Fig. 13. A mountain peak snow capped, and covered on the very crest by a cloud.
Some mountains are so lofty that it never rains upon them, but snows instead; and they are never free from snow, even in mid-summer. If one climbs to the top of such peaks he finds it always very cold there. While he is shivering from the cold he can look down upon the green fields where the birds are singing, the flowers blossoming and the men, working in the fields, are complaining of the heat.
One who watches such a mountain as this, or in fact any mountain peak, will notice that it is frequently wrapped in clouds ([Fig. 13]). Damp winds blowing against the cold mountains are chilled and the vapor is condensed. If one climbs through such a cloud, as thousands of people have done when climbing mountains, he often seems to pass through nothing but a fog, for really many clouds are only fogs high in the air. ([Fig. 14]).
But very often rain falls from these clouds that cling to the mountain sides. The reason for this is easy to understand. As the air comes against the cold mountains so much vapor is condensed that some of the tiny fog particles grow larger and larger until they become mist particles, which are too heavy to float in the air. They then begin to settle; and as one particle strikes against another, the two unite, and this continues until perhaps a dozen have joined together so as to form a good-sized drop, which is so heavy that it is obliged to fall to the ground as rain.
Fig. 14. Clouds clinging to the mountain sides. If one were climbing these mountains he would find himself, in passing through the clouds, either in a fog or a mist.
Let us now look at our summer storms. These do not form about mountain peaks; yet what has been said about the mountains will help us to understand such showers.
It is a hot summer day. The air is muggy and oppressive, so that the least exertion causes a perspiration, and even in the shade one is uncomfortably hot. Soon great banks of clouds appear ([Fig. 15]),—the "thunder heads,"—and people say "a thunder shower is coming, so that we will soon have relief from this oppressive heat." The clouds draw near, lightning is seen and thunder heard, and from the black base of the cloud, torrents of water fall upon the earth. If we could have watched this cloud from the beginning, and followed it on its course, we would have seen some facts that would help explain it. Similar clouds perhaps began to form over your head in the early afternoon and drifted away toward the east, developing into thunder storms many miles to the east of you.
On such a day as this, the air near the ground is so damp that it gives up vapor easily, as you can prove by allowing a glass of ice water to stand on a table and watching the drops of water gather there, causing the glass to "sweat" ([Fig. 9]). The sun beats down upon the heated ground and the surface becomes like a furnace, so that the air near the ground is warmed.