Apart from a large amount of liquid water, another of our planet’s defining features is its persistent cloud cover. The clouds are constantly changing; they come and go every day and can either make or break your day.
But have you ever noticed the enormous diversity in size and shape of clouds? If not, you are in for a real treat.
It turns out that there are many distinct forms of clouds.
What are the main types of clouds?
Even though clouds can be of any size or shape, they tend to fall into a few basic types. Luke Howard, in his seminal work “Essay of the Changes of Clouds” (1865), put clouds into three major groups: cirrus, cumulus, and stratus.
The first, cirrus (more accurately termed “cirro-form”), derives its name from the Latin word meaning “curl of hair.” Cirro-form clouds are white, and unsurprisingly, they resemble hair. They are high clouds, tending to form at around 20,500 to 45,000 feet (6.2 to 13.7 km). They are made up of ice crystals and tend to be high, wispy clouds that show up first before a low-pressure area like a storm system in the middle latitudes or a tropical system like a hurricane. Their delicate, feathery shape comes from wind currents which twist and spread the ice crystals into strands.
The second, cumulus, more accurately referred to as “cumulo-form,” are separate clouds that look like white cotton balls. They are low-level clouds, forming at around 6,500 feet or less. They show how the air in the atmosphere moves up and down or rises because of heat. Most of the time, they appear dense and may have defined edges. Cumulus clouds tend to have a flat base, which is where the moisture in rising air condenses.
Stratus, or “Strato-form,” are named from the Latin word for “layer,” and these clouds tend to look like a blanket because they are wide and spread out. They are also low clouds and are caused by rising air that tends to happen along and to the north of warm fronts. Strato-form clouds tend to have fuzzy edges, and because they are so thin, they do not produce much rain or snow.
There is also another major kind of cloud, nimbostratus, or “Nimbo-form.” According to Howard, these are a special category for rainy clouds that are made up of three types cirrus, cumulus, and stratus. He called this cloud-kind “nimbus,” which means “rain” in Latin (“stratus” is Latin for flattened or spread out). The vast majority of rain comes from nimbo-form clouds. The base of these clouds forms at around 2,000 to 10,000 feet (609 meters to 3 km) through the deepening and thickening of an altostratus cloud, often along warm or occluded fronts. These thick clouds are often associated with frontal systems brought by mid-latitude cyclones.
If the cloud is a cumulo-form but a rain cloud, it is termed cumulonimbus (more commonly called thunderclouds). These tend to form at less than 6,500 feet (1.9 km) on hot days when warm, wet air rises very high into the sky.
There are many other “subspecies” of clouds too, but these basically fall into one of the above major categories. These subclassifications of clouds are also basically based on their height above ground level.
But more on that in the next section.
What are the 10 different cloud types?
The International Cloud Atlas from the World Meteorological Organization says that there are more than 100 different kinds of clouds. That is a lot, but you’ll be relieved to know that they can all be put into one of 10 basic types based on how they look and how high they are in the sky.
These are: –
- Cumulus, stratus, cumulonimbus, and stratocumulus clouds that are below 6,500 feet (1,981 m)
- Middle clouds, which include altocumulus, nimbostratus, and altostratus, form between 6,500 feet (1,811 meters) and about 20,000 feet (6,096 meters)
- Cirrus, cirrocumulus, and cirrostratus are types of high-level clouds that form above around 20,000 feet (6,096 m)
All well and good, but let’s dig into the clouds in a little more detail, shall we?
1. Cumulus is the “classic” cloud
Cumulus clouds are basically the ones you probably learned to draw when you were young, and a type of cumulus cloud called stratocumulus stratiformis is the most common type of cloud. When the Sun shines on them, their tops are round, puffy, and bright white, while their bottoms are flat and mostly dark.
On clear, sunny days, cumulus clouds form when the Sun heats the ground directly below (diurnal convection). They are called “fair weather” clouds because of this. They tend to show up in the late morning, grow, and then go away in the late afternoon.
