Take some time each day to look at the sky. I am trying to begin each day with a few minutes outside, coffee in hand, looking up.
Block Island Sound, RI, September 7, 2013, by the author.
Thanks to the efforts of the late Jack Gordon's For Spacious Skies, I find myself looking at clouds more and more. But I make two mistakes: either (1) I enjoy their beauty without thinking about how they form and relate to weather, or (2) I think about how they form and relate to weather without enjoying their beauty. I probably get nearest the happy medium when, rather than simply gazing, or fixating on whether they're stratocumulus or altostratus, I instead see a cloud as dynamic atmosphere made visible: air roiling, temperature changing, and water molecules ricocheting around before my very eyes. To "see" clouds this way, you need to know a little physics and chemistry. And after another post or two, you'll be able to see clouds this way, too.
To start with, clouds do NOT form because the air is like a sponge that can hold only so much water vapor, and cooling the air is like squeezing the sponge more and more, yada yada yada. That was what I was taught, believed for many years, and taught many 7th graders (may they forgive me). As an explanation, it works pretty well, but is simply wrong. So what is the truth?
The first thing to understand is that water is evaporating and condensing all the time. That means that a cloud is less a "thing" than it is a dynamic PROCESS. To understand it, we will start with the fundamentals.
1. All molecules and atoms move, and they move all the time. Gas molecules bounce off each other, molecules packed together in a liquid jostle each other, and molecules locked into a solid simply vibrate--but all are in motion. In a gas, molecules are separated, so they zing around and ricochet off each other like pool balls--with the difference that, unlike on a pool table, the collisions of gas molecules are perfectly elastic--so the balls never "run out of energy" the way pool balls eventually do.*
2. Moving molecules have kinetic energy which is a consequence of their mass and speed. (The equation for kinetic energy is EK=mv2, which means that heavier, faster-moving molecules have more energy than lighter, slower ones. And the speed (v, for velocity) has a big impact on energy, because this number is squared.) We experience this energy of movement as temperature, because temperature is proportional to the average kinetic energy of a sample of molecules. In short, the warmer something is, the faster its molecules are moving. Try to picture this when you hold a hot cup of coffee!
3. Every water molecule has a little positive charge at one end, and a little negative charge at the other. Because of this, water molecules cling to anything with an electrical charge--including and especially each other. (Remember learning in school that "opposites attract."
What does all this have to do with clouds? Next time we'll look at evaporation and condensation.
Sunset over Block Island Sound, RI, September 7, 2013, by the author.
*By the way, those gas molecules are moving fast. As a rule, the speed of gas molecules is about equal to the speed of sound in that same gas. In air under normal conditions, that's rather more than seven hundred miles per hour. But they don't usually get too far, since on average air molecules are only spaced 10-5cm apart, so that a single molecule collides on average five times per nanosecond. (1ns=one billionth of a second.) That's fast-moving but mighty small game of pool!