Cirrus clouds take the most varied shapes, such as isolated tufts of hair--i. e., thin filaments on a blue sky--branched filaments in feathery form, straight or curved filaments ending in tufts (called _cirrus uncinus_), and others. Occasionally cirrus clouds are arranged in bands, which traverse part of the sky as arcs of great circles, and as an effect of perspective appear to converge at a point on the horizon, and at the opposite point also, if they are sufficiently extended. Cirro-stratus and cirro-cumulus also are sometimes similarly arranged in long bands. [Certain forms of cirrus are called “mares’ tails.” The long bands crossing the sky, as just described, are known as “polar bands” or “Noah’s ark.”] 2. CIRRO-STRATUS (CI.-ST.)--_A thin sheet of whitish cloud; sometimes covering the sky completely and merely giving it a milky appearance; it is then called cirro-nebula, or cirrus haze; at other times presenting more or less distinctly a fibrous structure, like a tangled web._ This sheet often produces halos around the sun or moon. 3. CIRRO-CUMULUS (CI-CU.) (_Mackerel sky_)--_Small rounded masses or white flakes without shadows, or showing very slight shadow; arranged in groups and often in lines._ 4. ALTO-STRATUS (A.-ST.)--_A dense sheet of a gray or bluish color, sometimes forming a compact mass of dull gray color and fibrous structure._ At other times the sheet is thin, like the denser forms of cirro-stratus, and through it the sun and moon may be seen dimly gleaming as through ground glass. This form exhibits all stages of transition between alto-stratus and cirro-stratus, but, according to measurements, its normal altitude is about one-half that of cirro-stratus. 5. ALTO-CUMULUS (A.-CU.)--_Larger rounded masses, white or grayish, partially shaded, arranged in groups or lines, and often so crowded together in the middle region that the cloudlets join._ [Illustration: A CLOUD BANNER OVER MT. ASSINIBOINE, CANADIAN ROCKIES. (_Photographed by Dr. C. D. Walcott._)] [Illustration: CIRRUS (with a few patches of lower clouds in the foreground). This is cirrus, but not of the “mare’s tail” variety. There are many distinct types of cirrus, which have sometimes been given separate names. (_Photographed at the Observatory of Trappes, France._)] The separate masses are generally larger and more compact (resembling strato-cumulus) in the middle region of the group, but the denseness of the layer varies and sometimes is so attenuated that the individual masses assume the appearance of sheets or thin flakes of considerable extent with hardly any shading. At the margin of the group they form smaller cloudlets resembling those of cirro-cumulus. The cloudlets often group themselves in parallel lines, arranged in one or more directions. 6. STRATO-CUMULUS (ST.-CU.)--_Large lumpy masses or rolls of dull gray cloud, frequently covering the whole sky, especially in winter._