Clouds may be the most important form of flow visualization available to everyone on the planet. Weather, including clouds, affects all of us, and being able to read the sky is tremendously useful for anybody who goes outside. But clouds mean much more than this to us. Clouds are often beautiful and sometimes frightening (Figure 1 might be both). Most of us have admired a spectacular sunset or spent time looking for recognizable shapes in clouds. Gavin Pretor-Pinney, in the Manifesto of the Cloud Appreciation Society asserts,
WE BELIEVE that clouds are unjustly maligned and that life would be immeasurably poorer without them.
We think that clouds are Nature’s poetry, and the most egalitarian of her displays, since everyone can have a fantastic view of them.
We pledge to fight ‘blue-sky thinking’ wherever we find it. Life would be dull if we had to look up at cloudless monotony day after day.
We seek to remind people that clouds are expressions of the atmosphere’s moods, and can be read like those of a person’s countenance.
We believe that clouds are for dreamers and their contemplation benefits the soul. Indeed, all who consider the shapes they see in them will save money on psychoanalysis bills.
And so we say to all who’ll listen:
Look up, marvel at the ephemeral beauty, and always remember to live life with your head in the clouds!
Whatever your reason, knowing how and why clouds exist will deepen your appreciation for them. In this section, you’ll learn to recognize different clouds, describe the air motion and atmospheric stability that govern the appearance of clouds, and even interpret weather data including Skew-T plots and wind soundings to predict and help identify clouds.
Names of Clouds
The first step is to recognize the basic clouds. Many of us learned the names of various types of clouds when we were children. How many can you recall?
Clouds are categorized using a Latin-esque terminology first developed by Luke Howard in 1802 . The names loosely depend on the height of the cloud’s base, and whether it is a flat layer or made of clumps. The 10 basic cloud types, or genera, are shown in Figure 2. In addition to belonging to one of the 10 genera, clouds also may be sub-classified by species and/or varieties. There are over 90 specific cloud names , but for now we’re only going to discuss the 10 genera. When we get further into the physics of clouds we’ll discuss some of those species and varieties.
If the cloud base is low — less than around a mile, or five to seven thousand feet, above ground level (and this number varies depending on the season and the latitude ) — then the cloud could be a type of stratus if the cloud is flat, continuous and covers a significant fraction of the sky. In this case, ‘stratus’ means ‘layer.’ Don’t get confused with stratus as in stratosphere, which is the layer of the whole atmosphere above the troposphere. The troposphere starts at ground level and contains almost all clouds and weather. Stratus layers are sometimes featureless, making it difficult to capture a compelling image of them. Figure 3 is an exception, showing layers of stratus, accented by stratus fractus clouds forming or dissolving (the accompanying report is excellent as well, if you are looking for a good example.)
The sometimes-boring appearance of stratus-type clouds also makes them tricky to classify: the height of the layer is hard to guess from visual appearance when you are under it. Here is a rule of thumb for stratus clouds — at least to tell cirrostratus from altostratus: cirrostratus, is a very high, flat, thin, cloud layer, made of ice crystals, so it will be a bright, fairly uniform veil over the sky. If you can see a visible thickening, where the thick part of the cloud is darker than the rest, then there are liquid water droplets present, and the layer will be at a lower, warmer altitude, and therefore be an altostratus. Altostratus occur between maybe 10,000 feet and 25,000 feet, while cirrostratus can start in the middle of that range; these altitude ranges overlap, and are only very approximate guides. It’s hard to tell the lowest level, the plain stratus, from altostratus by visual appearance, but if you have a handy high mountain range nearby and you can see the tops of the mountains, then it’s high enough to be altostratus. Figure 4 shows both types, so you can compare them. Nimbostratus is a stratus-type cloud that produces precipitation, rain or snow, that reaches the ground.
In contrast, if the cloud is composed of individual elements, or cloudlets as shown in Figure 5, then it is likely a flavor of cumulus, where ‘cumulus’ refers to ‘pile’, as in an accumulation of cloud stuff. There are a lot of flavors of cumulus! You can guess at the height classification using this rule of thumb. Well, rule of fingers, actually. Hold your hand out at arm’s length. Pick a cloudlet (single cloud, element or clump) right overhead, if you can. If the cloudlet is wider than three fingers together, it’s probably a low-level cloud and thus some type of cumulus. If the width is between one and three fingers wide, it’s likely a mid-level cloud, and probably an altocumulus (Figure 6). (Yes, alto should mean high, but in clouds it’s only middling.) If the cloudlets are smaller than one finger width, then it’s a true high-level cloud, so cirrocumulus, which are sometimes described as looking like scattered rice. If you’ve got thunder and lightning, you’ve definitely got a cumulonimbus (there is no nimbocumulus), which often stretch from a low level all the way up to the bottom of the stratosphere. It’s rare, but if you’ve got precipitation that reaches the ground coming from a cumulus cloud, but no thunder or lightning, then you would modify the name of the cloud by adding a ‘praecipitatio’ modifier, e.g. cumulus congestus praeciptitatio. Add ‘virga’ if it doesn’t reach the ground.
So, to get started, practice looking at clouds every time you go outside. Is it a stratus (flat) or cumulus (pile of cauliflower) type? The most common of all clouds is actually halfway in between: the stratocumulus (Figure 7). (No, sorry again, there’s no such thing as a cumulostratus.) Another in-between cloud is the cirrus, which are wispy or fibrous, so also neither stratus nor cumulus. Cirrus are made of reflective ice crystals, and are above the mid- and low-level clouds (i.e., not shadowed by them), and, thus, are bright in sunlight. No other clouds are wispy or fibrous, so cirrus are easy to ID.
Figure 8: “Weather 101: A Tutorial on Cloud Types” reviews basic cloud names in five minutes. National Weather Service Albuquerque
This should get you started. Figure 8 is a 5 minute video that reviews some of these basics. There are a couple of apps for your phone that can help. The Field Guide to Clouds mobile app shows examples of the basic clouds for comparison, plus other info about the cloud types. A much more powerful app is the Cloud-A-Day app, a product of the Cloud Appreciation Society (more about that below). The Cloud-A-Day app lets you upload a photograph, and then has an AI-assisted tool to help you identify your cloud.
There is a huge assortment of written information on how to identify clouds. My absolute favorite is The Cloudspotter’s Guide by British author Gavin Pretor-Pinney, the founder of the Cloud Appreciation Society . He founded the society during a tongue-in-cheek lecture in 2004, and got such an overwhelming response from cloud lovers that he has expanded it into a thriving enterprise. His Cloudspotter’s Guide gives accurate, detailed descriptions of clouds and cloud physics, interspersed with fascinating stories about humanity’s interaction with clouds in history and art. It is a delightful read, and I cannot recommend it enough.
The authoritative definitions of cloud genera, species, varieties and accessories are in the International Cloud Atlas , published by the World Meteorological Organization . The cloud classification summary is particularly useful .
Even if you know all the names (maybe you are a pilot), correctly identifying a cloud is no child’s game. Sometimes a cloud’s appearance will fit multiple definitions, at which point you’ll need more data to figure out what they are best named. For example, you might walk out into the rain, and not be able to tell from just looking whether you are under a benign nimbostratus or a dangerous cumulonimbus until you see lightning. You might have to watch for a while to see how the cloud is developing; is it growing, or shrinking? Is it moving, or just seeming to move or not move? Is it changing shape, from a sheet to cloudlets or vice versa? What altitude is it at? Is that altitude changing? Is there precipitation happening? Does the season make one or another cloud more likely? But the most important bit of data you may need is the reason the cloud is there at all.