How Do Clouds Form?

Cloud Formation
Normally, our sky explorations involve the Moon and planets. But the practical effects on our lives come from very different sky components—the ones called clouds. And now that summer’s officially here, the clouds have changed. Wherever you are right now, let’s walk outside or look out the window for some cloud gazing.

We take clouds for granted because the sky is part of nature and is always above us. But take a moment to watch these fluffy shapes form in the sky.

We’re not going to quiz you on the “cloud shapes” you may have learned in school (cirrus, cumulus, etc.). though the Almanac does have a Guide to Clouds for that.

Let’s just look up at the sky and watch the way a cloud changes. Slow down, observe, and then carry on with your day. (We would suggest you wear polarized sunglasses outside if you’ll be staring up at the sky, and to stare the opposite direction of the Sun.)

How Do Clouds Form?

In winter, clouds are mostly flattish sheets that give us overcast conditions. They move here from somewhere else, and often persist for a long, gloomy period of time.

Not now. In summer, we often grow our own clouds locally. On clear mornings, it’s fun to watch the process.

Let’s watch how a day in the life of a cloud …

  1. If there’s no early fog, the sky stays clear from sunrise until a few hours later, typically around 9:30 or 10 AM.
  2. Then, you see small isolated clouds appearing. What happened is not mysterious. The sun heated the ground, which warmed a patch of air over it, which then rose because warmer air is less dense, just like in hot air balloons. The bubble of warmer air keeps going up, passing through surrounding air that’s increasingly cool—typically chilling by 3 or 4 degrees Fahrenheit per thousand feet.
  3. Eventually, our bubble is cooled to its own saturation point, or dewpoint, since cool air cannot hold as much water vapor as warmer air. Suddenly, the air’s gaseous water changes to a gazillion tiny liquid drops, and now it’s a cloud. If you fly beneath it in a small plane, you can feel the bump as you penetrate this still-rising column of air, most pronounced over hot surfaces like urban roads and roofs, or over natural areas that are rocky, or hills angled into the sun. These get the first clouds each day. Conversely, vegetation and water doesn’t heat up very much.
  4. By noon, the clouds multiply and also develop impressive vertical dimensions, especially in humid conditions.
  5. By mid-afternoon, some may shed their water or even become thunderstorms. It’s so common and reliable at this time of year, it’s fun to observe. Meteorologists typically pin a probability on these daily chances you’ll get rain from the sequence.

How to Calculate the Chance of Rain

This brings up a practical question. Say the Weather Service says that the next three days each have a 30% chance of rain. You think, “My lawn could use a watering. I don’t care exactly when the rain comes, but what are the chances it’ll rain sometime, anytime, in the next three days, if each day has just a 30% chance?”

Here’s how to calculate the odds.

  1. Step One: Convert the situation into the chances that it won’t rain. In this case, if there’s a 30% chance it will rain each day, then there’s a 70% chance it’ll be dry.
  2. Step Two: Multiply each day’s chances. In this case, with three days with 70% dry-chances each day, you’d multiple 0.7 times 0.7 times 0.7 and arrive at a 21% chance it’ll be dry throughout the period.
  3. Step Three: Finally, subtract this from one (1 – 0.21 = 0.79) and you’ve got your answer: There’s a 79% chance it’ll rain sometime during the next three days. It’s quite likely.

See, that old high school math turns out to be useful after all.

Predicting the Weather

By observing clouds, you can often predict the incoming weather.

For example, above is photo of a cloud taken from the Almanac office in Dublin, New Hampshire; you can see the church steeple next door and the American flag in our town center.

You’re looking at a typical “mackerel sky,” so named because its wavy striped pattern resembles the markings on certain mackerel fish.

Several proverbs are related to this weather phenomenon, including:

Mackerel clouds in sky,
Expect more wet than dry.

The proverb is based in fact. The sight of mackerel clouds (also called “mare’s tails”) suggests that rain will arrive within a day or two, ahead of a warm front.

See the Almanac’s Cloud Guide with types of clouds and associated weather.

Source: Farmer's Almanac - Amazing Sky