All these periodical or varying winds may be very properly, from their time and character, be called the season winds of the earth, as another means of distinction from the constant trades: as they in part bring changes of season, and in part are brought that way.

Into the question of climate and seasons one other element enters, of especial importance in regard to those disturbances of the regular winds, which we call storms. That factor is the quantity of moisture in the atmosphere, and the consequent rainfall or snowfall of a region. Without this element, the phenomenal disturbances known as tornadoes would hardly occur: or if they did, there would be greater difficulty in ascertaining their approach.

Water, in its vapor state, is but three-fifths the weight of the air, and in consequence rapidly rises. This evaporation, as it is called, goes on at all times: even when the water is frozen. A very thin sheet of ice, hung in the open air, will finally disappear, even though the temperature be always below freezing.

Now, all the phenomena of rain, snow, and hail, that are brought by different seasons, in different climes, depend upon a single simple law: that warm air can hold a much greater quantity of vapor than cold air. The amount of moisture that may be held in suspension at different temperatures is as follows:

Temperature
of Air. 		Weight of vapor
in a cubic foot
of saturated air. 		Temperature
of Air. 		Weight of vapor
in a cubic foot
of saturated air.
20deg.Fahr.1.30grainsTroy.70deg.Fahr.8.00 grainsTroy.
32““2.13““80““10.95““
50““4.09““90““14.81““
62““6.15““100““19.79““

This gives a second reason why storms of wind and rain closely follow extremely hot weather.

Now, as the vapor is so much lighter than the air, their mixture must also be lighter. So any unusual amount of moisture is at once detected by the barometer, an instrument for measuring the pressure of the atmosphere. If the air grow moister, and therefore lighter, the barometer falls; a storm is approaching.

Since cold air can retain but little moisture, if a warm moist current be chilled, it must lose a part of its vapor, which at once falls to the earth as rain. If the cold be somewhat greater, the moisture is crystallized into snow. Greely’s observations at Fort Conger show that, varied as are the forms of snow crystals, those that fall during any particular storm are invariably of the same types, even though they may be collected from localities widely removed from each other. All crystals of snow are hexagonal in plan, but there is much variety in detail. The laws that produce one variety at one time, and a second at another, are not yet known.

The subject of hail is a peculiarly perplexing one to the meteorologist. Hailstones are more or less spherical in form, and are made of alternate layers of soft opaque ice, and hard clear ice. It is evident that they must acquire this structure by being whirled about between clouds of different temperature and density. Some have supposed that they are formed in a whirlwind, whose axis is horizontal, but for the present we must be content with Lord Dundreary’s explanation, for “it ith one of thothe thingth which no fellah can underthtand.”

Raindrops from a great height are larger than those from below, for they increase as they pass through the vapor-masses. As the warmest currents are also the highest, it will at once be understood why warm and tropical rains fall in large drops, while drizzling rains, mists, and fogs are characteristic of cold regions and cold seasons.