The Formation and Types of Clouds

Clouds have often been the source of prophecies and daydreams. They are beautiful, mystical, and fun to watch during a lazy afternoon. However, the formation and classification of clouds is a very scientific process.

The Formation of Clouds

Clouds are formed when water evaporates and rises into the atmosphere. As it rises, the moisture encounters cooler air and begins to condense. The water vapour attaches itself to any particle that might be in the air, such as dust, salt, or other debris. Once the vapour has cooled and enough water is present (the air mass becomes saturated), the cloud becomes visible to the naked eye. There are four processes that can achieve this, causing clouds to develop.

Orographic uplift is a process that occurs when air is forced to rise because of the physical presence of elevated land. As the air rises, the moisture condenses, causing clouds to form. This often occurs in more mountainous regions.

Convectional lifting is associated with a build-up of heat in a particular air mass. If an air mass becomes warmer and lighter than the surrounding air, it will begin to rise, expand, and cool. Clouds form when the air becomes saturated with moisture. Cumulous and cumulonimbus clouds are often formed in this fashion, and convectional lifting often occurs near the equator and in the interior of continents.

Frontal lifting (sometimes called convergence) happens when two masses of air collide. If one of these is cool and on is warm, then the warmer mass will typically rise. Of course, as this happens, the warm mass expands and cools, causing saturation. This often occurs at mid-latitudes, often causing the formation of cyclones.

Radiative cooling occurs when the surface of the Earth begins to lose its heat (such as at night). This heat is lost in the form of longwave radiation. The ground and the air directly above it begins to cool, but the air above is still warmer. Saturation usually occurs fairly quickly. This usually results in the formation of fog, which is itself a type of cloud.

Types of Clouds

Clouds are divided into three main categories based upon the process by which they are formed and the physical structure the cloud takes. Cirrus clouds are high altitude clouds and occur mostly as filaments. Stratus clouds tend to be found quite close to the surface of the earth. These clouds almost look like sheets. The best-known type of cloud is the cumulus cloud, which appears rippled, heaped, or rolled. These are the fluffy white clouds that are so often seen. Like stratus clouds, cumulus tend to be low-lying, though they can be found as high as 10,000 feet.

High-Level Clouds

When studying clouds and their formations, it is often easier to break them into groups depending on the altitude at which these formations occur. High-level clouds are considered to be those which occur at more than 20,000 feet above sea level. There are three main types of high-level clouds. These are: cirrus clouds, cirrocumulus clouds, and cirrostratus clouds.

Cirrus clouds, often abbreviated Ci, form in the coldest and highest regions of the troposphere. Water almost always freezes when it reaches this height, and so these clouds are usually formed of ice crystals. Cirrus clouds are wispy in appearance and are very often transparent. A single cirrus cloud generally does not bring rain, but a series of them can indicate an approaching storm, though this is usually followed by clearer skies.

Cirrocumulus clouds, often abbreviated Cc, are formed when moist air at a very high altitude reaches saturation, creating a series of ice crystals. There is a certain instability to this type of cloud, which gives it its cumuliform appearance.

Cirrostratus clouds, often abbreviated Cs, consist of a continuous layer of cirriform cloud covering a large area of the sky. These clouds are typically formed when convectively stable moist air cools to saturation at a high altitude, forming a layer of ice crystals. Cirrostratus clouds can be a warning of rain or snow, but only if they thicken to altostratus and eventually nimbostratus clouds as the weather front moves closer.

Mid-Level Clouds

There are two types of mid-level clouds: altocumulus and altostratus. These resemble cumulus and stratus clouds respectively, but they are formed in a lightly different way. These could tend to occur from 6500 to 20,000 feet above sea level.

Altocumulus clouds, often abbreviated Ac, are often associated with precipitation, but not always with a weather front. When they do bring rain or snow, it is usually as virga, which does not reach the ground. These clouds tend to be a smaller, more spread version of cumulus clouds. Like all cumulus clouds, there is usually a convective instability at the altitude of its formation.

Altostratus clouds, often abbreviated As, are formed when a large air mass is lifted to an altitude conductive to condensation, usually along a frontal weather system. It can bring rain or snow, and if the precipitation continues, the clouds will usually thicken into nimbostratus.

Low-Level Clouds

There are two types of low-level clouds: stratocumulus and stratus. These clouds usually occur below 6500 feet above sea level.

Stratocumulus clouds, often abbreviated Sc, are lumpy and trailing. They usually form in the slightly unstable air that follows a cold front. Sometimes, they will produce light rain or even snow, but it is unlikely that they will bring heavy precipitation.

Stratus clouds, often abbreviated St, form in low horizontal layers, and can have either a uniform or ragged base. Ragged stratus clouds often from in precipitation. The more uniform type of this cloud usually develops in more stable air conditions, such as maritime regions, and will usually produce a light drizzle.

Low-to-Mid-Level Clouds

There are two types of low-to-mid-level clouds: nimbostratus and cumulus. These clouds are typically found below 10,000 feet above sea level.

Nimbostratus clouds, often abbreviated Ns, tend to result in poor visibility and fairly constant precipitation. Though usually found above 6500 feet, it can thicken and drop below this level during precipitation.

The bright and cheery cumulus clouds, often abbreviated Cu, are usually associated with fair and pleasant weather. However, they can grow into cumulonimbus clouds, which usually produce storms. Though usually present at below 6500 feet, in low humidity, they can raise as high as 10,000 feet.

Vertically Developed Clouds

Cumulus clouds can sometimes develop into cumulonimbus clouds if the conditions are right, such as when the air mass is convectively highly unstable. Though typically found below 10,000 feet, these clouds can grow to 40,000 feet or higher. Cumulonimbus clouds almost always produce thunderstorms or rain showers of some kind, and can be accompanied by high winds. These types of clouds can also be indicative of potential tornadoes.

Many factors influence the development of clouds, including altitude, humidity, temperature, and moisture saturation, and each cloud type requires a specific combination of factors before formation can occur. The study of cloud formation and cloud type is a complex and interesting science.