2. Stratus clouds tend to form blankets across the sky
Stratus clouds look like flat, uniform layers of gray clouds that hang low in the sky. They look like a fog that wraps around the horizon (instead of the ground).
Stratus clouds form from large air masses that rise and later condense. They are common on gray, cloudy days when they might mist or drizzle lightly. If enough moisture is retained at the ground level, the cloud can transform into a nimbostratus.
3. Stratocumulus tend to be lumpy
If you took an imaginary knife and spread cumulus clouds across the sky without making a smooth layer (like stratus), you’d get something resembling stratocumulus clouds. These are low, puffy, grayish or white clouds that appear in patches with blue sky in between. When seen from below, stratocumulus clouds might look like dark honeycombs or a white blanket of stretched-out cotton. They are similar in appearance to cumulus clouds but bigger.
Stratocumulus are most likely to be seen on mostly cloudy days. They form when there is only a small amount of convection in the air and do not tend to produce much rain.
4. Altocumulus is roughly as wide as your thumb at arm’s length
Most of the clouds in the middle atmosphere tend to be what are called altocumulus clouds. You’ll be able to spot them because they look like large, round patches of white or gray in the sky or clouds that are lined up in parallel bands.
They look like the wool of sheep or the scales of mackerel fish, which is why people sometimes call them “sheep backs” and “mackerel skies.”
Interestingly, altocumulus and stratocumulus are often confused with each other. In addition to being higher in the sky, altocumulus can be identified by the size of their cloud mounds. Put your hand up to the sky and in the direction of the cloud. If the mound is the size of your thumb, it’s an altocumulus cloud. (It’s probably stratocumulus if it’s about the size of a fist.)
Altocumulus usually grow by convection, after rising damp air mixes with descending dry air and are often seen on warm, humid mornings, especially in the summer. You may also see them out in front of cold fronts, which means that cooler weather is coming. When altocumulus appears with another cloud type at the same time, a storm often follows.
5. Nimbostratus bring the rain
When the sky darkens, it is most likely because of the presence of nimbostratus clouds. They can reach from the lower and middle levels of the atmosphere and are thick enough to block out the Sun.
Nimbostratus is the most common type of rain cloud. You’ll see them when it’s raining or snowing steadily over a large area or when it’s expected to rain or snow steadily over a large area.
They form by a gradual accumulation of moisture over a large area as a warm frontal system lifts the warm and moist area higher up in the atmosphere, where it condenses. They can also form from other types of clouds, such as a descending altostratus.
6. Altostratus form mid-level sheets of clouds
Altostratus clouds look like gray or bluish-gray sheets that cover part or all of the mid-level sky. Even though they cover the sky, you can usually still see the Sun as a dimly lit disk behind them, but often not enough light gets through to make shadows on the ground. They may spread over thousands of square miles.
Altostratus tends to form before a warm front or preceding nimbostratus clouds. They can also happen with cumulus when a cold front is coming. They don’t give heavy rain, but they often signal that nimbostratus clouds – and lots of rain – are coming.
7. Cirrus clouds are made of ice crystals
Cirrus clouds are thin, white, wispy clouds that move across the sky. Cirrus clouds are made of tiny ice crystals instead of water droplets because they are found above 20,000 feet (6,096 m), where it is cold, and there is little water vapor.
Cirrus clouds usually form when the weather is nice. They can also form ahead of warm fronts and big storms like nor’easters and tropical cyclones, so seeing them can also mean a storm is coming.
“Mares’ tails (cirrus) and mackerel scales (cirrocumulus) make lofty ships to carry low sails,” says an old proverb that sailors learned to tell them when rainy weather was coming.
8. Cirrocumulus are basically mini-clouds
Cirrocumulus clouds are small, white patches of clouds often arranged in rows that live at high altitudes and are made of ice crystals. The small cloud mounds of cirrocumulus are called “cloudlets,” and they often look like grains. They are much smaller than the cloud mounds of altocumulus and stratocumulus.
