Page:Popular Science Monthly Volume 54.djvu/99

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91
THE CAUSE OF RAIN.

hygrometer sometimes remains near saturation without there being precipitation of vapor; and, supposing that the temperature is near 3° or 4° C, which is about the mean temperature of the year, it will require a cooling of only one or two degrees centigrade at most for the air to be unable to hold all its vapor and for the excess of it to be transformed into rain. This is confirmed by experiment and observation.

I will mention a remarkable example illustrating this point. Not rarely, when the west wind is blowing violently on the top of the Puy-de-Dôme, an east wind, blowing opposite to it, prevails at Clermont. Then an eddy is formed behind the plateau and the chain of puys that runs from north to south, a little west of Clermont. This eddy gradually becomes a vast whirlwind with a horizontal axis, several leagues long, a few kilometres wide, and seven hundred or eight hundred metres high. It commonly gives rise to an abundant and continuous formation of black clouds, which appear in an instant along its length, following its intersection with the upper current. The phenomenon is frequent, and is sometimes produced under very interesting conditions, as on a certain day when the temperature at Clermont was five degrees above zero, centigrade, while the hygrometer indicated that the air contained seven tenths of the quantity of vapor required for saturation. Under such conditions the temperature on the Puy-de-Dôme would have only had to be a very little above the freezing point for the vapor of the horizontal eddy to be transformed into rain on meeting the upper current coming from the west. Now, on the top of the mountain the thermometer marked 4° C. below the freezing point. Hence, every time the lower east wind increased a little, this having the effect of carrying the vapor and the air of the lower regions a little higher, the black clouds could be seen developing with a recrudescence of intensity. A few instants afterward a torrential rain fell at Clermont.

In some cases—and such frequently occur in summer—the mingling of strata of air of different temperatures is effected by ascending currents. The sky is clear; the moist air in contact with the soil is warmed under the action of the sun, rises, and more or less quickly reaches a much colder stratum. Light mists are formed; they may frequently be seen rising and spreading out over the warmer or moister spots. On the flanks of the Puy-de-Dôme one may often find himself among ascending currents of this sort which succeed one another intermittingly when the air is calm, after a rain; they rise with a velocity of four or five metres at least per second.

These fogs finally become stationary in a region of the same density with themselves. There they accumulate and form a cloud or a group of clouds that go on developing. When penetrated by the