We often briefly glance at the sky in the morning to see how cloudy it is before heading off to work, school, or college. However, different cloud types can tell us more about the weather than we may think.

Some cloud formations can look almost identical to the casual observer. To be honest, they can even be confusing for some seasoned weather enthusiasts.

And to make things more confusing, you have the odd occasion where you experience light rain or drizzle without a cloud in sight. This phenomenon is called a sunshower or serein.

After doing some research, one will also find several different classification systems used for categorizing or grouping different types of clouds together.

The World Meteorological Organization (WMO) classifies clouds into 10 different types, known as genera. They are further grouped according to the altitude in the atmosphere at which they usually occur, namely high, middle, low, and multilevel clouds.

This is the system we will be using for describing the different cloud types in this article.

Before we delve deeper into the different types of clouds and the characteristics that make them unique, we first need to define what exactly a cloud is:

Cloud Definition​​​​​​​​​​

​A cloud is the visible physical manifestation of millions of small water droplets or ice crystals, or a combination of both, suspended in the air, which forms after water changes from its gaseous state into its liquid or solid state. Clouds occur at different altitudes and in different forms, depending on how and where they develop.

Clouds are mainly the result of evaporation and condensation. There are several ways in which these processes can take place, but one of the most common patterns they follow occurs as follows:

As the Sun heats the surface of the Earth, over land or water, it also warms up the air directly above it. Since warm air is lighter than cold air, it starts to rise into the atmosphere as it expands. The heat also causes moisture on the ground and surface water to evaporate, adding to the water vapor already present in the air.

Since air temperature drops with altitude, the rising warm air starts to cool down. Once it cools down to the point where the water can no longer stay in its gaseous form, condensation takes place, and small water droplets and/or ice crystals form, depending on altitude and temperature.

The clouds we observe are a manifestation of these millions of water droplets and ice crystals suspended in the air.

We use these four groupings to organize the different cloud types as follows:

1) High-level clouds

• • • Cirrus Clouds
    • Cirrocumulus Clouds
    • Cirrostratus Clouds

2) Mid-level clouds

• • • Altocumulus Clouds
    • Altostratus Clouds

3) Low-level clouds

• • • Stratus Clouds
    • Stratocumulus Clouds

4) Clouds with vertical development / multi-level extent

• • • Cumulus Clouds
    • Cumulonimbus Clouds
    • Nimbostratus Clouds

By looking at each cloud type individually, one will gain a better understanding of its characteristics and development.

Clouds are classified as high-level clouds when they occur at altitudes of 6096 meters (20 000 feet) or higher. At these heights, temperatures are below freezing point, meaning the moisture in the clouds is in the form of ice crystals or supercooled water droplets.

(Supercooled water droplets are water droplets that remain in liquid form below freezing point. Upon contact with ice crystals or any other object like dust or pollen, however, they immediately freeze into crystal form.)

Due to the height of these clouds, they don’t produce precipitation of any kind that reaches the ground. They are, however, sometimes seen as early indicators of stormy weather to follow later.

The clouds most commonly found at this height are Cirrus, Cirrocumulus, and Cirrostratus clouds:

Cirrus clouds are the familiar thin, feathery-looking clouds you see high up in the sky on an otherwise clear and sunny day. Some parts of these flaky clouds have an almost transparent look due to their light nature.

As is common with many high-level clouds, they often occur during clear and pleasant weather conditions. However, as I already pointed out, they may be early indicators of stormy weather or warm fronts.

Cirrocumulus clouds are a variation of Cirrus clouds. They are patchy-looking clouds which are often arranged in rows. Some meteorologists see them as a degraded form of Cirrus clouds.

These high-altitude clouds are small and very short-lived and are sometimes referred to as cloudlets. As with Cirrus clouds, they appear and are associated with clear and pleasant weather conditions.

Cirrostratus clouds can be best described as a transparent veil of clouds. Unlike the two previously mentioned high-level clouds, they have a very smooth appearance.

The veil of milky-looking clouds can sometimes cover almost the entire sky. Due to the refraction of light by ice crystals, these clouds form the rainbow-colored halos we often see around the Sun.

These clouds are an indication of high moisture levels in the upper atmosphere, which often precedes the arrival of a warm front.

Mid-level clouds usually occur at altitudes of between 2000 meters (6 500 feet) and 6096 meters (20 000 feet).

As they appear much lower in the atmosphere, some condensation occurs above the water’s freezing point. This means middle clouds contain a mixture of ice crystals and water droplets.

Apart from appearing lower in the atmosphere, they are also denser than higher-level clouds. This means they are less prone to being transparent and allowing sunlight through.

Middle clouds seldom produce any rainfall. They do, however, sometimes create what is called virga. (Virga is rain or snow that starts falling but evaporates before reaching the ground.)