Cirrocumulus clouds don’t bring rain and don’t last long, but you can see them in the winter or when it’s cold but clear.
Cirrocumulus clouds can look similar to altocumulus clouds but are a more uniform color. Cirrocumulus clouds can come after cirrus clouds during a warm frontal system, but they do not interact with other types of clouds to form larger clouds.
9. Cirrostratus can form halos around the Sun
Cirrostratus clouds are clear, white clouds that cover or almost completely cover the sky. They have a sheet-like appearance and are quite translucent. To spot them if the right time of day or night, look for a “halo,” which is a ring or circle of light around the Sun or Moon.
This is a sure sign that you have cirrostratus. These halos are made when the Sun’s light bends around the ice crystals in the clouds. This is similar to how sundogs are made, but the halo is a full circle instead of just two sides.
Pure white cirrostratus clouds are a sign that there is a lot of water in the upper atmosphere, indicating that a warm front is coming and that rain is expected.
Cirrostratus clouds can turn into altostratus clouds if they descend to a lower altitude and most commonly move in a westerly direction.
10. Cumulonimbus tend to look like anvils
Cumulonimbus clouds are one of the few types of clouds that cover all three layers. They look like cumulus clouds, which are where they come from, but they rise up into towers with tops that bulge out like cauliflower. The tops of cumulonimbus clouds are always shaped like an anvil or a plume. Usually, their bottoms are cloudy and dark.
Cumulonimbus clouds are thunderstorm clouds, so if you see one, you know that wet weather is on the way (often short but heavy periods of rainfall, hail, and possibly even tornadoes).
Cumulonimbus clouds are particularly common in the afternoons of summer and spring months when the Earth’s surface releases heat.
Is fog a kind of cloud?
In short, yes, it is.
Fog is, technically speaking, a special kind of stratus cloud. Yet it is important to note that fog can also vary in form and kind.
Fog is caused when water vapor condenses and when relative humidity reaches nearly 100%. Most of the time, fog rolls in when the air cools to the dew point. That cooling could happen on a calm, clear night. The heat that has built up during the day will radiate from the ground, making it cooler near the ground.
Fog generally forms as water vapor condenses around microscopic particles such as dust, air pollution, and salt (at sea).
It could get so cold that it would start to rain. This kind of fog is called “radiation fog” because it is caused by radiational cooling, which is when heat from the ground cools the air. This kind of fog often forms in valleys because that’s where the wind is most likely to be calm. If the wind is gusty, this kind of fog won’t form because the air stays mixed and won’t likely cool to the dew point as quickly.
When warm air moves over a colder surface, it makes another kind of fog. When the warm air hits the cold surface, it will cool down, and when its temperature drops to the dew point, fog will form. During a winter thaw, when warm air flows over a frozen or snow-covered surface, this fog often forms.
The way fog forms over the ocean and along the coast are often the same. Warm air flows over the surface of the ocean, which is cold. As soon as the air cools to its dew point, fog will cover the surface of the ocean and the shores around it.
If the air is tropical and has a high dew point and a lot of water vapor, it won’t take much to cool it down to its dew point. (Of course, it also helps if the water is cold.) Fog along the coast is most common in early summer. All of these types of fog are called “advection fog” because warm air moves to colder places.
How high up are different cloud types?
The Arctic Circle (approximately 66.30°N) in the Northern Hemisphere and the Antarctic Circle (approximately 66.30°S) in the Southern Hemisphere are used to divide the Polar Regions from the Temperate Regions. The Tropic of Cancer (approximately 23.44°N) in the Northern Hemisphere and the Tropic of Capricorn (approximately 23.44°S) in the Southern Hemisphere mark the boundary between the Temperate and Tropical Regions.
The exact line between these areas changes from day to day and from season to season. In both hemispheres, the jet stream runs roughly between the Polar Regions and the Temperate Regions. The Sub-Tropical Jet Stream runs roughly between the Temperate Regions and the Tropical Regions.
