Nacreous Clouds

Also known as polar stratospheric clouds, these iridescent clouds form at 15 to 25 kilometers high in the polar stratosphere and are mostly seen in winter at high latitudes especially in the Scandinavian region and Northern Canada. On account of their high elevation and the curving of the Earth’s surface, they obtain sunlight from under the horizon and reflect it to the ground, shining radiantly well before sunrise or after sunset.

Altocumulus Mackerel Sky

A mackerel sky forms at medium altitude level of 2.4 to 6.1 kilometers and is a sign of unstable humidity. If the atmosphere beneath is stable and no moist air moves in, the climate would almost certainly stay dry. On the other hand, if moist air would rise reaching this layer, combining with the instability of its surface temperature, thunderstorms would often result. The term “Mackerel sky” was derived from the fact that the cloud pattern looks quite similar to the markings of an adult king mackerel.

Noctilucent Clouds

Also called Polar Mesospheric Clouds, they are the highest clouds in the Earth’s atmosphere and occur at an altitude of 75 to 85 kilometers, almost on the verge of space. They have a glowing appearance since they are illuminated by sunlight from below the horizon; and can normally be seen during summer months at latitudes of between 50 and 70 degrees north and south of the equator.

Cirrus Uncinus

Also known as mares’ tails, these thin wispy clouds form at very high altitudes on the edge of the troposphere. They are normally observed when a warm or occluded is nearing and usually indicate that there would be rain. Uncinus is Latin for curly hooks.

Lenticular Clouds

Officially known as altocumulus standing lenticularis, they are stationary lens-shaped clouds that develop at high elevations, and are usually aligned at right-angles to the direction of the wind.

Roll Clouds

A type of arcus cloud, a roll cloud is a low, horizontal, tubular cloud associated with the gust front of a thunderstorm. They often appear to be rolling on its horizontal axis, as cool sinking air undercuts warm rising moist air that condenses forming clouds, which rolls in response to the surrounding wind.

Shelf Clouds

Another type of arcus cloud, a shelf cloud is a low, horizontal wedge-shaped cloud. Unlike a roll cloud, a shelf cloud is attached to the bottom of the parent cloud (most commonly a thunderstorm). The cloud’s upward motion can often be seen on the leading/outer part of the shelf cloud, while its base often appears turbulent and boiling.

Cirrus Kelvin-Helmholtz

One of the most unique cloud formations, Cirrus Kelvin-Helmholtz appear as a series of horizontal spiral of cloud. However, these clouds are hardly ever observed since they tend to disperse only a few minutes after forming.

Mammatus Clouds

Also called mammatocumulus, they are opaque or semitransparent lobular-shaped cloud structures hanging under the base of other clouds. They often signal a particularly strong storm when they form under cumulonimbus, that’s why aviators are strongly warned to steer clear of such clouds. The name “mammatus” came from the Latin word “mamma” meaning “breast”, because the shape of these clouds resembles that of the human female breasts.