Under this heading we shall find the atmosphere playing a very important part. The air is composed of oxygen and nitrogen with some carbonic acid gas and aqueous vapour. We have, under the Chemistry section, discussed these constituents which unite to make up the air or atmosphere in the following proportions:—
| Oxygen | 210·0 |
| Nitrogen | 775·0 |
| Aqueous Vapour | 14·2 |
| Carbonic Acid | 0·8 |
| 1000·0 |
It is a fact that all over the world the same chemical result is found. Whether we bottle up the air in the valley, or, as Gay-Lussac did, go up to an elevation of 21,000 feet in a balloon, we shall find the air of the same chemical composition. In Europe, Asia, Africa, and America, it is all the same. The pressure is less as we ascend, and we cannot manage to breathe in very high altitudes so well as upon the ground for which we were fitted, but the air is the same.
The atmosphere, then, is not always equal in density, nor is it quite transparent. The light from sun and stars is, to a certain extent, lost, and it has been calculated that the sun’s rays lose one-fifth part of their brightness passing through the atmosphere. We all know what the air is. We breathe it, we feel it blowing, we witness its effects. Were it not composed as it is we should die or go mad; plants would not live, and the earth would become a desert. Air is everywhere—invisible; a so-called empty vessel is full of air because an animal will live in it till the atmosphere has become vitiated by the carbonic acid from the lungs. Yet air, or rather its watery portion, is visible when condensed.
Vapour is not perceptible. But how does it become so? We cannot see the air, how can we see a portion of it? We can answer this question by illustration. The steam from an engine is not visible on a very hot day. But when the day is damp and dull the vapour is condensed, and becomes visible; then air appears and is resolved into vapour again. This change was effected by heating water and then cooling it, when it came back to water again. This watery vapour is always present in the atmosphere. Heat, also, is present in the atmosphere, and the sun is the origin of that heat.
Fig. 714.—High tide and storm on the coast of Schleswig.
Heat, we know, is the effects of the rapid motion of small particles of matter, and is radiated from our bodies—so we feel cold; it reaches our bodies, and we feel warm. So air is heated or cooled by the sun, not in its absence, except when the earth and air have been so warmed during the day that the heat is given out by them long after sunset. We have read of the pressure of the atmosphere in the Physics section, and that warm air is lighter than cold air, as shown in the ascension of the Montgolfier balloon. It is this variation of temperature of the atmosphere that gives birth to one great meteorological agent—viz., the Wind, which we will now consider.
Winds and Air Currents.
We can easily illustrate the cause of winds. Suppose we have a hot room and a cold one, and we suddenly open the door of communication between them, the heated air which has risen to the ceiling of one room will rush out through the upper part of the opening of the door, and the cooler current will flow in just above the floor. If we place a lighted candle in the upper and lower part of the opening we shall see the flame tending outwards from the heated room, and in an opposite direction from the cold room. In the centre of the open door there will be but slight disturbance. So it is in nature. The warm air ascends, cooler air rushes in to fill the space, and a storm or a breeze is created. The balance must be restored. The upper current probably moves one way, and the lower the other way. Thus clouds are said to be “coming up against the wind” when they are moving in an upper current, or in a different direction to that the wind is blowing just above the earth’s surface.