Snow sometimes reaches us in a partly melted condition; under these circumstances it is called sleet, and snow being much lighter than rain (ice is lighter than water), it descends less directly, and represents about one-tenth the depth of the rain-fall. The use of snow in warming the earth is universally acknowledged, and as it is such a bad conductor, a man in a snow hut will soon become unpleasantly warm.
Fig. 723.—Ice crystal.
Ice is only water in another form, and snow is ice; and it is the air in the snow that gives it warming properties. These are all simple facts, which any one by observation and careful reading and study may soon ascertain for himself. We have another frozen fall of water from the clouds—viz., hail, which may possibly be the development of sleet.
Hail is formed by the falling rain being frozen in its descent, or when different currents meet in the atmosphere. A hail-storm is accompanied with a rushing sound, as if the hail-stones were striking against each other. They are very destructive, and actual hail showers occur in summer more frequently than in winter, and a peculiarity noticeable with regard to hail is its infrequent occurrence during the night.
Records of destructive hail storms are plentiful. The hail assumes a great size, weighing sometimes as much as two ounces, and measuring several inches round. Thunder and lightning are very frequent accompaniments of hail showers.
Dew is moisture of the atmosphere deposited on a cool surface—another form of condensation, in fact. Cold water in a tumbler will produce a “dew” upon the outside of the glass when carried into a warm atmosphere. Such is the dew upon the grass. It is produced by the air depositing moisture as it becomes colder after a warm day when much vapour was absorbed. Warm air can hold more water than cold air, and, the saturation point being reached, the excess falls as dew at the dew (or saturation) point. We have previously remarked that one use of clouds was to prevent rapid radiation of heat which they keep below. Under these circumstances—viz., when a night is cloudy—we shall find much less dew upon the grass than when a night has been quite clear, because the heat has left the atmosphere for the higher regions, and has then been kept down by the clouds; but on a clear night the air has become cooled rapidly by radiation, and having arrived at saturation point, condensation takes place.
Dew does not fall, it is deposited; and may be more or less according to circumstances, for shelter impedes the radiation, and some objects radiate less heat than others. Hence some objects will be covered with dew and others scarcely wetted.
When the temperature of the air is very low,—down to freezing point,—the particles of moisture become frozen, and appear as hoar-frost upon the ground. Thus dew and hoar-frost are the same thing under different atmospheric conditions, as are water and ice and vapour.
We have now come round again almost to whence we started. We have seen the land and water, and the parts that water, in its various forms, plays upon the land, and its effects in the air as rain, etc. We have noticed the winds and air currents as well as the ocean and its currents. We know what becomes of rain and how it is produced, and how the sea works upon the shore, and how clouds benefit us. There are besides some less common phenomena which we will now proceed to examine.