10. Having dealt at some length with inversion movements, we must now describe another process which has a great influence on cloud formation. It has already been stated that the pressure distribution is not equable all over the globe, but that there are various areas of low and of high pressure. These will be treated of hereafter, but it will be necessary here to state that they produce more or less horizontal currents over the globe, which move over one another with varying velocities and directions. Now the nearer a current is to the earth's surface, the more it is retarded by friction with the earth. The amount of this friction depends on the velocity of the current, and on the irregularities of the earth's surface. Every one is familiar with the fact that the wind often blows with greater strength on the open surface of the sea than at inland localities, and over an open plain than over broken or well-timbered country; also with the fact that at the summit of a tower more wind is experienced than at the base. This retardation, caused by friction with the earth, makes itself felt at a very great height. Consequently, as we rise higher, and the density of the air becomes less, the currents move faster.

Now the effects of friction are similarly felt at the contiguous surfaces of two currents moving with different velocities and directions. It has been found that these effects are similar to those produced by the action of wind over water, but, generally speaking, of far greater magnitude.

If two horizontal or nearly horizontal currents,

differing in velocity, or in direction, or in both velocity and direction, move the one over the other, the particles of air will intermingle to a certain extent at the surface of contact, and whirls, ripples, and waves will be produced, whose size and shape depend on the respective velocities and directions of these

currents.

Now, if the upper current be colder than the lower, as will usually be the case, and if the lower current contain water-vapour near its maximum

COLDER CURRENT

WARM MOIST CURRENT

FIG. 2.-INTERFRET ACTION PRODUCING CLOUD.

tension, it is evident that condensation will very likely take place over the crests of these waves. For these portions of the lower current are pushed far up into the colder upper current, and thus have their temperature reduced. To this process we give the title "Interfret" (Fig. 2).

11. If, however, the lower current be colder than the upper, and the latter is charged with vapour near its maximum tension, we shall have condensation taking place in the hollows of the waves instead of in their crests. To this phenomenon we give the title "Reversed Interfret" (Fig. 3).

We may thus have waves of cloud formed over the sky, either in long parallel rows, in which case the currents are from nearly the same or from nearly opposite points of the compass, or with no actual arrangement in parallel lines, but cut up into small waves, ripples, and vortices, like a choppy sea." In this latter case we have innumerable patches and flecks of cloud so often seen in fairly quiet weather in

summer.

66

12. There is yet another process of cloud-formation

WARM MOIST CURRENT

COLDER CURRENT

FIG. 3.-REVERSED INTERFRET PRODUCING CLOUD.

which is distinct from those already mentioned. This process is one belonging almost exclusively to very high and cold strata of air where there is seldom much moisture.

If the temperature of the air containing aqueous vapour is very low, and the tension of the vapour is near its maximum for that temperature, a sufficient decrease of temperature will congeal some of the vapour at once into ice. More usually, however, the vapour would first be condensed into water, and then, on decrease of temperature, this would be congealed into ice.

Thus, if water-vapour, from any cause, exists in strata of air of very great altitude and of very low temperature, and has a tension near its maximum for that temperature, and any reduction of temperature takes place (as by Interfret actions), this vapour will be either at once congealed into ice or first condensed. into water. In either case, in the rarefied atmosphere in which it exists it will fall by its own weight through strata of air below. The rapidity of descent will further decrease the temperature and ensure congelation into ice. Now, as a general rule, the nearer the falling particles get to the earth, the slower, warmer, and denser are the currents with which they meet. Consequently, the lower the particles descend, the more they lag behind the more rapidly moving particles above, and the higher the temperatures which they experience. They thus form a twisted or curved cloud, which hangs from a point in the sky and disappears again into vapour below. The interpretation of this curve will be treated of in a later chapter. To this method of cloud-formation we give the term "Inclination."

If the reader remembers that ice particles are less dense than water particles, and that their form usually presents a greater surface to the resistance of the air, also that in falling they meet with air of ever-increasing density, he will have no difficulty in understanding how clouds of this kind can exist for great lengths of time at very great altitudes where the pressure and the temperature are very low.

13. A few words may now be said on some other

.

effects that are produced by aqueous vapour in the atmosphere. Whereas dry air absorbs very little of the heat which passes through it to and from the earth, it is believed that aqueous vapour will arrest a considerable amount of this heat. In any case it is certain, from chromatic and other observations, that a portion of the vapour is often condensed sporadically on dust particles present in the air, for radiation from these particles lowers the temperature of the surrounding air. In this state it acts as a very important screen for the organic life existent upon the globe. The fierce heat of warm climates at periods when the atmosphere is extremely dry is the cause of the severe effects experienced by organic life when this veil is partially withdrawn. On the other hand, the excessively humid atmosphere prevalent in the same or similar latitudes when the tension of aqueous vapour approaches its maximum is favourable to organic life in general, although its enervating and depressing influence happens to be injurious to European man. At very high altitudes the solar rays beating on the mountaineer out of the dark steely sky are almost insufferably hot, and cause severe sunburns on the face and hands, whilst the temperature of the atmosphere is many degrees below zero.

Actual cloud also acts as a screen to solar and

terrestrial radiation, as may be seen. on a hot summer's day by the cooling effect of the passage or formation of a thick cloud, or by the rise of temperature and disappearance of dew produced on a winter's night by the clouding of the sky.