As you may recall from my previous posting, Writer Under Pressure, I have been struggling with understanding the basics of atmospheric pressure. I pored over many a book to try to find the right metaphor, image, equation that would set off the "aha!" for the Accidental Naturalist. The problem with metaphors and similes, I found, is that they don't hold up under scrutiny. Atmospheric pressure is sort of like blue Jell-O, sort of like a bag of potato chips, sort of like balls bouncing on a pool table, sort of like an ocean of air. But not really and not if you want to add all sort of other necessary ingredients to your atmosphere--heat, wind, aerosols, moisture, clouds. Adding heat to blue Jell-O is not a pretty sight.
Thinking that I needed to knuckle down and understand the physics of atmospheric pressure, I began reading about gravity, mass, weight, temperature, volume, Boyle's Law, Charle's Law, and the essential behavior of air molecules.
In no time, I was confronted by "force per unit area," 14.7 psi, hectopascals, V a P, V a T, and the like. Eventually, I understood what all of this meant, but I certainly do not want to write a book about clouds using this kind of language. Nor would you want to read it.
Nobel-Prize Winning physicist Richard Feyman said "The glory of mathematics is that we do not have to say what we are talking about." The more I knuckle down, the more I believe the atmosphere works in such glorious ways that describing it is beyond words and mathematics equally. This last statement is the equivalent of a Hallmark card that starts out "Words cannot begin to describe...." and then goes on and on ad nauseum.
This morning, I found two descriptions of the behavior of air molecules that worked for me. Neither metaphor nor equation, they are are simple descriptions that rely on well-chosen verbs and adjectives. And, most importantly, the authors write about air with a sense of awe and a light touch.
John Suchoki writes this in Conceptual Chemistry:
"Think of the molecules of air inside the inflated tire of an automobile. Inside the tire, the molecules behave like zillions of tiny Ping-Pong balls, perpetually moving helter-skelter and banging against the inner walls. Their impacts on the inner surface of the tire produce a jittery force that appears to our coarse sense as a steady outward push. Averaging this pushing force of a unit of area provides the pressure of the enclosed air."
Pure poetry! For me, this entire paragraph behaves like air molecules inside my enclosed brain: zillions, Ping-Pong, helter-skelter, banging, jittery.
In Meteorology Today, C. Donald Ahrens writes this:
"Air molecules are in constant motion. On a mild spring day near the surface, an air molecule will collide about 10 billion times each second with other air molecules. It will also bump against objects around it--houses, trees, flowers, the ground, and even people. Each time an air molecule bounces against a person, it gives a tiny push. This small force (push) divided by the area on which it pushes is called pressure."
And this:
"Air molecules not only take up space (freely darting, twisting, spinning, and colliding with everything around them) but...these same molecules have weight. In fact, air is surprisingly heavy. The weight of all the air around the earth is a staggering 5600 trillion tons...The weight of the air molecules acts as a force upon the earth."
Darting, twisting, spinning, colliding...I get it!
And, in describing the differences in density of air molecules between the troposphere (the "bottom" 11 km of our atmosphere, the layer closest to earth) and the thermosphere (the "top" layer above 85 km), Ahrens writes:
"The low density of the thermosphere also means that an air molecules will move an average distance...of over one kilometer before colliding with another molecule. A similar molecule at the earth's surface will move an average distance of less than one millionth of a centimeter before it collides with another molecule."
Now I am beginning to see and feel the invisible atmosphere. It is making my head spin. And, it is making me hungry.