Altocumulus clouds are a very common sight across the world. They are characterized by their woolly, round or oval-shaped appearance. These patchy clouds have a white to light gray appearance and are sometimes formed in parallel rows.

Often observed during warm and humid mornings in the middle levels of the atmosphere, these clouds can signal the onset of thunderstorms or cold fronts. The time of year and your location will determine the type of weather to expect.

The uniform, grey blanket of cloud cover that often fills the entire sky is a trademark feature of Altostratus clouds.

These mid-level clouds are much denser than the similarly shaped Cirrostratus clouds found higher up in the atmosphere, meaning they are less transparent and don’t allow shadows to be cast on the ground. They are still thin enough to be able to see the sun through them.

Altostratus clouds are frequently associated with light rain, but due to their height and density, they are not able to produce heavy rain.

Low clouds are formed at altitudes below 2000 meters (6 500 feet). At these lower heights, the clouds consist mainly of water droplets.

(Cold winter months with subzero temperatures are the exception when you will find ice crystals present in these low-altitude clouds.) 

Stratus clouds are made up of thin layers of clouds that are formed close to the ground. They are mostly featureless with a grayish color. 

One of their standout features is taking up large portions of the sky at a time. (Often stretching from horizon to horizon.)

Stratus clouds are closely related to fog. In fact, fog is nothing more than a form of stratus cloud that forms at ground level. 

The precipitation associated with these dreary-looking clouds mostly consists of mist or a light drizzle.

These low-lying, puffy-looking clouds are spaced closely together, with small pieces of blue sky visible in between them. When viewed from below, they have a honeycomb appearance.

With colors ranging from white to grayish, and their tendency to cover substantial parts of the sky, people often associate rain with these clouds.

In reality, stratocumulus clouds are pretty benign when it comes to precipitation. A light drizzle may be the most you will get out of this cloud system. 

Multi-level clouds are clouds that have a large vertical buildup. They are called multi-level clouds because of their ability to spread through the lower, middle, and upper cloud levels.

The clouds are characterized by vertical air movements called updrafts. These vertical currents can spread moisture upwards through the cloud system into the upper regions of the atmosphere.

The combination of updrafts and downdrafts in multi-level clouds creates an environment that can result in severe weather events. This includes heavy rainfall, hailstorms, and even tornadoes that form within multi-level clouds.

Not all multi-level clouds develop to this extent, though, and can be completely benign, as our first example will illustrate.

The light, puffy-looking clouds scattered across the sky are arguably the most well-known of all the cloud types. (It’s probably the first image that comes to mind when you think of a cloud.)

They are instantly recognizable with their white, fluffy, round tops and flat bottoms. They are fairly evenly spread out with a fair amount of blue skies visible between them. (Their shape is often compared to that of a cauliflower.)

Cumulus clouds often appear during sunny days early in the day and disappear towards the evening. With little or no precipitation associated with them, they are often referred to as “fair weather clouds”.

Starting out as a humble cumulus cloud, strong vertical air movement (updrafts) combined with enough moisture in the air allows this type of cloud to develop. Cumulonimbus clouds are seen as your typical storm clouds.

They start at a low cloud level and can grow and expand up to the highest level. It is within this space, dominated by updrafts and downdrafts, that all the elements necessary for the development of a storm system are formed.

When viewed from a distance, cumulonimbus clouds appear to have a low dark base, with the clouds above it building up to great heights, creating a spectacular towering effect.

The lower levels of a cumulonimbus cloud consist mainly of water droplets, while the upper level, where temperatures are well below zero, consists mainly of ice crystals and supercooled water.

As far as precipitation goes, these clouds are known for producing heavy rainfall and hailstorms. They are also responsible for producing violent winds, and it is within this cloud system that tornadoes can occur.

Nimbostratus clouds typically cover the entire sky. It is a dark, thick layer of clouds, capable of completely blotting out the sun. 

Starting at a low level and building up in height, the clouds are usually loaded with moisture and associated with long periods of persistent rain or snowfall. That is why it is known as your typical rain cloud, with the precipitation usually spread out over a large area.

Alongside cumulonimbus clouds, nimbostratus clouds are almost guaranteed to provide the area they cover with a substantial amount of precipitation. 

However, they do not have the uniquely identifiable shape of cumulonimbus clouds, and it’s harder to judge where the rainfall will take place due to the large area it covers.

Breaking it up into proper categories helps you to better distinguish the different clouds from one another. You should now be able to start telling the major cloud systems apart.

You will be forgiven for still finding it difficult to tell certain clouds apart. Some of them are almost indistinguishable under some conditions. Even experienced observers still have a hard time sometimes telling them apart.

But you know what they say, “Practice makes perfect”.  

In a separate article, I go into a little more detail and take you through the steps to help you identify cloud systems. I also explain how you can use your cloud knowledge alongside your home weather station to make better forecasts. You can find that article here.

Until next time, keep your eye on the weather!