You see dark grey clouds looming overhead. So, will it rain or will it not?

For those of us living in Seattle, that question arises pretty often. Wintertime often brings dark grey clouds. And the question arises each morning: Should I take a raincoat to work? Or should I not?

The forecasters are not particularly helpful. Often, they suggest a 50% probability of rain. Does that really help?

The answer to the raincoat question, I think, is not complicated, but depends on three concepts that may not be so familiar.

  1. The first concept: what keeps clouds aloft in the first place? Clouds are built of water droplets. And, since droplets are denser than air, they ought to fall to the earth. But, evidently, they might not. Otherwise, we’d never see clouds in the sky. The reason clouds remain aloft is because they repel the earth. The earth is negatively charged, and the clouds are negatively charged. So they repel, and the clouds remain elevated. Clouds that are highly charged repel more than less-charged ones; so they situate themselves higher in the sky. That’s why you can sometimes see stacks of clouds, each with differing amounts of negative charge. The main point is that clouds are negatively charged.

  2. The second concept: in those dark clouds, the net negative charge is less than in the white ones. Darker means the sun can’t get through because droplets are more tightly packed. And the way that happens is through more intervening positive charges, which pull the negative droplets closer together. But more intervening positive charge also means lower net negativity. And with less negativity, there’s less repulsion. So, those dark rainclouds generally sit lower than the lighter clouds. When clouds begin to obscure the mountain peaks, you know it might rain.

  3. The third, and critical concept: induction. What’s induction?—often called Faraday induction after the Scottish scientist who first discovered it. Well, consider an isolated charge, say negative. Place that negative charge near some neutral material. The negative charge attracts the material’s positive charges, drawing them to its near surface. That creates an attraction between the material and the isolated charge, drawing them closer to one another.

For the strength of that attraction, distance matters. If the isolated charge sits at an appreciable distance from the material, the inductive effect will be puny. If the charge is nearby, then the effect will be more pronounced. The closer the charge, the stronger the attraction.

So, how do these three concepts help us understand the decision to rain or not to rain? Think of the raincloud as an isolated charge. The negative charge of the raincloud will induce a positive charge immediately below the raincloud. Those localized charges are well documented. They don’t negate the overall negative charge of the earth because they accumulate only locally, just beneath the cloud.

Through induction, those induced positive charges attract the negative cloud. They pull the cloud downward. That brings the cloud closer to the earth, which induces even more positive charge below, which draws the cloud even farther downward. So the effect gets progressively stronger.

Eventually, the pull may get so strong that the contents of the cloud get pulled towards the earth. That’s rain.

Several features of the process are noteworthy.

  • First, it’s not the entire cloud that gets drawn. It’s the lower droplets. Those lower droplets experience the highest attraction, so they’re pulled from the cloud proper. That’s why it rains in droplets, and, thankfully, not as a large cloud dump.

  • Second, those drops don’t just fall. Remember: they’re pulled to the earth. That being the case, you’d expect that the falling speed might exceed the speed expected merely from the pull of gravity. And that’s confirmed. High-speed video shows that droplets can fall at speeds up to ten times the free-fall velocity. So clearly, those droplets are drawn toward the earth. That’s why you can sometimes experience “pounding rain.”

  • Third, you can understand why, as you may drive through mountain passes, you can often see clouds sticking to the mountains. It occurs in patches. By induction, the clouds induce opposite charges on those mountains; and so they stick. It’s not complicated.

Well, I’m not sure that all of this helps us decide whether or not to carry an umbrella to work, but I do hope they help us understand what turns on the rain. It’s not a mysterious celestial switch of some kind; it’s grounded, I believe, in simple physics.