Fig. 732.—In the northern Seas.
Weather permitting, we can go out and study the clouds as described in the foregoing chapters, or consult the barometer, and see which way the wind blows. The child will tell us that a high “glass” means fine weather, and a low barometer indicates rain, but this is only relatively true. A high glass may be falling, a low glass may be rising. A sudden fall or a sudden rise are indicative of bad, windy weather, or a short-lived fine period. The glass may rise with a northerly wind, and rain will supervene, so careful observation is necessary before one can obtain even a superficial knowledge of the weather. (See subsequent observations on “Weather.”)
The Americans telegraph the results of their observations of coming storms across the Continent, corrected by the signs noticed and recorded by vessels arriving in New York. Thus they are frequently very accurate; steady application and observation at Sandy Hook must give them a great deal of useful information for the “forecasts.”
The word Climate is derived from the Greek klima, a slope; and thus at a glance we perceive how the aspect it presents to the rays of the sun in the earth’s revolutions, must affect the “climate” of a country. Of course the position of any portion, the elevation and locality of the mountains, have also a share with the soil, winds, rains, and sea-board, in determining the climate of any region. Many points have already been touched upon in former chapters. Temperature, moisture, and vegetation are the chief natural features which determine climate, and we must find out the position of the land with reference to the sun first, to ascertain the climate.
The more vertical the sun is the hotter the atmosphere, for the rays strike directly upon the earth, which radiates the warmth received. These heat rays are, as we know, invisible. The hottest portion of the earth must be at the equator for the sun is overhead, and the rays beat down directly upon the earth. The sun is also nearer than when at the horizon, and less rays are absorbed by the atmosphere. The longer the day the greater the heat.
Fig. 733.—In the southern steppes.
Temperature is registered by observation of the thermometer, and the distribution of heat is represented upon a chart across which lines are drawn at places of equal temperature. These lines are called “isothermal.” There are also terms to denote equal winter temperature and the average summer heat—isochimines and isotheres respectively.
Temperature decreases as we ascend from, and increases as we descend into, the earth. This fact proves that the air is not warmed by the sun’s heat, but by radiation from the ground. As we ascend we reach the line of perpetual snow, which varies in different parts of the globe. In the tropics it extends from 15,000 to 18,000 feet; but it varies even in places of the same latitude, according as the towns are inland or on the coast, as in the Pyrenees and Caucasus, where there is a difference of three thousand feet in the snow limit.