Page:The American Cyclopædia (1879) Volume XI.djvu/445

 METEOROLOGY 433 those below which the phenomena of twilight, the aurora, and shooting stars take place, and these are generally confined to an altitude less than 100 m. above the earth's surface. At an altitude of 12 m. the air has a density one tenth of that which it has at the surface of the earth ; that is, at that altitude the average barometric pressure would be three inches, and it is not likely that any changes take place at this height that are appreciable at the surface of the earth. The elevations of the mountains on the earth's surface bear a very appreciable ratio to the altitudes of the cloud-bearing and storm-producing strata of the air ; in fact, these latter nearly all lie far below the summits of the highest ranges ; and, as a consequence, the distribution of the ele- vated portions of the continents is a very im- portant factor in meteorology. Of equal or even more importance is the relative position of the land and oceans. Not only is the ocean the principal source of the moisture in the atmosphere, but its influence is felt in a very different way. The power of absorbing and radiating solar heat is very great for all vegeta- ble structures, such as the forests and prairies present; solid earth and rocks possess these properties in a far less degree, and even lower than these in the scale must be placed the ocean water. On the other hand, the specific heat of the earth is so much less than that of water, that the same amount of heat received from the sun upon each will affect the temper- ature of the air over the land nearly twice as much as that over the ocean. 2. Temperature. Atmospheric temperatures are almost uniform- ly measured by means of the mercurial or the spirit thermometers, the air thermometers be- ing at present employed only in researches of extreme delicacy. (See THERMOMETER.) The heat found within our atmosphere may be con- sidered as coming from six sources : 1, that peculiar to the interior of the earth ; 2, that received from the stars ; 3, that received from the moon ; 4, that received from shooting stars; 5, that produced by friction of tides, winds, &c. ; 6, that received from the sun ; all which should be added to that originally possessed by the atmosphere. Considering the original heat as tending continually to be radiated and lost in space, the true meteorological problem is to determine how this loss is made up from the six sources just enumerated. 1. That the interior of the earth is in general warmer than the surface is shown by observations of tem- perature in deep springs, wells of water, and mines, and by the phenomena of volcanoes and geysers, and is very plausibly demonstrated by all seismological theories. (See EARTHQUAKE.) As an average for the whole earth, and espe- cially applicable to the temperate zone, the temperature at a depth of 80 to 100 ft. is con- stant, showing that at this point we reach the surface of equilibrium between the conduction of heat from the interior outward, and the effect of the radiation from the earth's sur- face. The only known exception to this rule is found in the northern portion of Siberia, where at a depth of 400 ft. the temperature is still 10 below the freezing point. On ac- count of the slow conducting power of the materials of the earth, the superficial layer of 80 ft. in thickness should be viewed by the meteorologist not as a medium through which any important part of the interior heat of the earth is conducted to the atmosphere, but rath- er as a storehouse of the solar heat that enters into the ground, and thus equalizes the daily and annual changes of temperature. Never- theless, the quantity of heat conveyed into the atmosphere by the circulation of air within mines, by hot springs, and by. volcanoes, is perhaps barely appreciable, and is for the en- tire globe possibly equivalent to an elevation, of T ^-o F. in the temperature of the entire at- mosphere. 2. Since the earth is in the cen- tre of a celestial vault thickly studded with stars, each of which radiates heat in all di- rections, its tendency is to assume a tempera- ture higher than that which prevails through- out the interstellar spaces; the latter being estimated by Pouillet at about 222 F., while the temperature of the earth due to the stellar heat would be 128 F. 3. The heat radiated from the moon to the earth, though exceeding- ly small, has been measured by Lord Kosse ("Philosophical Transactions," 1873). The greater part of this heat is absorbed by the aqueous vapor of the atmosphere before it can reach the earth's surface. The principal effect of the lunar heat is therefore probably ex- erted in warming the upper layers of air, and in dissipating the clouds. The surface of the moon as heated by the sun attains to its max- imum temperature a few days after the full moon, at which time its influence on the clouds and on the rainfall is probably barely appre- ciable at stations favorably situated, though Klein (1868) and Wierzbicki (1873) have shown it to be inappreciable in the interior of Europe. 4. The numerous shooting stars that daily en- ter the earth's atmosphere bring into it an ap- preciable quantity of heat, as is evident by their own combustion and dissipation ; but as yet no accurate knowledge on this point is available. This heat is a simple case of the conversion of force into heat, and its amount could be calcu- lated had we any positive information as to the mass of the meteors. 5. A small fraction of the energy of the earth's daily rotation is by friction converted into heat, which passes from the ocean to the atmosphere and exerts a slight warming influence. For the further consideration of this obscure subject, see Fer- rel, "Tidal Researches" (Washington, 1874). 6. Solar radiation produces on the earth both chemical and optical as well as thermal effects. This is subject to very slight fluctuations, con- nected in some unknown way with the varia- tions of the solar spots ; the frequency and ex- tent of the latter vary in a period of 11-^ years, and possibly also in a period of 55 or 56 years.