101: Clouds II

The first cloud etage are the low clouds. These genera have bases below 2km or so, but are not in direct contact with the ground. Some of these clouds cover a great vertical extent, even reaching into the lowest part of the Stratosphere. It is because of this variable growth that the etage is defined by the cloud base, not their tops.

 

 

Cumulus (Cu)

Fair weather Cumulus humilis

Developing Cumulus mediocris and maybe some congestus in the background

Visible satellite image of cumulus

Infrared satellite image of cumulus

These cumuliform clouds are the quintessential fair weather clouds and likely the most iconic of the genera. There are three distinct species of cumulus based on their degree of development. Cumulus humilis are small clouds that often occur on otherwise clear days. Cumulus mediocris clouds have clearly developed some and often are seen on days with a large number of convective clouds, including thunderstorms. The largest are Cumulus congestus, often referred to as towering cumulus. These are typically significantly taller than they are wide and may appear to consist of several convective towers. Occasionally these will produce light showers, but more importantly, should they continue to develop they will become an entirely different genus: cumulonimbus. Due to consisting of individual pockets of rising air, individual cloud elements are often hard to discern on satellite imagery. When they do show up, they typically appear very bright on visible images, but on infrared images their brightness will depend on the height of the cloud tops, with tall cumulus congestus appearing the brightest.

Stratus (St)

Stratus clouds

Thick stratus layer species called Stratus opacus

An extreme case of Stratus undulatus

Visible satellite image of stratus

Infrared satellite image of stratus

If cumulus are the classic fair weather cumuliform clouds, then stratus is the epitomic dreary day stratiform cloud. These low clouds often cover large areas and sometimes appear nearly featureless, as is the case with Stratus nebulosus. At other times, weak atmospheric waves will cause wave-like features in the cloud base; such is the case with Stratus undulatus. Generally stratus layers are thin enough to allow the sun or moon to shine through and they rarely produce any type of precipitation. Stratus appears on visible satellite images as a smooth layer of cloud that often can be clearly seen appearing to conform to terrain. Since stratus is such a low cloud, its top will not be significantly cooler than the ground, making it very hard to distinguish on infrared imagery.

Stratocumulus (Sc)

Stratocumulus

Stratocumulus

Stratocumulus undulatus

Stratocumulus

MODIS image featuring stratocumulus along the coast

Infrared satellite image of stratocumulus (the dark grey cloud layer along the coast)

This genus is essentially a cross of the first two genera. Stratocumulus cloud layers are often formed when very weak convection has occurred in or under a stratus layer and caused it to bunch up some into obvious individual cloud elements. These are the most common genus of cloud on Earth, especially over the ocean. One reason for this is the tendency for the ocean to be warmer than the air, particularly during the winter; this causes weak convection into an often pre-existing stratus layer. In general, the presence of stratocumulus indicates a stable air mass. Thus, when these clouds are seen feeding into a tropical cyclone, it is likely that the storm has moved into a highly unfavorable air mass and will likely be deteriorating. Since there is only a small amount of separation between the cloud elements, stratocumulus will often appear very similar to stratus on visible satellite images except under very high resolution. Like stratus, stratocumulus is often hard to detect on infrared images since they are so low.

Nimbostratus (Ns)

Nimbostratus

Nimbostratus, note the water on the roof

Nimbostratus showing some detail on its underside

Nimbostratus in satellite images

Nimbostratus are the producers of long episodes of light to moderate precipitation. From the ground they appear very similar to stratus, except often even more uniform and generally much thicker, such that the sun or moon usually cannot be seen. This genus is sometimes classified under middle clouds or a category for clouds of great vertical extent. This is probably because they often will initially form from middle clouds and then extend downward as they begin to precipitate. Nimbostratus will often appear similar to stratus on visible imagery, but on infrared images they will be significantly brighter due to their much higher cloud tops.

Cumulonimbus (Cb)

Large cumulonimbus with its characteristic anvil shape

Tornado extending from the mesocyclone at the base of a super cell cumulonimbus

Wall cloud extending from the mesocyclone at the base of a super cell cumulonimbus

Wall cloud extending from the mesocyclone at the base of a super cell cumulonimbus

Visible satellite image of cumulonimbus

Infrared satellite image of cumulonimbus

Visible satellite image of cumulonimbus super cells

Infrared satellite image of a group of cumulonimbus (center)

These are the big weather makers. Cumulonimbus form from cumulus clouds that have grown considerably in vertical extent. Clouds of this genus produce heavy showers and occasionally severe weather. The backbones of these clouds are the robust updrafts caused by the rapidly rising air. These updrafts will continue upward until the air ceases to be buoyant, such when the air encounters an inversion layer, such as the tropopause: the permanent inversion at the base of the stratosphere. At this point the cloud will spread out horizontally creating an “anvil” shape. In some cases, the main updraft of the cloud will extend a little ways into the stratosphere purely due to its upward momentum, creating what are called “overshooting tops”. What goes up must come down, and the air that has reached its limit at the end of the updraft will have had much of its water vapor condensed, which falls as precipitation. The spent air will begin descending, creating a downdraft. This downward flow of air will occupy the same space as the updraft and begin to weaken it. Eventually the updraft will collapse altogether, causing the cloud to “rain itself out” and dissipate. If the cloud grows tall enough, the upper portion of it will consist of ice crystals, at which point the cloud is referred to as “glaciated”. Under the right conditions, some rotation will become associated with the updraft, called a mesocyclone, and in the process, separate the updraft from the downdraft. At this point the cloud is referred to as a “super cell” storm. These systems last much longer than the average cumulonimbus because the up and downdrafts don’t compete with each other, preventing the updraft from rapidly collapsing. Super cell storms are the most dangerous type of cloud, since they often spawn strong tornadoes, heavy rain, frequent lightning, and large hail. A full discussion of these clouds would take one or two full posts to cover so I’ll leave it here since this post is specifically about the types of clouds. On satellite images individual clouds can often be discerned due to their size. In visible imagery cumulonimbus appear very bright, especially if they are glaciated. At high enough resolution, the overshooting tops will be visible as small bumps on the top of the cloud. These clouds are also easily identified on infrared images due to the very cold temperature of their tops.