You have probably heard some of the old wives tales about the look of clouds, halos around the moon, even colors of the sky being indicators of weather to come. They are often based on a lot of truth and can, indeed, provide a useful forecast.
Taking note of what the clouds look like, their shape, movement, and even altitude is such an interesting and useful way to understand what is happening in the atmosphere.
Let’s first brief you on how clouds form in general. Invisible water vapor (a gas) condenses into visible water droplets or ice crystals. There are also tiny particles floating around in the air, like salt and dust. These particles allow that water vapor to condense onto something and grow bigger. More and more of these small droplets bump into each other, creating even bigger drops. Eventually, they are big enough together to form clouds we can see. Over time these droplets can become big enough to form rain!
I’m going to tackle one type of cloud, the cumulus cloud, and how 3 of its forms can help us predict the weather.
*Fun Fact: Light and fluffy cumulus clouds are not *light* at all! Researchers have calculated that the average cumulus cloud weighs about 1 million pounds! That’s equivalent to about 40 school buses, or close to 100 full grown elephants! Amazing since they appear to float, light as a feather!
First up: cumulus humilis. This is the ‘cotton ball’ form and it indicates fair weather. Why the fair weather forecast? These clouds, much smaller and less dense than their thundering, precipitating cousin, the cumulonimbus, don’t have the depth or lifespan long enough to produce big enough droplets. There can be a few reasons for this: a drier air mass (less available moisture to make the droplets), a stable atmosphere (meaning it would inhibit vertical motion. Instability would allow the cloud to grow bigger and taller which could lead to precipitation), and/or a relatively short lifespan (watch a time lapse of the fair-weather clouds. You can see them bubble up and down quickly, even though they seem to be like stable parade floats in the sky!). Add in relatively drier air, a stable atmosphere, and a shorter existence and you have yourself some nice weather. A fair weather forecast!
Next: towering cumulus, aka ‘cumulus congestus.’ As its name implies, these cumulus clouds grow big and tall, piling up, and can indicate rain and/or storms in the forecast. Unlike its fluffier counterpart, the cumulus humilis, these form in an unstable environment. This means they don’t have the atmosphere keeping it down or limiting its vertical growth. Under these conditions, they rise up thousands of feet and can do so in a fairly short period of time (on the order of half an hour!). The result is more droplet collisions, more opportunities to create bigger droplets and eventually precipitation. Hang around long enough, and you may find yourself in a cumulonimbus (thunderstorm)! If you see these towering clouds early in the day, they are a sign of atmospheric instability. Figure on some showers and perhaps even thunderstorms in the forecast!
And lastly: alto cumulus. The ‘alto’ part of the name comes from the fact that these clouds are higher up than a regular cumulus cloud, in the mid levels (anywhere from a few thousand feet up to as high as 18,000ft. up). There are lots of variations of this type, but you may immediately recognize the quilting effect of one version, like a patchwork of fluff. They signal a presence of instability and moisture in the mid levels. Like other higher-level clouds, they can indicate approaching weather, a change in the forecast. When a warm front approaches, for example, you’ll often seen high clouds with an increasingly lower cloud deck as time goes on. Eventually, you may see precipitation, colder weather, etc. As for the patchwork version of altocumulus: while these clouds themselves don’t precipitate, see them on a warm, sticky morning and you may see rain and/or storms later in the day!
What’s your favorite type of cloud? Let me know in the comments below!