|When in doubt, head to the children's books.|
I have been avoiding dealing with pressure. I believed that if I just relaxed and flipped through my collection of meteorology books, I would eventually understand atmospheric pressure. I did not. What’s worse, the more I tried to understand it, the more I realized I didn’t understand about the atmosphere—what’s in it and how it works.
Unfortunately, I cannot legitimately write about clouds without understanding pressure. I turn first to my lay pal, Eric Sloane who addresses the topic of pressure in an early chapter of his Weather Book. The chapter is called “Air Has Weight.” It sounds simple enough. Many of you might be saying, “Of course air has weight! What’s the problem?” Try explaining “air has weight” to a first grader, an alien, or a writer without a science background. Soon, you’ll find yourself in a very very long and frustrating conversation in which you use words such as density, latent heat, mass, low, high, and perhaps adiabatic lapse rate. You will revert to metaphors which will only make matters worse because the atmosphere is really not like the ocean nor do air molecules really behave like billiard balls.
Eric Sloane feels my pain. After a few paragraphs of simple-enough explanation, he writes, “But no matter how many times we say that ‘air has weight,’ it’s just too difficult a picture to visualize at once.”
“One of the reasons that we find it hard to visualize air having weight,” he continues, “is because atmospheric weight is always referred to as pressure. (The weight of gas presses out in all directions). The pressure of air is caused by the weight of all the air above that presses down on whatever air is below.”
|Image courtesy www.science-of-speed.com|
And here is what they tell me and thousands of young readers.
“Air pressure is the force of air pressing down on the ground or any other horizontal surface.” [I close this book and set it on the table in a vertical position. It does not float away.]
“Atmospheric pressure is simply the weight of air. You don’t feel it because the pressure is distributed in all directions and there’s also air inside your body pushing outward….” [How many 11-year-old boys are going to snicker at this one and then let out a huge burp (or worse)?]
“Pressure is the result of weight.” [Huh?]
“Because air is a fluid, the pressure it exerts comes from all directions.” [Air is a fluid? When did I learn this? I guess the same month I forgot “fluid” is not synonymous with “liquid.”]
“Air pressure is the weight of the air pressing down on the earth.” [This contradicts statement above].
“Because air has weight, it is constantly pushing down on everything on earth.” [There is some cause-and-effect fallacy going on here…]
“High pressure areas (or highs) are like mountains of air. That means there’s more air above you. So your pressure is higher.” [Nothing like a tautology to clears things up!]
“Atmospheric pressure is the weight of the atmosphere on a specific point of the Earth’s surface. [And judging from what follows this, I think the author of this statement panicked and slunk behind calculation jargon.] “Measurement is done with a barometer and can be expressed in different units of measure, either hectoPascals (hPa) (formerly called millibars (mbar)) or millimeters of mercury (mmHg). Average pressure at sea level is 1013 hPa of 760 mmHg.”
“Think of air as a three-dimensional pool table where all the balls are moving and bumping into one another constantly. The effect of all this moving and bumping is pressure.” [The only problem with our 3-D pool table is that it occupies a single plane. The atmosphere does not. So difficult is it for us to visualize air and air pressure that many authors draw metaphors and similes to familiar objects that lure us into thinking we “get it.” If you “get it” in this example, you’ve got it wrong. And did anyone think of a 2-D pool table when reading this?
Are you as confused as I am?
Wait. It get’s worse. The average atmospheric pressure at sea level is 14.7 psi [per square inch]. The average person has 3000 square inches of skin, so rounding up, 15 psi x 3000=45,000 pounds of pressure per person. But, we “don’t feel this” my pile of books tells me because a) the air in our bodies exert equal and opposite pressure, b) we are used to it, c) we are structured to withstand the pressure, d) pressure is not actually applied downward as most of my books would have me believe, but equally to all sides of an object.
I cannot get my head around this at all. When I mention to my husband and 19-year-old son the fact that we are living under 45,000 pounds of pressure, they think I am making this up, have added a few zeroes, or do not have a science background as usual. I tell them I learned this on YouTube courtesy Julius Sumner Miller, the American physicist and TV personality whose popular educational programs (including "Why Is it So?") were broadcast from the 1950s-1980s. A student of Einstein, Miller is marvelously enthusiastic, brilliant, and "mad."
They look stunned. And then I tell them that “somehow” the air in our bodies exerts 45,000 pounds of pressure to balance this out. They give me the “of course” look as if they had known this all along and were just temporarily flabbergasted. When I ask where they think all that pressure is coming from, they mumble something about lungs and molecules and then quickly change the subject.
I feel really bad for all the children who read these books and are tacitly asked to accept what they should question, what should make them slack-jawed with wonder and curiosity. You tell them "air has weight" and they buy it. I must have bought it, too, but now I want an explanation or a full refund.
Istart working on an explanation. After a lovely breakfast at the San Francisco Street Bakery this morning, I confessed to my friends, Maxine and Ray, that I was struggling with pressure and wondering why we all weren’t plastered to the sidewalk under 45,000 pounds of atmospheric pressure. Maxine jumped at the chance to explain which she did—first without metaphors and then with. She tried using a bag of potato chips (so fun to pop!) to stand for air under pressure.
What happens, she asks me, when you take the bag of chips on an airplane?
I eat them, I reply.
My husband eats them?
I eat them, Ray chimes in, laughing.
The bag expands! Maxine shouts as if that explains everything.
Because you’re flying at a high altitude and the air pressure is lower, the air in the bag can expand because there is no pressure against the air inside the bag.
So imagine the cells in your body are little bags of potato chips, pressing against the pressure of the atmosphere.
I can’t do that, Maxine.
She looks at me, disappointed but not daunted.
I need a new metaphor, I say. But don’t tell me that the atmosphere is like an ocean of air and we live at the bottom of this ocean because when I am swimming in the ocean I float at the top…and I cannot get to the bottom of the ocean without wearing cement shoes. This image ruins the ocean for me as a source of pleasure.
How about Jell-o? she asks. Imagine the atmosphere like Jell-o—surrounding you on all sides, putting equal pressure on all sides your body equally.
I can imagine myself in a swimming pool of blue Jell-o. I get this. And then one of those green skinned grapes appears in the Jell-o—the kind of grape that came in the can of fruit cocktail and was mixed into Jell-o when I was a kid, presumably to make the Jell-o a health food. And then I try to remember the other fruits—little squares of peaches, a maraschino cherry, a pale yellow pear cube. I imagine them all suspended in a blue atmosphere of Jell-o. And then I realize I am no longer listening to Maxine. She notices there is not a light bulb going off above my head.
You know what you need? she asks not waiting for my answer. You need to rent that movie with
I am not sure how to take this.
But I think Jell-o is a really good starting metaphor to explain the all-directional and equally applied force of atmospheric pressure on a body. I am not sure the Jell-o-as-metaphor will hold up under pressure from temperature, density, and altitude.
Getting clouds in there will be tricky, too.
I will work on it and keep you posted.