Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/132

Rh 122 METEOROLOGY [BAROMETRIC surface of the globe. The following are the mean varia tions of pressure from observations made on board the &quot;Challenger,&quot; September 1 to 12, 1875, in mean latitude 1 8 S. and long. 150 40 W, the mean being 29 928 inches : Inch. Inch. Inch. 2A.M. -0-012 10A.M. 0-032 GP.M. -0-028 4 ,, -0-022 Noon 006 8 ,, 004 6 ,, 0-003 2p.M. -0-043 10 ,, 0&quot;013 8 ,, 0-028 4 ,, -0-055 Midnight 012 The most striking feature in these oscillations is the amplitude of the range from the A.M. maximum to the P.M. minimum, amounting to O OST inch, and the rapidity of the fall from 10 A.M. to 2 P.M. The same feature appears in all means deduced from observations made at least 12 on each side of the equator. From October 12 to 22, 1875, in mean lat. 35 1 S., long. 134 35 W., the mean atmospheric pressure was 30 298 inches, and the difference between the A.M. maximum and the P.M. minimum was only 036 inch ; and from July 12 to 19, 1875, in mean lat. 36 16 N. and long. 156 11 W., the mean pressure was 30 328 inches, and the difference between the A.M. maximum and P.M. minimum was only 025 inch. Thus, with a mean pressure in the Pacific about lat. 35-36 1ST. and S. much greater than near the equator, the oscillation is much less, being in the North Pacific less than a third of what occurs near the equator. Similarly, this oscillation is small (or even smaller) in the high-pressure areas in the North and South Atlantic as compared with the same oscillation near the equator. It is well known that aqueous vapour absorbs the heat rays of the sun considerably more than does the dry air of the atmosphere ; how much more physicists have not yet accurately determined. Consequently air heavily charged with aqueous vapour will be heated directly by the sun s rays as they pass through it in a greater degree than comparatively dry air is. Now it is shown further on that the prevailing surface winds outflow in every direction from the areas of high mean pressure in the Atlantic and Pacific about lat. 36 N. and S. Since, notwithstanding, the pressure continues high, it necessarily follows that the high pressure is maintained by an inflow of upper currents, and as the slow descending movement of the air connects the inflowing upper currents with the outflowing prevailing winds of the surface, it follows that the air over high- pressure areas is very dry, and that it is driest where pressure is highest and the high-pressure area best defined. Hence over the best-defined anticyclonic regions the air will be least raised in temperature through all its height by the heat rays of the sun. On the other hand, between these high-pressure areas of the great oceans there is a belt of comparatively low pressure towards which the north and south trades pour their vapour unceasingly. The atmosphere of this belt of low pressure is thus highly saturated with aqueous vapour which rises in a vast ascending stream of moist air to the higher regions of the atmosphere. These equatorial regions thus present to the sun a highly saturated atmosphere reaching to a very great height. It is in these regions therefore that the atmosphere will be most highly heated by the sun s heat rays as they pass through it. One of the most striking facts of meteorology is the suddenness with which this barometric oscillation increases in amplitude on entering on these parts of intertropical regions ; and the rapidity with which its amplitude diminishes on advancing on the high-pressure regions of the horse latitudes is equally striking. The following are the mean oscillations in the middle regions of the four great oceans about lat. 36 from the A.M. maximum to the P.M. minimum about the time of the year in each case when the sun is highest in the heavens : South Pacific, 0*036 inch ; North Pacific, 025 inch; South Atlantic, 024 inch; and North Atlantic, 014 inch. These amplitudes diminish as the ocean becomes more land-locked with continents, or as the anticyclonic region becomes better defined and currents of air are poured down more steadily from the higher regions of the atmosphere. If the temperature of the whole of the earth s atmosphere were raised, atmospheric pressure would be diminished, for the simple reason that the mass of the atmosphere would thereby be removed to a greater distance from the earth s centre of gravity. Quite different results, however, would follow if the temperature of only a section of the earth s atmosphere were simultaneously raised, such as the section comprised between long. 20 and 60 W. The immediate effect would be an increase of barometric pressure, owing to expansion from the higher temperature ; and a subsequent effect would be the setting in of an ascending current more or less powerful, according to the differences between the temperature of the heated section and that of the air on each side. These are essentially the conditions under which the morning maximum and afternoon minimum of atmospheric pressure take place. The earth makes a complete revolution round its axis in twenty-four hours, and in the same brief interval the double-crested and double-troughed atmospheric diurnal tide makes a complete circuit of the globe. The whole of the diurnal phenomenon of the atmospheric tides is there fore rapidly propagated over the surface of the earth from east to west, the movement being most rapid in equatorial regions, and there the amplitude of the oscillations is greater than in higher latitudes under similar atmospheric, astronomical, and geographical conditions. Owing to the rapidity of the diurnal heating of the atmosphere by the sun through its whole height, some time elapses before the higher expansive force called into play by the increase of temperature can counteract the vertical and lateral resist ance it meets from the inertia and viscosity of the air. Till this resistance is overcome, the barometer continues to rise, not because the mass of atmosphere overhead is increased, but because a higher temperature has increased the tension or pressure. When the resistance has been overcome, an ascending current of the warm air sets in, the tension begins to be reduced, and the barometer falls and continues to fall till the afternoon minimum is reached. Thus the forenoon maximum and afternoon minimum are simply a temperature effect, the amplitude of the oscillation being determined by latitude, the quantity of aqueous vapour overhead, and the sun s place in the sky. All observations show that over the ocean, latitude for latitude, the amplitude of the oscillations is greater in an atmosphere highly charged with aqueous vapour and less in a dry atmosphere. It is also to be noted that in very elevated situations, particularly in tropical regions, the amplitude is greater proportionally to the whole pressure than at lower levels. This is what is to be expected from the law of radiant heat by which more of the heat rays of the sun is absorbed by the air, and particularly by its aqueous vapour, mass for mass, in the higher than in the lower strata. When the daily maximum temperature is past, and the temperature has begun to fall, the air becomes more condensed in the lower strata, and pressure consequently at great heights is lowered. Owing to this lower pressure in the upper regions of the air, the ascending current which rises from the longitudes where at the time the afternoon pressure is low flows back to eastward, thus increasing the pressure over those longitudes where the temperature is now falling. This atmospheric quasi-tidal movement occasions the P.M. increase of pressure, which reaches the maximum from 9 P.M. to midnight, according