Lenticular Clouds (Altocumulus lenticularis): Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. If the temperature at the crest of the wave drops to the dew point, moisture in the air may condense to form lenticular clouds. As the moist air moves back down into the trough of the wave, the cloud may evaporate back into vapor. Rain or snow can occur from these storms. They are stable at lower altitudes and unstable at higher altitudes. They can be up to 12,000 meters or 40,000 feet.
Undulatus Asperatus: They appear dark and storm-like, they tend to dissipate without a storm forming. The ominous-looking clouds have been particularly common in the Plains states of the United States, often during the morning or midday hours following convective thunderstorm activity. They appear below 2000 meters or 6000 feet, but when they are in the form of altocumulus, they can appear higher than 2000 meters or 6000 feet. They occur when there is unstable air aloft and stable air at the surface.
Noctilucent Clouds: They are unstable clouds and do not have an involvement in producing weather.They occur at 76,000 meters to 85,000 meters or 250,000 feet to 280,000 feet and can only be seen in latitudes 50 to 70 degrees north or south of the equator.
Fallstreak Hole Clouds: They are large circular gaps that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation particles. When a portion of the water does start to freeze it will set off a domino effect, due to the Bergeron process, causing the water vapor around it to freeze and fall to the earth as well. This leaves a large, often circular, hole in the cloud. These clouds are stable clouds and they are in altitudes of 6.5 to 8.5 kilometers or 22,000 to 28,000 feet.
Mammatus Clouds: Mammatus are most often associated with the anvil cloud and also severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds. These are unstable clouds and are often located in altitudes of 6,500 feet to 20,000 feet or 1,980 meters to 6,096 meters.
Wave Clouds: A wave cloud is a cloud form created by atmospheric internal waves. The atmospheric internal waves that form wave clouds are created as stable air flows over a raised land feature such as a mountain range, and can form either directly above or in the lee of the feature. As an air mass travels through the wave, it undergoes repeated uplift and descent. If there is enough moisture in the atmosphere, clouds will form at the cooled crests of these waves. In the descending part of the wave, those clouds will evaporate due to adiabatic heating, leading to the characteristic clouded and clear bands. The cloud base on the leeward side is higher than on the windward side, because precipitation on the windward side removes water from the air. They are located at high altitudes and specific values of heights are not listed.
Cloud Iridescence: Cloud iridescence is the occurrence of colors in a cloud similar to those seen in oil films on puddles, and is similar to irisation. It is a fairly uncommon phenomenon, most often observed in altocumulus, cirrocumulus and lenticular clouds, and very rarely in Cirrus clouds. They are unstable clouds and are located a very high altitudes.
Roll Clouds: A roll cloud is a relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be “rolling” about a horizontal axis. They are a solitary wave called a soliton, which is a wave that has a single crest and moves without changing speed or shape. It is at very low altitudes and it is an unstable cloud.
Pyrocumulus Clouds: A pyrocumulus, or fire cloud, is a dense cumuliform cloud associated with fire or volcanic activity. A pyrocumulus is similar in some ways to a firestorm. However, one may occur without the other. A pyrocumulus cloud is produced by the intense heating of the air from the surface. Pyrocumulus Clouds are unstable up to a point where they reach stability and they are very low due to the origin of coming from a volcano.
Actinoform Clouds: An actinoform or actiniform cloud is a collection of low clouds that take a distinct shape. They are named after the Greek word for “ray” due to their radial structure. Actinoform clouds can spread out over 300 kilometers across and thus cannot be easily seen with the naked eye. They have heights less than 2 kilometers or about 6,561 feet. They are stable clouds and do not account for any type of weather.
Wave Windows: In meteorology, wave windows are atmospheric standing waves. Both lee waves and the rotor may be indicated by specific wave cloud formations if there is sufficient moisture in the atmosphere, and sufficient vertical displacement to cool the air to the dew point. Waves may also form in dry air without cloud markers. Wave clouds do not move downwind as clouds usually do, but remain fixed in position relative to the obstruction that forms them. They are stable clouds and are located at high altitudes.
Pileus Clouds: A pileus, also called scarf cloud or cap cloud, is a small, horizontal, altostratus cloud that can appear above a cumulus or cumulonimbus cloud, giving the cloud a “hoodlike” appearance. They are formed by strong updrafts acting upon moist air at lower altitudes, causing the air to cool to its dew point. They are usually indicators of severe weather, and a pileus found atop a cumulus cloud often foreshadows transformation into a cumulonimbus cloud, as it indicates a strong updraft within the cloud. They are unstable and are present at lower altitudes.
Polar Stratospheric Clouds: Polar stratospheric clouds or PSCs, also known as nacreous clouds, are clouds in the winter polar stratosphere at altitudes of 15,000 to 25,000 meters or 49,000 to 82,000 feet. They are stable clouds and do not associate with weather patterns.
Morning Glory Clouds: Morning Glory clouds can most often be observed in Burketown, Australia in September to mid-November, when the chance to see it early in the morning is approximately 40%. A Morning Glory cloud is a roll cloud that can be up to 1,000 kilometers long, 1 to 2 kilometers high, often only 330 to 660 feet above the ground and can move at speeds up to 37 miles per hour. Sometimes there is only one cloud, sometimes there are up to eight consecutive roll clouds. These are stable clouds and do not associate with weather.
Shelf Clouds: A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. A shelf cloud is attached to the base of the parent cloud, which is usually a thunderstorm. Rising cloud motion often can be seen in the outer part of the shelf cloud, while the underside often appears wind-torn. This clouds are unstable and are present at low altitudes.