Certain cloud types appear to have very distinct shapes and sizes. Arcus clouds are one such example, but these cloud formations may also be an indication of something much more ominous approaching.

Under the right conditions, many cloud formations make for some of the most striking photographs you can take. Some clouds, though, are so visually impressive and awe-inspiring that a picture doesn’t do it justice. Arcus clouds are one such example.

This article focuses on what arcus clouds are, how they form, as well as the different types of arcus clouds.

Arcus cloud formations may be responsible for some of the most spectacular views one can observe from the planet’s surface, but there is more to this meteorological phenomenon than merely its appearance.

Before we look at its development and characteristics in more detail, we first need to establish what arcus clouds are:

Arcus Cloud Definition

Arcus cloud is the umbrella term used in meteorology for low-lying accessory clouds that spread out horizontally, usually from the boundary of a larger storm system, like a thunderstorm. These cloud formations have a visually striking appearance and typically form as shelf or roll clouds.

The summary provides a concise but cryptic definition of what an arcus cloud is and how it forms. A broader definition is needed to fully understand this type of cloud formation.

Arcus clouds are low-altitude clouds with the cloud base forming at a height of approximately 2 kilometers (6500 feet).

Arcus clouds are divided into two main categories: Shelf Clouds and Roll Clouds. As a result, arcus clouds are either wedge-shaped or in the shape of a horizontal tube-shaped column (depending on whether a shelf or roll cloud develops).

The two major types of clouds associated with the formation of arcus clouds are cumulus and cumulonimbus. Notably, the severe updrafts and downdrafts present in cumulonimbus clouds are responsible for the creation of many spectacular arcus formations.

Unlike the cumulonimbus clouds that form the basis for many arcus clouds, arcus clouds themselves develop and spread out in a horizontal fashion.

Arcus clouds pose no direct danger in the form of precipitation or strong winds, but in many cases act as a precursor to approaching thunderstorms and severe weather.

Although the two types of arcus clouds, shelf and roll clouds, have unique characteristics and a specific way in which each one develops, they both have a similar origin.

Cold air flows out from the storm front and spreads horizontally, forcing the warmer air at the surface upward. The arcus cloud forms along this boundary of rising warm air and advancing cold air.

The primary way in which arcus clouds form can be summarized in the following steps:

1. 1. Thunderstorms are characterized by strong updrafts and downdrafts in the storm cloud. It is the strong downdrafts present at the leading edge of a thundercloud that are primarily responsible for the creation of arcus clouds.
   2. Cold air, cooled by altitude and precipitation, is carried to the ground by downdrafts, from where it spreads out horizontally in front of the storm system.
   3. The heavier cool air spreads out quickly over the ground and pushes underneath the warmer, moist air, lifting it into the atmosphere.
   4. As the warm air rises and cools down, condensation takes place, which leads to the formation of arcus clouds with their unique shape and characteristics.
   5. Depending on the specific atmospheric conditions and location, this process leads to the formation of wedge-shaped shelf clouds or cylindrical roll clouds.

Both shelf clouds and roll clouds have a unique appearance with characteristics of their own, which we will address in the following section.

As stated in the summary, arcus clouds can be divided into two primary types of formations:

• 1. Shelf Clouds
  2. Roll Clouds.

These two types of arcus clouds may have a similar origin, but appear substantially different, with distinct characteristics that can best be understood by looking at each cloud formation individually.

The most common type of arcus cloud is the ominous-looking shelf cloud that usually precedes large thunderstorms. When discussing arcus clouds, this is the type of cloud formation that generally springs to mind. 

A shelf cloud is a type of arcus cloud that is characterized by its visually striking wedge shape and horizontal development. It typically develops at low altitudes from the leading edge of its parent cloud, known as a cumulus congestus cloud.

Shelf clouds usually develop out of the parent cloud, called a cumulus congestus cloud. Congestus clouds are cumulus clouds that are taller than they are wide, in other words, with strong vertical development. They are also often a precursor to cumulonimbus clouds.

The familiar ragged-looking underside of a shelf cloud is a result of turbulent winds, as well as wind shear caused between the updrafts and downdrafts.

Arcus clouds in the form of shelf clouds are synonymous with a derecho, a widespread and potentially devastating storm system. You can read all about it in this article.

