Let’s Talk About Clouds: An Introduction
I have always been fascinated by the clouds, and I have fond memories of laying in the grass and identifying shapes and figures in the clouds as a child (although to be honest, I still try to spy fun shapes in the clouds from time to time!). The towering cumulus clouds that I saw so frequently growing up in southern Alabama were the reason I decided to take my first meteorology course, and I know that I’m not the only one enchanted by their majestic beauty - there are millions of cloud posts on Instagram from across the world!
I think what intrigues so many of us about clouds is their innate complex and varied nature. They can grow tens of thousands of feet tall, be lumpy and bumpy or flat as a pancake. They provide relief from the heat of direct sunlight, but they can also trap warmth - leading to higher temperatures. They can be carried by winds at speeds up to 150 miles per hour - or they can remain stationary as the winds pass through them.
So what are clouds? Clouds are a visible collection of very small water droplets or ice particles in the atmosphere. They can weigh tens of millions of tons yet still float in the atmosphere. They can reach heights up to 60,000 feet in Tropical regions, 45,000 feet in Temperate regions, and 25,000 feet in Polar regions. An example of the three regions is shown below, though it is important to note that the division between these regions varies from day to day and season to season based upon locations of the jet and sub-tropical jet streams.
How do clouds form?
For a cloud to become visible, only two ingredients are necessary - water and nuclei. Water is always present in the atmosphere in one form or another, but water molecules are too small to bond together to form cloud droplets. They need an object with a radius of at least one micrometer that they can bond with - these are called nuclei.
The relative size of water molecules to condensation nuclei. Courtesy NOAA NWS JetStream.
Nuclei are very small solid and liquid particles, made up of things such as tiny specks of wind blown soils, ocean spray, or smoke particles from fires. These nuclei are hygroscopic meaning they attract water molecules. Called "cloud condensation nuclei", these water-molecule-attracting particles are about 1/100th the size of a cloud droplet upon which water condenses. Therefore, every cloud droplet has a speck of dirt, dust or salt crystal at its core. But, even with a condensation nucleus, the cloud droplet is essentially made up of pure water.
However, having water attracting nuclei is not enough for a cloud to form. The air temperature needs to be near or below the saturation point. Called the dew point temperature, the point of saturation is where evaporation equals condensation. Thus, a cloud forms when a block of air (called a parcel) containing water vapor has cooled below the point of saturation. Air can become saturated in many ways, though the most common way is through lifting of air from the surface up into the atmosphere. Once the parcel reaches saturation temperature (100% relative humidity) water vapor will condense onto the cloud condensation nuclei resulting in the formation of a cloud droplet. Of course, the atmosphere is constantly moving, and cloud droplets are constantly forming and dissipating as condensation and evaporation are continually occurring.
Though most clouds form by rising air, they can also form when the air gets colder for other reasons. This can happen overnight as the air is cooling, or when warm air moves over a cold lake or a snow-covered surface.
In an ideal atmosphere the saturation level of a parcel with a surface temperature of 85°F and a dew point of 65°F will cool to the saturation point at about 4,000 feet in elevation. Courtesy NOAA NWS JetStream.
Types of Clouds
Though clouds to seem to have inifinite shapes and sizes, there are actually four main types of clouds: cirro-, cumulo-, strato-, and nimbo-.
Cirro-form - The Latin word “cirro” means curl of hair. Composed of ice crystals, cirro-form clouds are whitish and hair-like. There are the high, wispy clouds to first appear in advance of a low-pressure area such as a mid-latitude storm system or a tropical system such as a hurricane.
Cumulo-form - These are the fluffy clouds that look like cotton balls; they show us that vertical motion or the thermal uplift of air is taking place in the atmosphere. They are normally dense with sharp outlines (great for finding shapes!) The base of cumulus clouds are generally flat and occurs at the altitude where the moisture in rising air condenses.
