III.] RAIN AND DEW. 43 



a direction opposite to that of the wind which happens to be 

 blowing over the surface. It is these clouds that are sup- 

 posed to be made up of minute ice-particles (p. 41), since 

 they produce, when they come between us and the sun or 

 moon, those coloured circles which are known as halos. 



Very different from the cirrus is the well-known acmuhes,^ 

 a dense cloud which forms towering heaps of convex or 

 concave form resting on a nearly horizontal base. Different 

 again are the continuous sheets of cloud which are often to 

 be seen extending widely in a horizontal direction, and are 

 known consequently as the stratus.'^ 



It often happens that the clouds under observation will 

 not fall into any place in the foregoing classification. 

 Instead of belonging definitely to any one class they may 

 combine the characters of two or more groups, and in such 

 cases expressive designations are framed by combining the 

 preceding elementary terms. Thus, the beautiful effect of 

 what is known popularly as the "mackerel sky" is due to 

 numerous detached clouds of the composite forms termed 

 cirro-cumulus. In like manner we may have a cirro-stratus 

 and a cumulo-stratus, but such words sufficiently explain 

 themselves. The dull rain-cloud, termed the fiifnbus,^ is 

 a composite form sometimes described as a cumulo-cirro- 

 stratus. It is the dark grey cloud, or system of clouds, 

 from which rain is actually falling. Before leaving the 

 nomenclature of clouds, it may be useful to remark that 

 the term scud is frequently applied to any loose detached 

 clouds which drift rapidly before the wind. 



In meteorological reports, it is convenient to express 

 approximately the proportion of sky which, at any given 

 time, is covered by cloud. This is effected by using an 



* Cumulus, a heap. * Stratus or stratum, a bed. 



" Nimbus, a rain-cloud. 



