We Love Weather Exclusive

What Clouds Tell Us About the Forecast

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!


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27 Comments


  1. Hi, this morning I saw the sky full of alto cumulus clouds, but I couldn’t remember what that might mean for todays weather. I live in Illinois about 26 miles east of St Louis, our weather was okay until afternoon and now it is completely overcast and humid. What am I missing?

  2. My favorite type of cloud is cumulonimbus. I LOVE the intensity of seeing something so large and powerful! It’s very mesmerizing, and I always get excited about thunderstorms anyway!

  3. I am such the all around weather geek. About around in the early 90ies. I looked up into the sky and noticed the color of the sky was really odd looking. It was like a blue that I have never seen before. I was listening to the weather report and they said that something was brewing. I decided to get ready for the worst. (well the best for my weather geekness) I got a fire going, water saved up, candles out on the table, made sure the batteries were in site. Got some activities ready for the kids, windows taped up…… I am glad I did that cause when the winds started up they escalated to a big wind event. A few neighbors windows were broken. Mine were saved. We were without power for a few days. I cooked meals on the coleman and made foil dinners in the fireplace. I stress highly never use generators inside the house. But I really stress again to look at the sky, get to know what she is doing and why.
    About some five years ago or so I was happy as a clam on facebook. Then at around 4PM I heard a solitary hugeeeee clap of thunder. Then that was it…but when the sun went down the show got underway in a huge way! This lasted a couple days. I was loving it in a big way! โ›ˆ๐ŸŒฉ

  4. I love nature so much… but as we should, she created everything for us. so its only right we show our passion of respect… this website is so awesome!

    1. @maria-larosa, there is a new type of cloud according to “Whose Line Is It Anyway?” It is called “giganticus erecticus”, or a giant scary-looking cloud minus the wall cloud. #cloudcomedy ๐Ÿ˜€

  5. I saw a strong (NWS Via TWC) thunderstorm come ashore last month while vacationing in Ft. Walton, FL. I was running back and forth from the balcony to the TV with my camera. I was very excited.

  6. alto cumulus. @2lookingup I’ve been seeing them all week as well.. I dont think I’ve ever seen them before as often as I have this week.. beautiful clouds, also mysterious on the amount of time they’ve been occurring.

    1. They come in so many varieties! I can’t help but look up and think about all the ways the air is behaving to get the alto cumulus to look the way they do.

  7. I love cumulnimbus clouds how they rise up so high up in the troposphere. They even sometimes break through the stable air in the stratosphere. Also how they have other clouds within that cloud. I love the anvil how it’s spreads when it reaches the tropopause. Also what weather it creates within the cloud and I lovery it’s shape.

    1. Yes! Here in Atlanta, too! It’s been a story stretch for sure. Lots of opportunities to observe these types of clouds.

  8. @maria-larosa, I adore the mammatus clouds because of their pouch-like features after severe storms pass. Dr. @twcerikanavarro, I totally love altocumulus clouds because they remind me of their fish scale texture that resembles a mackerel. Is there any way that I can purchase a copy of the updated “International Cloud Atlas”? #clouddialogue101 โ˜๏ธ๐ŸŒซ๐Ÿ ๐ŸŽ

    1. I agree! There is something so cool, mysterious, fascinating about mammatus clouds. They’re very dramatic! As for the atlas, I’m not sure. Good question!

    2. Those are some really cool looking clouds. But for some reason they give me a headache. I don’t know why, could be the pressure they are creating.