There were clouds across the moon: cirrostratus thickening to become altostratus.

Altostratus may be composed of ice crystals.

The altostratus and cumulus congestus combination is more common ahead of an advancing cold front.

Here is where the clouds get their marching orders and shape assignments: cumulonimbus, altostratus, cirrus.

The sun can be seen through thin altostratus, but thicker layers can be quite opaque.

In some ice crystal altostratus, very thin, rapidly-disappearing horizontal sheets of water droplets appear at random.

Altostratus (As)

Altostratus is formed by the lifting of a large mostly stable air mass that causes invisible water vapor to condense into cloud.

The first clouds to form along the sloping surface of the cold air are high cirrus, which thicken to cirrostratus and altostratus.

This cloud typically forms from altostratus in the middle altitude range then subsides into the low altitude range during precipitation.

When a warm front approaches, cirrostratus clouds deepen and lower into altostratus, and rain usually begins 12 to 24 hours later.

An elevated portion of a frontal zone forces broad areas of lift, which form clouds decks such as altostratus or cirrostratus.

There was only one cloud to be seen, but it covered the entire soggy Irish sky, dropping rain all day - a "drizzling altostratus", since you ask.

Altostratus and nimbostratus clouds always have this physical appearance without significant variation or deviation and, therefore, do not need to be subdivided into species.

A further thickening of the altostratus is often accompanied by virga and the arrival of precipitation is imminent.

Similar clouds found at intermediate heights carry the prefix alto- resulting in the genus names altostratus and altocumulus.

They are prefixed by alto-, yielding the genus names altocumulus (Ac) and altostratus (As).

If it is a warm front, the cirrus clouds spread out into cirrostratus, which then thicken and lower into altocumulus and altostratus.

The variety undulatus (having a wavy undulating base) can occur with any clouds of the species stratiformis or lenticularis, and with altostratus.

A thickening and lowering of cirrostratus into mid-level altostratus is a good sign the warm front or low has moved closer and precipitation may begin within 24 hours.

In the middle étage are the alto clouds, which consist of the cumuliform cloud altocumulus and the stratiform cloud altostratus.

Cirrocumulus in its pure form is actually a high cumuliform genus, and cirrostratus is stratiform, like altostratus and lower based sheet clouds.

Nimbostreatus fractus is a variation of nimbostratus and will sometimes occur as a weather front clears, these being areas of dark rain bearing clouds with lighter altostratus between.

Elevated portions of weather fronts (which are three-dimensional in nature) force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus.

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.