The clouds classified as middle level clouds are those you will which have a base of no less than 6,500 to 23,000 feet. Middle clouds will appear to look very bright and are generally much less fragmented in their appearance than most other clouds, which is just to say that upon inspection they will appear full rather than have “holes” in their body. This is due to having a high ratio of ice crystals in their composition as well as their distance from the Earth. They can also appear as sheets which are elongated and thin (At times the sun or moon can be observed through this type of formation) or cumuliform which are thicker and appear more puffy in which the sun and moon cannot generally be observed through. Something else to keep in mind is middle clouds move with the air currents at that level which may be contrary to the currents at the lower surface level which is another good hint to separate them from low clouds, in addition to their perceived slow movement.
There are three basic types of middle clouds: Altocumulus, Altostratus and Nimbostratus. Within those three types there are sub-categories which narrow the field even further into a total of nine middle clouds. Sometimes they will appear quite similar, however each has certain distinctions which set them apart.
The first variety is altocumulus which is the middle cloud most commonly confused with a lower level cumulus cloud. Like it’s low level counterpart the formation is puffy like balls of cotton and it will appear light on the bottom and dark at the top in almost all cases. Altocumulus will vary in thickness and can appear broken or smooth when mature. Altocumulus clouds are further considered unstable as they often form from dissipating storm clouds which is another tip to keep in mind when you are taking your observation. A thick altocumulus cloud is a good indication that bad weather may be approaching.The think and thick type of altocumulus are each considered a distinct type.
Altocumulus Lenticular shares the same properties as the regular altocumulus cloud except that when it forms it takes on the appearance of a lens. This formation will be smooth and flat when observed from the ground. An altocumulus lenticular will most often appear thin and broken, but the distinguishing factor is that most times it appears as if it is sitting in one spot refusing to move. In stable conditions this cloud can gently rise or fall in place. When it descends it will generally dissipate, seemingly right before your eyes.Another good hint to identifying this is it will appear to have one somewhat pointed edge or “wedge” which air flows “out” of and an opposite end which is fuller that air flows “in” through.
Altocumulus Castellanus otherwise known as accas is low cloud eight, and will appear like any other cumulus cloud with its bright base. The distinguishing feature is that it looks like an old castle with turrets rising from the top. Accas is one of the easiest clouds to spot in the sky because it is so unique. It is sometime confused on first glance with a towering cumulus (TCU) however while TCU would be a castle with only one turret, accas will have at least two in addition to sitting higher in the sky.
Altostratus clouds will look like long smooth gray bed sheets laid out in the sky. These will usually develop very small and then fill in and spread in all directions rather rapidly, by nature standards at least. These “sheets” can be thick or thin but will generally not appear to have breaks which allow the sun to be seen although at times that will possible, especially when the cloud is in it’s earliest stages of development. Altostratus cloud formations are actually several layers of the same cloud type that form independently, but begin “fusing” together as they mature making one immense cloud. When this cloud is observed in the aftrnoon it is usually a good indication of evening rain. Another form of altostratus will appear to be a combination altostratus/altocumulus in that it is an immense gray sheet, but has a lens like look with those “in and outflow” features. As with altocumulus, the thin iand thick variety of this formation are each considered a distinct type.
Nimbostratus clouds are the most difficult to describe, and are even difficult to photograph. These are wide sprawling gray or white sheets which will often sprawl over better than 15,000 feet of sky. The first good indication you are viewing a nimbostratus aside from it’s size is that they hold so much condensation they are thick enough to blot out the sun in a sense, which is to say you will find almost no breaks in at any point. Another great indicator is prolonged precipitation. A problem identifying nimostratus is that many different clouds will often form below it and obscure it from full view. More often than not, the oly way to really be sure you are viewing a nimbostratus cloud is to watch it form from a stratocumulus or altocumulus, as few develop independently of those two types of cloud.
Identifying the middle clouds can be a challenge as so many of these evolve and de-eveolve from each other making them appear to be the same. With some careful observation over the course of a few days you will soon be able to pick out the features of each and before long it becomes second nature to just look up and pick each apart. Remeber each has certain distinct patterns and formations and it is a piece of cake!