Shelf clouds appear at the leading edge of this dangerous storm front, and although they pose no danger themselves, their appearance points to the looming threat of the derecho that follows close in its footsteps.

As a shelf cloud passes overhead, it is usually followed by a dark, tumultuous section of sky commonly known as the whale’s mouth in meteorological circles. This stretch of weather slots in between the appearance of shelf clouds and the arrival of the thunderstorm.

(The dark, turbulent stretch between the edge of a shelf cloud and a thunderstorm is sometimes characterized by a distinct wavy appearance, known as asperitas clouds. These clouds don’t appear that often, but are almost as spectacular as the shelf cloud itself.)

The formation of a shelf cloud is identical to the process described earlier in this post. From the fifth point, though, the process is unique to the development of shelf clouds:

As the cold air from the outflow boundary (leading edge of the storm) moves forward, it tilts the rising warm air along its boundary.

It is along this boundary between the warm updrafts and cold downdrafts that shelf clouds form. As the warm air continues to rise along this boundary, it cools down, and condensation takes place, which results in the formation of a shelf cloud.

A lesser-known type of arcus formation is called a rolling cloud. It is the second of the two types of arcus clouds.

Although a roll cloud is classified as an arcus cloud, it differs significantly from its more famous cousin, the shelf cloud. To understand its unique characteristics, it first needs to be defined more clearly:

A roll cloud is a rare type of arcus cloud characterized by its round, tube-shaped formation, which forms at very low altitudes and appears to rotate on its horizontal axis. It acts as a single wave known as a soliton and, unlike a shelf cloud, develops completely independently from other clouds.

As already stated, the appearance of roll clouds is rare. It is no less spectacular than well-documented cloud systems, but it remains an uncommon occurrence. The biggest reason for their scarcity is that the weather conditions have to be close to perfect for them to occur.

In the WMO’s Cloud Atlas, it is now officially called volutus clouds. Although it is still viewed as a type of arcus cloud, the World Meteorological Organization (WMO) recently classified it as an entirely separate cloud species.

What makes a roll cloud unique is that it appears as a solitary, independent cloud, completely detached from any parent cloud. On very rare occasions, one can view several roll clouds appearing in succession.

Roll clouds also act like a soliton when it comes to their motion. A soliton is a single wave with a single crest that moves ahead without changing its speed or size (which sums up the movement of a roll cloud).

It is evident that roll clouds have characteristics that distinguish them from shelf clouds, even though they still fall within the same family of clouds.

Although the formation of a roll cloud follows the same process described earlier in this post, it also differs in a significant way:

A roll cloud forms completely independently of any larger cloud system. In many cases, it occurs without any other significant cloud development in sight.

Although not physically attached to a parent cloud, a roll cloud still forms at the leading edge or gust front of a storm system.

In some instances, the downdrafts that accompany a storm system form some distance in front of the storm cloud’s edge. As a result, a roll cloud can form at the boundary between updrafts and downdrafts without the presence of the larger storm cloud.

Sometimes, though, a thunderstorm will clear up and dissipate completely, leaving only the updrafts and downdrafts behind. In turn, they can lead to the formation of roll clouds in otherwise clear and fair weather.

Arcus clouds are fairly easy to identify once you know what to look for. They typically appear as low, horizontal cloud formations along the leading edge of a storm system.

Shelf clouds are wedge-shaped and attached to a larger storm cloud, often stretching across the horizon like a dark, advancing wall.

Roll clouds, on the other hand, appear as long, tube-shaped clouds that seem to rotate along their horizontal axis and are often detached from other cloud formations.
In both cases, their sudden appearance and dramatic structure are often a clear indication of approaching unstable weather conditions.

It is clear that all arcus clouds follow a similar pattern when it comes to their development and the factors involved in the process. The physical manifestations of the two types of arcus clouds, however, are dramatically different.

Shelf clouds develop their familiar ragged wedge-shaped form, while roll clouds are characterized by their round, tube-shaped formation. Shelf clouds further develop at the leading edge of a storm cloud, while roll clouds form independently from any other cloud.

In conclusion, this article focused on explaining what an arcus cloud is, how it develops, and highlighting the different types of arcus clouds.

Until next time, keep your eye on the weather!