The pressure of the atmosphere varies some what from day to day, and even from hour to hour, as well as with the latitude and with the height above the sea. For scientific purposes the normal atmospheric pressure is now gen erally taken to be equal to the pressure due to a column of pure mercury, 760 millimetres (29.9212 inches) high, at theievei of the sea, in latitude 45'; the mercury being at the tem perature 32° F. The pressure so defined is called an °atmosphere); or, more briefly and conveniently, an eatmo.a The °atmosphere) of pressure, as so defined, is nearly equal to a pressure of 1,000,000 dynes per square centi metre, and it has therefore been proposed to take 1,000,000 dynes per square centimetre as the standard atmosphere of pressure, calling it an °absolute atmosphere?' because the dyne is a unit in the °absolute system) of units. This proposal has not yet been adopted by physi cists to any great extent. See UNITS.
Knowing the pressure exerted by the atmos phere upon each square inch of the earth's surface to be about 14.7 pounds, and knowing the dimensions of the earth, it is not difficult to calculate the total weight of the entire atmos phere. The calculation when performed, shows that the mass of the atmosphere is about 1/1,000,000th of that of the whole earth.
If the atmosphere were of uniform density, it would be easy to calculate the height to which it extends. We should only have to divide the pressure upon one square inch of the earth's surface by the weight of a cubic inch of the air, and the quotient would be the height of the atmosphere, in inches. Thus a cubic inch of air, at a pressure of 30 inches of mercury and at the temperature of freezing water, weighs about 0.000749 of an ounce; and as a column of mercury 30 inches high exerts a static pressure of about 235.8 ounces, it fol lows that if the atmosphere were homogeneous (that is, of uniform density throughout), its height would be about 253.8+ 0.000749 = 314,000 inches, or 4.97 miles, when the air has a temperature of 32° F., and the barometric pressure is 30 inches. The height so calcu lated is convenient for use in certain physical computations, and is called the °height of the homogeneous atmosphere." If we turn from this problem to the more difficult one of de termining the actual height of the atmosphere, we find that no satisfactory results can be given. As we go up, the strata become rarer and rarer, for the reason that the lower layers are weighed down and compressed by those above, and at increasing heights there is less and less air above, to exert this compression. At great heights the atmosphere becomes more and more attenuated, and thins out by in sensible gradations into a perfect vacuum. There is no definite boundary, immediately below which there is an atmosphere, and im mediately above which there is none. Glaisher and Coxwell, in their famous balloon ascension of 5 Sept. 1862, attained an actual elevation of over 29,000 feet, and observed a barometric height of 9.5 inches (corrected) ; but it is certain that the atmosphere extends far higher than this. Some estimates, based on the cal culated heights of shooting stars when they first become luminous, place the limit at which the atmosphere has a density sufficient to pro duce any observable effects at about 200 miles; but, as has been pointed out above, all esti mates of this kind are necessarily indefinite and unsatisfying. (For some of the questions
raised in connection with the limits of the atmosphere, see GASES, KINETIC THEORY OF).
The atmosphere, as might he expected from its relatively great depth, exhibits an absorp tion spectrum (see SPECTROSCOPE), and this vanes to a certain extent from time to time. A portion of this absorption spectrum is due to the presence of water vapor, and the °rain bands" in the spectrum have been utilized to a limited extent (though not very generally) in connection with weather predictions. It is also known that the atmosphere is less trans parent to the rays at the blue end of the spectrum than to those in the middle and toward the red end. The experiments of Prof. S. P. Langley, on the expedition of the United States Signal Service to Mount Whitney, demonstrated that this selective absorption is so great that the sun would appear distinctly bluish, instead of white or yellowish as it does under actual conditions, if we could see it from a point outside of our own atmosphere. Consult Langley, searches on Solar Heat' (1884).
Little is yet known concerning the electrical phenomena of the atmosphere. In clear, calm weather, the atmosphere appears to be always positively electrified, with respect to the earth, and the difference in potential increases greatly during snowstorms and high winds. In thun der storms it is subject to sudden and violent oscillations, as might be expected. Many the ories have been proposed to account for the electrification so observed, particularly for the enormously high potentials that are in evidence during thunder storms; but none has yet met with general acceptance. It was formerly thought that the evaporation and condensation of water had much to do with it, but no ex perimental evidence has been adduced to justify this hypothesis, although physicists have given it the most careful attention. Bartoli and Pet tinelli made exhaustive experiments in connec tion with it, both with water and with organic compounds; but always without obtaining any favorable results. Kelvin, MacLean and Gall observed electrification when dry air bubbled through a liquid, the air being electrified nega tively in the case of pure water, and positively in the case of sulphuric acid or salt water. Apparently these arc all friction phenomena, and it is not certain that they have any bearing on the electrical phenomena of the atmosphere. We know, from numerous experiments, that dust facilitates the condensation of aqueous vapor, and numerous authorities have en deavored to trace a similar connection between dust and the development of high electric po tentials in the atmosphere. No certain results have been attained, however, as may be judged from the fact that in the 12 years immediately preceding 1902, no less than 25 new thunder storm theories were proposed, 6 of these being published during the year 1895. See also METEOROLOGY ; WIND.