Fig. 718.—Cirrus cloud.
The cirro-stratus is perceived in long parallel lines, and indicates rain; when made-up rows of little curved clouds it is a certain prophet of storm, and when viewed as haze is also indicative of rain or snow. “Mock-suns” and halos are often observed in the cirro-stratus.
The nimbus is the rain-cloud, or condition of a cloud in which rain falls from it. It is upon this rain-cloud we can perceive the rainbow, and on no other cloud, but otherwise only in the sky.
We have now seen the varieties of cloud and their common origin with fogs and mists, which differ from them only in the elevation at which they come into existence, according to the condition of the atmosphere.
The uses of clouds are many and varied. Their first and most apparent use seems to be the collection and distribution of rain upon the earth. But besides this, they shelter us from the too great heat of the sun, and check the evaporation at night. Supposing we had no clouds we should have no rain. If we had no rain the earth would dry up, and the globe would appear as the side of the moon appears—a waterless desert. The invisible vapour in the atmosphere will produce cloud, but the moon can have no atmosphere in that sense. Vapour will also absorb heat, and intercept the sun’s heat rays, acting much as clouds do in preventing radiation and great changes of temperature.[33]
All animals and plants depend upon moisture in the atmosphere as much as upon the varying degrees of warmth. A dry east wind effects us all prejudicially; warm, soft airs influence us again in other ways. Air will be found drier as a rule in continents than in islands or maritime districts, and this will account for the clearness of the sky in continental regions. Fogs and mists arise when the air is what is termed saturated with moisture, and colder than the earth or waters upon it. So the celebrated and dangerous fogbanks of Newfoundland arise from the warm water of the Gulf Stream, which is higher in temperature than the air already saturated. And the same effect is produced when a warm wind blows against a cold mountain; the air is cooled, and condenses in cloud.
The cooling of the breath by the exterior air is exemplified in winter when we can perceive the vapour issuing from our mouths as we speak.
