Besides the ten main classes of clouds, a few distinct minor varieties are recognized by all meteorologists. Among these is the “lenticular cloud”; an isolated small cloud, which frequently shows iridescence, and the shape of which has been compared to that of a lens or an almond. This cloud may remain stationary, or nearly so, but it really marks the position of a billow in a stream of air, the moisture condensing at one edge of the cloud and dissolving at the other. Another distinctive and rather rare form of cloud, seen chiefly in connection with thunderstorms, is _mammato-cumulus_, likewise known as “pocky cloud,” “festoon cloud,” “rain balls,” etc. It consists of numerous sacklike or udderlike protuberances, convex downward. When a stream of moist air is forced to ascend in passing over a mountain its moisture is often condensed by the process of dynamic cooling, already explained, and a “cloud cap” is seen over the summit. In local weather lore such caps are generally regarded as a sign of rain. These clouds attached to mountains were called “parasitic clouds,” by writers of a century ago, who proposed some naïve explanations of them. Occasionally a “cloud banner” streams far to the leeward of the mountain. One of the most famous and striking of cloud caps is the “tablecloth” that spreads over Table Mountain, near Cape Town, when a moist wind blows in from the sea. Sometimes the local topography causes the wind that has swept up over a mountain to form a second “standing” wave to the leeward of the summit, and this may also be marked by a cloud, which, like the cloud cap, presents a delusive appearance of permanence, while it is, in reality, in constant process of formation on the windward side and dissipation on the leeward. The two clouds thus formed, one over the summit and the other to the leeward, are often seen at Table Mountain, and are further exemplified in the celebrated “helm and bar” of Crossfell, in the English Lake District. In the case of a wind blowing athwart a ridge or mountain range, a bank of cloud may extend along the whole crest, as in the “foehn wall” that appears along Alpine heights when the foehn wind is blowing. Some day meteorology will be taught in art schools, for the same reason that anatomy now is. When that blissful day arrives painters will probably show us skies less at odds with nature than those that deface the work of artists of all degrees of celebrity, including the “old masters.” CHAPTER VII PRECIPITATION Meteorologists have been in much perplexity over the naming and classification of the various deposits of atmospheric moisture known collectively as “precipitation.” The subject is one to which a good deal of attention has been paid in recent years, but it must be admitted that, even at the present time, the terminology of this group of atmospheric phenomena is not yet satisfactorily settled, either in English or in any other language.