Strato-form - This name originates from the Latin word “layer” because these clouds are generally broad and fairly wide spread, like a blanket. They result from non-convective rising air and tend to occur along and to the north of warm fronts. The edges of strato-form clouds are diffuse, unlike the cumulo-form clouds.
Nimbo-form - This is a special type of cloud which combines cumulo-, cirro-, and status clouds. “Nimbus” is Latin for rain, and the vast majority of precipitation occurs from nimbo-form clouds
There are also ten basic combinations of clouds which are combinations of two or more of the above categories; all cloud images below are courtesy NOAA NWS Jetstream. From the World Meteorological Organization's (WMO) International Cloud Atlas, the official worldwide standard for clouds, the following are definitions of the ten basic cloud types. Cirrus (Ci), Cirrocumulus (Cc), and Cirrostratus (Cs) are high level clouds. They are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.
High Clouds
Cirrus (Ci)
Cirrus clouds are detached, white, delicate clouds, mostly white patches or narrow bands. They may have a fibrous (hair-like) and/or silky sheen appearance. They are composed of ice crystals. Cirrus near the horizon are often a yellowish color, and before sunrise and after sunset, they are often bright yellow or red.
Cirrocumulus (Cc)
These are thin clouds that look patchy, like a sheet or layer of clouds that don’t provide shade. They are composed of very small elements that look like grains or ripples.
Cirrostratus (Cs)
These transparent clouds, have a smooth appearance and nearly always cover the whole sky. During the day, when the sun is sufficiently high above the horizon, the sheet is never thick enough to prevent shadows of objects on the ground. Cirrostratus nearly always produce a “halo” around the sun or moon.
Mid-level clouds
Altocumulus (Ac), Altostratus (As), and Nimbostratus (Ns) are mid-level clouds. They are composed primarily of water droplets though, they can also be composed of ice crystals when temperatures are low enough.
Altocumulus (Ac)
The most common mid-level cloud, these clouds are white and/or gray and can appear patchy, sheet-like, or layered, generally appearing as plates or rolls. They often appear at different levels at the same time and can appear with other cloud types
Altostratus (As)
These gray or bluish cloud sheets or layers totally or partially cover the sky, though they are thin enough to reveal the sun. They are too thick for shadows to be visible. Virga (precipitation which falls but evaporates before it hits the ground) can sometimes be seen hanging from these, and may even reach the ground at times, causing very light precipitation.
Nimbostratus (Ns)
This thick, dark gray cloud layer results from an altostratus layer thickening. The layer is diffused by falling rain or snow, and is thick enough to blot out the sun. The cloud base lowers as precipitation continues. Both Altostratus and Nimbostratus can extend into the high level of clouds.
Low-level clouds
Cumulus (Cu), Stratocumulus (Sc), Stratus (St), and Cumulonimbus (Cb) are low clouds composed of water droplets. Cumulonimbus, with its strong vertical updraft, extends well into the the high level of clouds.
Cumulus (Cu)
These detached, generally dense clouds with distinct outlines develop vertically in the form or rising mounds, domes or towers with bulging upper parts often resembling a cauliflower. The sunlit parts of these clouds appear mostly bright white, while their bases are relatively dark.
Cumulonimbus (Cb)
The “thunderstorm” cloud, this heavy and dense cloud appears as a mountain or huge tower. The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume. The base of the cloud is often very dark, with low ragged clouds that may or may not touch the base. These clouds can produce precipitation, hail, and tornadoes.
Stratocumulus (Sc)
These clouds appear gray or white and in patches, sheets, or layers and almost always a dark honeycomb appearance with regularly arranged, small elements.
Stratus (St)
These clouds are generally gray, with a uniform base. They may produce drizzle, ice prisms, or snow grains. When the sun is visible through this cloud, its outline is clearly discernible. Often when a layer of Stratus breaks up and dissipates blue sky is seen.
Today we’ve covered the basic types of clouds, but in the future we will dive into more unique cloud features such as fallstreaks and mammatus - stay tuned!