The height of the top of the troposphere varies with latitude and by season. The troposphere, in case you are not aware, is effectively the lowest region of the atmosphere and accounts for roughly 80% of all mass of the atmosphere.
In general, as the height of the tropopause goes down, so do the heights at which clouds form.
The only exception is for low clouds, whose cloud bases are generally found within the first 6,500 feet (2,000 meters) above the ground in each region.
Cumulus and cumulonimbus clouds sometimes have bases that are higher than 6,500 feet (2,000 meters). In the non-mountainous parts of the southwest United States during the summer, the base of these convective clouds will be well into the mid-level cloud range.
Cumulus clouds have been seen with bases as high as 9,000 feet (2,750 meters) over North Central Texas, and thunderstorms have been seen with bases as high as 11,000 to 12,000 feet (3,350 to 3,650 meters) near San Angelo, Texas.
This happens when, even though the air near the ground is dry, the air in the middle levels is pretty moist and unstable. Because the lower level is so dry, air needs to rise up to two miles (3 km), and sometimes even more, before it cools enough to condense.
How do clouds form?
A cloud is defined as “a visible aggregate of minute droplets of water or particles of ice or a mixture of both floating in the free air.”
Each droplet has a diameter of about a hundredth of a millimeter, and there are around 100 million droplets in every cubic meter of air. The droplets are so small that they can stay liquid even when the temperature is -22°F (-30 °C). When they do reach these low temperatures, we tend to call those drops “supercooled droplets.”
Ice crystals, which can be about a tenth of a millimeter long, make up clouds in the upper, colder parts of the atmosphere.
When water vapor in the air condenses into water droplets or ice crystals, clouds form. For this to happen, the air must be saturated, which means it can’t hold all the water it has as vapor, so it starts to condense out into a liquid or solid.
There are two main ways to reach a point of saturation.
The first is adding so much water to the air, such as through evaporation, that the air can’t hold anymore.
The second is by cooling the air until it reaches its dew point, which is the temperature at which condensation happens, and the air can’t “hold” any more water.
This occurs because the air can only hold a certain amount of water vapor at a certain temperature. In general, the air can hold more water vapor when it is warmer. So, a lower temperature makes the air less able to hold water vapor, which leads to condensation.
This is the most common way clouds form, and it has to do with air rising in the lower atmosphere. As air rises, the lower pressure in the air causes it to expand.
The energy used to expand causes the air to cool. In general, air will cool by 1.8 °F (1 °C) for every 330 feet (100 meters) as it rises. The rate of cooling will depend on how much water is in the air or how humid it is. Air that is moist may cool more slowly, for example, at a rate of 0.9°F (0.5 °C) per 330 feet/100 meters.
So, as air rises up, its ability to hold water vapor decreases. This leads to condensation. The condensation level is the height at which the dew point is reached, and clouds start to form.
There are five things that can cause air to rise and cool down:
- Heat on the surface: The Sun heats up the ground, which warms the air around it and makes it rise. Most of the time, the columns that rise are called thermals.
- The land: When there are mountains or hills in the way, the air is forced to rise. It’s called “orographic uplift.” Orographic means relating to topographic features like hills or mountains, etc.
- Frontal: A mass of warm air rises above a mass of cold, dense air. A “front” is the name for the boundary.
- Convergence: When two streams of air from different directions meet, the air is forced to rise.
- Turbulence: A sudden change in the speed of the wind with a height that makes the air turbulence.
Another important thing to consider is that water vapor needs something to condense around in order to turn into liquid water. There are millions of little salt, dust, and smoke particles floating in the air.
These tiny particles are called “condensation nuclei“, and they allow condensation to happen when the air has reached saturation.
Amazing really.
And that, nephophiles (someone who loves clouds), is your lot for today. Now you are equipped with all the basic information you’ll need to identify most clouds you’ll ever see in your normal day-to-day.
So, why not show off to your friends and family by pointing them out next you are out and about?
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