Arcus Clouds: What They Are and How They Form

To us, clouds always look like giant pillows in the sky. Arcus clouds are the very definition of a comfortable pillow-looking cloud. Clouds can look dreamy and light, or moody and atmospheric.

What is certain, though, is that whether they are light or dark, dreamy or moody they all make for great photographs! 

Have you ever wondered what it is that makes clouds different? You’d be forgiven for thinking this: “I mean, aren’t all clouds just clouds?”. We totally understand why you would think that!

arcus cloud formation

You may be surprised to find out that the answer is no. All clouds are different. They could have different shapes, different colors, and may even be formed differently.

As you may have guessed by the title, in this article, we will tell you all about the wonderful Arcus cloud and what makes it so different from the others. 

Of course, all clouds come in different shapes and sizes, and all clouds are beautiful, but there is nothing so photogenic as the long rolling cylindrical shape of the Arcus cloud formation.

These low lying clouds have been a favorite of photographers for as long as cameras have been around, and when you see one it is easy to understand why. 

As we have already mentioned, the focus is going to be on arcus clouds. We are going to tell you all about them, explaining what they are and how they form. So sit back, relax, and imagine you are floating on a fluffy cloud as you read our informative article. 

What is an Arcus Cloud? 

Arcus clouds, as we said in the introduction, are a low lying cloud. They form a horizontal shape in the sky and tend to be an accompaniment of another type of cloud called Cumulonimbus.

There are two types of Arcus clouds. These are called shelf clouds and roll clouds. Roll clouds are the most distinctive ones, forming the long cylinder shape we mentioned earlier in the article.

They look rounded and neat. Shelf clouds have a fluffier appearance, looking more like a typical cloud. Because Arcus clouds lie low, they are very atmospheric to look at. Two explore what they are in more detail, it will be most helpful to explore both roll and shelf clouds separately. 

Roll clouds

As we mentioned earlier in the article, roll clouds have that very recognizable cylindrical shape. They are very neat in appearance.

As clouds go, this type is very rare (which is a shame when you think about how cool it looks!) because the weather conditions that are associated with it need to be perfect to get them to appear.

It’s like waiting for a rare bird sighting, or waiting for Christmas as a kid! They are so rare, in fact, that they now have their own classification. They are what are known as Volutus clouds.

They are still a type of Arcus cloud, but because of their appearance as a single independent cloud, they have been given their own cloud specification by the World Meteorological Organization. Unlike the other types of Arcus clouds, these ones are not attached to a parent cloud. 

Shelf clouds

Shelf clouds are another type of Arcus clouds. They are far more common than Roll clouds, but nonetheless, they also have an impressive appearance. These clouds are wedge shaped, and typically they are followed by dark tumultuous weather, such as those seen in storms.

Like roll clouds, these are also horizontal in shape. Unlike roll clouds, however, shelf clouds are generally attached to a parent cloud, usually cumulus congestus clouds.

They tend to have ragged ‘fluffy’ looking edges, especially on the underside of them. This raggedness is caused by the winds that tend to accompany them. 

Now that we know exactly what an Arcus cloud is, and the two main types of them, we can explore how they are formed. 

How do Arcus Clouds form?

To get a better understanding of Arcus clouds, it is important to look at how they form.

As we have mentioned already, the type of arcus cloud known as the roll cloud (or volutus cloud if you want to get technical) is a very rare sight. On the other hand, shelf clouds are a little more common.

First, we will explore the roll cloud and how it is formed. 

Formation of a roll cloud

These rare occurrences can only form when the conditions are absolutely perfect. One of the most famous occurrences is the ‘Morning Glory cloud’ as it has been dubbed, that can be seen in Australia, in Queensland to be precise, every year in October.

They tend to occur near the coast and are a result of the downdrafts of a storm. By this, we mean that sometimes, downdrafts in a storm system can occur in front of the storm cloud.

When this happens, a roll cloud is given the perfect opportunity to form between the updraft and downdraft. They do so independently and are not attached to the storm cloud in any way.

Sometimes, this can even occur after the storm cloud has passed. This happens when a storm system leaves behind their up and down drafts, allowing the roll cloud to form even when there seem to be no other clouds around.

The presence of the roll clouds can look eerie, particularly if they are not surrounded by any other type of cloud as the storm has dissipated.

However, they are not usually a sign of any inclement weather, and should not be a cause for concern but instead a cool phenomenon that you can happily post on your social media! 

Formation of a shelf cloud

Shelf clouds differ from roll clouds, even though they are both technically Arcus clouds. Whilst roll clouds look ominous but are actually harmless, shelf clouds do typically signify something a little more serious.

They are synonymous with a type of storm known as a derecho. Anyone who has experienced a derecho will know just how much of a bad omen these shelf clouds can be! Derechos are a very destructive type of storm that can cause a lot of damage to homes, landscapes, and even human life.

Shelf clouds can appear at the beginning of these storm systems, and whilst the clouds themselves can cause no harm, they do herald bad news when they are accompanied by these storm clouds. 

However, it is not all doom and gloom (although these clouds do look rather gloomy!). Seeing a shelf cloud formation does not necessarily mean that you will definitely experience a derecho.

In the most simple terms, shield clouds are a type of arcus that form when air that has been cooled by the rain meets warm, moist air. This rain cooled air pushes the warm, moist air out of its way to make room for it (we imagine the air as a sassy icon shouting “move it, warm air, cool air is coming through!”, you don’t have to imagine that but it helps).

The meeting of these two types of air then forms the cloud. Typically they form as part of the bigger cloud system called cumulonimbus, but they can also be seen along the gust fronts of the cumulus cloud formation. 

If you happen to see a shelf cloud, the sort of weather you can expect is very different from the clear weather that can be seen with a roll cloud. Expect dark, gloomy skies, and then if not a full blown storm, you will certainly get wind followed by rain which is often heavy. 

Conclusion

...And that concludes our cloud lesson! Arcus clouds, as we are sure you will agree after reading this article, are a very impressive cloud formation. We are sure you are eager to get cloud spotting now that you know all about these photogenic feats of nature. 

To recap, remember, arcus clouds come in two different types: roll clouds and shelf clouds. Both of these types of arcus cloud are very different, but both equally wonderful to look at, and even more wonderful when you understand how they are formed.

Roll clouds are the rarer of the two and are usually associated with coastal regions of the world such as the coast of Queensland in Australia, the English Channel, the Shetland Islands, and California.

If you are ever lucky enough to see one, don’t you worry, we know they look ominous but they are typically harmless and make for a great photo opportunity. 

Shelf clouds are a little different. These wedge shaped formations are typically bearers of bad news. Whilst they are awesome to look at, they can often signify that bad weather is on its way so please take care should you ever notice one when you are out and about.

They look super eerie though, especially when followed by dark, tumultuous skies, so be sure to snap a quick photo if it is safe to do so. 

We hope you enjoyed learning about Arcus clouds today - Happy Cloudspotting!

About the Author Marvin J. Snyder

I'm the research analyzer and data interpretation here at Weather Station Lab. I test various weather stations and share my conclusion here. Since my childhood, I had a passion towards weather and I'm always fascinated by that. Eventually, I pursued Ph.D. in Atmospheric Science from the University of Arizona. I hope my contribution will help you to know more about weather stations. Read more about us, here