CHAPTER VI

CLOUDS OF INCLINATION

87. We now come to deal with the last great genus of clouds in our classification. Though fewer in the number of their varieties, less striking to the ordinary observer's eye, and perhaps less common as a whole, yet clouds of Inclination are perhaps more important and more indispensable to the correct forecast of weather than any others. For this reason it seems desirable, before describing each separately, to give a short general description of the structure and characters of the whole class.

Clouds of Inclination resemble rather the clouds of Interfret than the clouds of Inversion in general appearance. It is scarcely likely that any but the most unpractised observer would confound Cirrus and Cumulus, yet it is an unquestionably difficult task to describe the distinction between Cirro-macula and Stratus Maculosus. In fact, so close is the analogy between some of the Interfret clouds and some of the clouds of Inclination, that we cannot help seeing that the phenomena of Interfret play an important part in that part of cloudland which is the domain of Cirrus. But, on the other hand, the

general process which gives rise to the name "Inclination" is of a nature entirely distinct from and much simpler than that of Interfret or of Inversion.

88. This process, already briefly treated of in chap. i., may be described as follows:

When, from any causes, a layer of air containing water-vapour has been carried to a very high altitude into layers of air which are very rarefied and of very low temperature, condensation will commence as soon as this layer has been sufficiently cooled, and this will be helped by Interfret action between it and the surrounding layers. The minute water particles thus formed immediately fall by their own weight through the cold layers, and are almost as immediately frozen into ice dust. If their fall still continues, they are carried through currents which grow gradually warmer but move gradually slower and slower, owing to friction, until finally they experience a temperature which forbids their existence as ice particles, and converts them first into water particles, and then, probably very soon, into their old form of watervapour. The cloud thus formed will obviously present a curved form to the eye of an observer, for each portion will lag behind the more rapidly moving portion above it. This curve has already been noticed in connection with Stratus Præcipitans, and it is the careful observation of this "slant" twist" that gives us so much information as to the force and direction of the upper currents.

or

89. The process above described is that of the simplest type of Inclination clouds, and it is evident

that this process may be much complicated. The particles may, as it were, be checked almost at once in their fall, or may, after falling a considerable distance, be met by cross currents of warmer or colder air, and thus Interfret action at different levels may take place; moreover, the whole process may occur in connection with great Inversion movements. In fact, as will be seen later, the very existence of a quantity of moisture at such a high altitude is generally due to due to one of these latter

movements.

The above description also applies to the normal formation of clouds of Inclination, but it will be evident that the same result would be obtained if the particles, instead of falling downwards, were emitted in an upward direction. Whether it be

a great Inversion movement, or whether it be a geographical obstruction such as a mountain, that is the cause of an ascensional moist current, the curved form of the cloud produced will be reversed, but otherwise exactly similar to that produced by falling particles. In the former case the cloud produced is a true Inclination cloud, in the latter it is Stratus Præcipitans (§ 49).

90. But the physical constitution of clouds of Inclination, in that they consist almost entirely of minute spicules of ice, is a feature which distinguishes them especially from other forms of cloud. And it is in their electrical relations that we observe a special contrast. For water spherules, owing to their spherical shape, are adapted to hold the maximum

charge of electricity, whereas the points of the ice needles discharge the electricity in streams, and the needles arrange themselves end-on in lines or threads which are approximately parallel to one another. Thus clouds of Inclination are far more perfect conductors of electricity than those clouds which are composed of particles of water, and their character in this respect plays a most important part in the production of rainfall. With few and rather unimportant exceptions the rain-producing clouds are composites formed either by the close approach, or still more commonly, by the actual union of clouds of Inclination with other cloud-forms. It is when one of the huge clouds of Inversion, heavily laden with a great charge of electricity, has towered up and invaded the territory of the clouds of Inclination that the roar of battle begins. If we watch him carefully, we see that the top of his mighty head has suddenly lost its rounded appearance and assumed a hairy look, as if some invisible hand had plucked him by the beard and he is bristling with anger. But what has really happened is this-he has met with or made. for himself a good conductor, in fact, has become cirrified." This conductor, consisting of ice needles, gradually draws off the charge of electricity, and this lessens the mutual repulsion of the water particles, which therefore unite, forming drops, and fall to the earth as rain. It is for an exactly similar reason, as has been mentioned in § 71, that a large disruptive discharge to earth is generally followed by a heavy dash of rain. It is obvious also that if for any

I

reason the water particles become frozen at any lower part of the cloud, that part at once conducts away electricity, and thus we have a fall of rain or snow without any actual invasion of the Cirrus level. Of course such a phenomenon is not common, but the fact that it does sometimes happen tends greatly to support this theory.

We thus see that it is in their electrical relations with other clouds that clouds of Inclination have such an important effect on weather.

Nubes Fulgens

91. It would not be strictly true to say that Luminous cloud is a cloud of Inclination, but it is convenient to notice it under this class, as it is certainly a cloud of the "upper currents." This cloud has received much attention of late years, owing to its mysterious and remarkable character. It exists at an altitude far greater than that of any other cloud associated with our own planet. It appears to the casual observer as self-luminous, and suggests the idea of luminous light. But, from various reasons, the conclusion has been drawn that Luminous cloud merely reflects the rays of solar light; and we might compare its appearance to the mirrored volcano gleams often mistaken for flame in the dust cloud hanging at night over the crater of a burning mountain. It has not been shown to be distinctly associated with auroral or sun-spot phenomena. Though perhaps not composed of