Introduction to Clouds

Most clouds form as air rises, expands, and cools. If sufficient moisture is present during this process, the lift will produce condensation resulting in the clouds we see in the sky.

Atmospheric stability is an important concept in the understanding of cloud formation.

As we discuss stability, we must make a couple of assumptions. First, we assume that there is no mixing of air between the risking/sinking parcel of air and the surrounding environmental air, and secondly, that there is no transfer of heat energy between that parcel and the surrounding environmental air.

That having been said, a parcel of air is considered to be stable if, when forced to rise, it becomes the same temperature or colder than the surrounding environmental air, through adiabatic cooling. When this condition is met, the parcel of air will have the tendency to either hold its position (if it is the same temperature as the surrounding environmental air) or sink to its original position (if it is colder than the surrounding environmental air).

Instability, on the other hand, is when if forced to rise, the temperature of an air parcel (even with adiabatic cooling!) remains warmer than the surrounding environmental air. When this condition is met, the parcel of air, because it is warmer than the surrounding environmental air, continues to rise higher and higher in atmosphere (as long as its temperature remains warmer than the air it is rising through).

A good rule to remember is that warmer environmental air aloft tends to enhance atmospheric stability while colder temperatures aloft tends to enhance instability.

As air rises and condensation occurs in the atmosphere, sensible heat (absorbed through the latent heat of evaporation) is released within a rising air parcel (through the latent heat of condensation), which results in enhanced instability.

Some clouds form in more stable atmospheric environments, where upward motion in the atmosphere may be very weak; others form in explosively unstable environments, where lift and upward motion is extreme.

The first cloud classification scheme was developed by French naturalist Lamarck in 1802 but the system never received much attention. Luke Howard, an English naturalist, developed a cloud classification system one year later in 1803. Some expansion of Howard's system were made by Abercromby and Hildebrandsson in 1887. It is this expanded system of classifying clouds that is still largely in use today.

Howard's cloud classification system used Latin words to describe clouds as they appear to ground observers. He used stratus (Latin for "layer") to describe sheet-like (stable) clouds. In addition, he employed cumulus (Latin for "puffy") to denote a cloud with some puffy vertical (unstable) extent. The word nimbus (Latin for "violent rain") means a rain cloud, while the word cirrus (Latin for "curl of hair") denotes a wispy high level cloud. These were the basic cloud forms in Howard's system, while other clouds in the classification scheme are developed by combining the basic cloud types presented above. A good example would be the cumulonimbus cloud, which is a rain cloud with a "puffy" character with vertical extent.

In addition to the basic cloud types, there are other more unusual types of clouds or cloud accessory features that deserve our attention. There is a list of the more common cloud accessory features (modifiers) after the listing of basic cloud types.