[Illustration: Fig. 28 ] There are really four simple cloud formations and three compound formations: =1. The Cirrus Cloud.= (Fig. 24.) The Cirrus cloud is always seen high in the sky and at a great elevation. Its formation is fibrous and it is particularly characterized for its many varieties of shapes. It also has a marked delicacy of substance and it is pure white. =2. The Cumulus Cloud.= (Fig. 25.) The Cumulus cloud is of moderately low elevation. It is a typical cloud of a summer day. It may be recognized by little heaps or bushes rising from a horizontal base. In summer-time we are all familiar with the cumulus clouds rising with the currents of air in huge masses. They form one of the most accurate indications of fair weather when you see them gradually dissolving. Sometimes these clouds become very large, and, while the texture is generally of a woolly white, naturally, when they assume such large sizes, they gradually change in color to a darkish tint. =3. The Stratus Cloud.= (Fig. 26.) This is the opposite of the Cirrus cloud, because it hangs the lowest of all, in gray masses or sheets, with a poorly-defined outline. =4. The Nimbus Cloud.= (Fig. 27.) Any cloud can be classed as a nimbus cloud from which rain or snow is falling. Of the Compound Clouds we have: [Illustration: Fig. 29 ] 1. The Cirro-Cumulus Cloud (Fig. 28), which has all the characteristics of both the Cirrus and the Cumulus. The most characteristic form of this cloud, and the one most commonly known, is when these clouds form small round masses, which appear to be cirrus bands broken up and curled up. This is what people call the “mackerel” sky. 2. The Cirro-Stratus Cloud (Fig. 29), which is known when the clouds arrange themselves in thin horizontal layers at a great elevation. [Illustration: Fig. 30 ] 3. The Cumulo-Stratus (Fig. 30) is the cumulus and the stratus blended together. Their most remarkable form is in connection with approaching thunder storms, and are often called thunder heads. They rapidly change their outline and present a beautiful spectacle in the sky at times. The Cirrus, Cirro-Cumulus and Cirro-Stratus are known as the upper clouds and the others are known as the lower. ATMOSPHERIC DISTURBANCES Disturbances of the atmosphere are classified as follows: Cyclonic, or low area storms, or anti-cyclonic, or high area storms. [Illustration: Fig. 31 ]