does not really exist. The manner in which particles

may be emitted in an upward direction will appear more clily clearer when we come to deal with clouds of Inclin

ation.

The showers resulting from the formation of Stratus Præcipitans are often frequent, thick, and wetting, but the rain-drops are never of large diameter, and the snowflakes are seldom of great size, although the size of snowflakes is not dependent on precisely the same circumstances as those which determine the diameters of rain-drops. In the winter of higher latitudes a rather thick local snowshower is not very uncommon even when Stratus Quietus is alone concerned in the production of the sub-variety here treated of.

50. As may be inferred from the fact that it results from different forms of Stratus, Stratus Præcipitans has an extensive range of altitude. In dealing with the most valuable information supplied by this cloud we confine ourselves to forms at a considerable distance from the earth's surface. Solitary nubecules of high Stratus Maculosus occasionally discharge a little snow, which is evaporated soon after it commences to descend. Out of a dense and compact body of similar cloud, the base of which is, of course, at a relatively low level, precipitation on a much larger scale sometimes occurs. Both phenomena may now and then be noticed at the same time, but in the instance of closely-welded Stratus Præcipitans, if the latter be at a considerable height, but little of the precipitation reaches the earth.

Now let us observe the curves taken by the visible lines of the descending particles. These bear a true resemblance to the lines of falling rain, hail, or snow beneath a Cumulo-nimbus. But they afford a better illustration of the physical formation of the principal clouds of Inclination. When precipitation of small particles, particularly of ice, either commences at the exact position in which vapour ceases to be vapour and is converted into cloud, or begins to take place out of an already formed cloudlet, or out of an extensive layer of cloud, the visible line of the precipitation is, at the point where it commences, nearly perpendicular, for gravity produces the descent of the particles, and there is very little difference of horizontal movement in the closely-contiguous airlayers. But as the particles proceed downwards, attaining their terminal velocities under increasing atmospheric pressures, they encounter an increasing difference in the horizontal movements of the air, due to increasing friction and to the effects of viscosity. The tail of the precipitation-line is therefore no longer perpendicularly beneath the head, and since the upper currents move, in a majority of cases, more rapidly than the lower, the tail commonly lags behind the head. Differences of direction of movement cause the whole curve to exist no longer in a plane perpendicular to the earth's surface. The twist thus given to the precipitation-lines belongs to the discussion of clouds of Inclination, and forms one of the most interesting as well as the most useful of the subjects dealt with in this monograph. It is im

portant that the student of weather should look carefully at the interesting and somewhat beautiful Stratus Præcipitans whenever it occurs at a considerable distance above the earth, not with the immediate object of forecasting weather, but with the aim of understanding the primary cause of the formation of clouds of Inclination.

CHAPTER V*

CHARACTERS AND CONTRASTS-CLOUDS OF INVERSION

51. BEFORE proceeding with a detailed description of the clouds under this heading, it may be advisable, even at the price of repetition, to place before the reader the broad distinction in formation between these clouds and those dealt with in the previous chapter. This distinction is, that whereas clouds of Interfret are caused by the interaction of more or less horizontal currents of different velocities or directions, clouds of Inversion are caused by condensation in an upward direction in more or less vertical currents, although the shape of the clouds of the latter class. may be affected by differences of velocity and direction in the horizontal movements of the air.

Cumulo-rudimentum

52. Many misleading titles, such as Fracto-cumulus and Cumulo-frustum, have been applied to this wellknown cloud, but these, as we shall see later, lead us to erroneously presuppose that the cloud is a brokendown and degraded form of Cumulus.

The range of altitude of the rudimentary cloud

F

is small. The mean altitude, as given in Table III., is 1500 feet (458 m.) above mean sea-level. But in reference to this variety of cloud it is important to observe that in the tropical and extra-tropical latitudes the level is higher in summer than in winter, and that the alteration of level is greater over the land than over the open sea. For the heating of the floor of the atmosphere causes vapour to remain in the state of vapour near that floor, and raises the level at which condensation is possible; and it is the atmosphere which exists over a land-surface which has its temperature specially raised by insolation, since the solid surface conducts less heat? downward than the liquid, and parts with more heat in an upward direction.

It will be sufficient to state here first, that this variety occurs in rather different atmospheric conditions from those of actual Cumulus, and, secondly, that it is also Cumulus in a rough and unwrought state. These two statements are by no means irreconcilable. When fragments of cloud are carried along by wind in the rather low layers of the atmosphere, they commonly adopt the form characteristic of this variety, and the majority of them undergo no further development. A minority, however, under a slight change of condition, are the nuclei or embryos of the true cumulus clouds, which certainly differ considerably in form and appearance from the ova in which they originated.

53. Cumulo-rudimentum is one of the most frequent clouds of the globe. It is common in the