, and how strong the wind would be.

    However, I did not go as far as to claim that my ability to predict today's weather with my left foot extended to an ability to use it for predicting the state of the earth's climate system in the year 2100.

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    Replies
    1. I feel you, at times the stuff that broke and the arthritic joints will decide to all hurt more, same day. Hey, what’s going on???? Turns out, change in air pressure happened. Best explanation I’ve found so far.

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    2. I also get headaches when the barometric pressure drops rapidly (like when we get those juicy winter storm). I know a few people in the same boat.

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  5. Brilliant, B.B. I laughed out loud at your last sentence.

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  6. It's my understanding that there is some technique of siphoning water over a height slightly greater than 34 feet, and I read in the Guinness book of records, I think is was, that that discovery has been patented. I don't know how it works but I suspect that it utilizes intermittent momentum-driven flow in some way. Anyone care to chime in?

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  7. An intense storm hit Bellevue at around 11:50 AM today. A rumble here and there, but the most impressive was the wind... had the rain going nearly vertical! The temperature gauge at our house also dropped by 10F. Impressive! (for washington state)

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  8. You might be interested in this -- just found it after reading your post earlier today.

    "Man Attempts to Drink From the World’s Longest Straw...a 34 foot flexible straw"
    Video at https://www.youtube.com/watch?v=hPlZHGdooo0 [8:01]

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Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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