BAROMETRIC OSCILLATIONS.] METEOROLOGY 121 of the night, because if the temperature falls a little below the dew-point the liberation of heat as the vapour is condensed into dew speedily raises it, and if it rises higher the loss of heat by radiation speedily lowers it. This consideration suggests an important practical use of the hygrometer, it being evident that by ascertaining the dew-point the approach of frost or low temperature likely to injure vegetation may be foreseen and provided against. Diurnal Oscillations of the Barometer. The general character of the daily oscillations of atmospheric pressure is shown by the two curves of fig. 2. The solid line gives the mean oscillation for Bombay and the 4 A.M 10A.M. 4 P.M. 10 P.M i 1 1 1 1 1 lx^ v 1 1 1 1 1 1 1 1 1 1 /
/ V /
/ " A z
/. 2 / . X
/ v /
/ V-. .-/ ^
" /
z V- ,x 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 I 1 1 I FIG. 2. Daily Oscillations of Atmospheric Pressure. dotted line that for Vienna, these two curves being to a large extent typical of diurnal barometric oscillations in tropical and temperate regions as regards the two maxima and minima and the time of their occurrence. A series of twelve maps of the globe were prepared for June, showing, for all stations whence observations have been obtained, the deviations at noon, 2 P.M., 4 P.M., &c., Greenwich mean time, from the mean daily pressure ; and four lines were drawn indicating the positions of the two daily maxima and minima at these hours. For fully 30 D north and south of the equator the lines of maxima and minima run north and south, but in higher latitudes these directions are changed, and the changes are chiefly conspicuous as regards the A.M. maximum and the P.M. minimum. Thus, for example, at 6 P.M. (G. M. T.) the line of P.M. minimum is for the latitude of London near 16 W. long. ; in 30 N. lat. it is in 35 W. long., in which the line runs south as far as 30 S. lat. ; its course thence turns south-westwards to near the Falkland Islands, 60 W. long. Hence in June the P.M. minimum occurs about three hours earlier in the Falkland Islands than to the south-west of Ireland, thus showing in a striking manner the influence of season on this phenomenon. In the middle and higher latitudes in summer, proximity to the sea delays the time of occurrence of the A.M. maximum and the P.M. minimum; whilst in continental situations the A.M. maximum occurs much earlier than in lower latitudes, and the P.M. minimum nearly as late as at places near the sea. In cases where the lines of maxima and minima cross a region such as southern and western Europe, whose surface is diversified by large tracts of land and sheets of water, the deflexions are of a remarkable character. The retardation of the time of occurrence of the A.M. maximum is greatest in situations which, while eminently insular in character, are at the same time not far from an extensive tract of land. Of this Holland presents the best example in Europe ; and there the A.M. maximum, which at Paris occurs at 8 A.M., does not occur at Utrecht till 9.30 A.M., at Amsterdam till 12.30 P.M., and at Helder till 2 P.M. There is thus as regards the same diurnal pheno menon in June a difference of six hours between Paris and Helder. Sicily and the south of Italy on the one hand and Madrid on the other present also the most striking contrasts. Again at Sitka (56 50 N. lat., 135 W. long.), which has one of the most truly insular climates in the world, the A.M. maximum is delayed to 2.30 P.M. whereas at Astoria, ten degrees to southward, it occurs at 9.30 A.M., and at Fort Churchill, in California, as early as 7 A.M. There is thus as regards the same phenomenon a difference of 7 h 30 between Sitka and Fort Churchill. From hourly observations made in this month at the base, the top, and two intermediate points on Mount Washington (N. H.) it was found that the time of occur rence of the A.M. maximum at the base of the mountain, which is 2898 feet above the sea, was 8 A.M. ; at 4059 feet, 10 A.M.; at 5533 feet, 11 A.M.; and at the top, 6285 feet, noon. Hence, as regards the time of occur rence, the influence of an isolated mountain like Mount Washington brings about a result similar to what is observed in insular situations. But the analogy is even closer. In insular climates the minimum in the early morning is very greatly in excess of that in the afternoon ; and the same relation is observed on the top of Mount Washington, where the former is - 020 inch and the latter 004 inch. Again in continental climates the minimum in the early morning is much the smaller of the two, and the same relation was observed at the base of the mountain, where the observed minima were respectively O OOG inch and 020 inch. The differences presented by the daily curve of pressure at the top as compared with that at the base of the mountain have their explanation in the effects which follow the diurnal range of temperature. As the temperature is at the minimum at the time of least pressure in the morning, the atmosphere is more condensed in the stratum between the base and the top, and conse quently the barometer at the top reads relatively lower. As the temperature continues to rise during the day, the stratum of air above the base of the mountain expands, thus placing more air above the barometer at the top, so that, while at the base pressure begins to fall at 8 A.M., at the top it continues to rise till noon, simply from the mechanical upheaval of the air owing to the higher tempera ture. In the afternoon, when the minimum at the base falls to - 020 inch, it is only - 004 inch at the top, this relatively higher pressure at the top being due to expansion from temperature. The peculiar feature of the pressure curve at the top is essentially a temperature effect. The diurnal oscillations of the barometer occur alike over the open sea and over the land surfaces of the globe. The atmosphere over the open sea, as already shown, rests on a floor or surface subject to a diurnal range of temperature so small as to render that temperature practically a constant both day and night. This considera tion leads to the vital and all-important conclusion that the diurnal oscillations of the barometer are not caused by the heating and cooling of the earth s surface by solar and | terrestrial radiation and by the effects which follow these diurnal changes in the temperature of the surface, but that they are primarily caused by the direct and immediate heating by solar radiation, and cooling by nocturnal radia tion to the cold regions of space, of the molecules of the air, and of its aqueous vapour. These changes of tem perature are instantly communicated through the whole atmosphere from its lowermost stratum resting on the earth s surface to the extreme limit of the atmosphere, which the flight of meteors proves to be not less than 500 miles. There are important modifications affecting the amplitude and times of occurrence of the four prominent phases of the phenomena observed over land surfaces, the temperature of which is being superheated during the day and cooled during the night ; but it is particularly to be noted that the barometric oscillations themselves are inde pendent of any changes of temperature of the floor on which the atmosphere rests. Let us first look at the phenomena in the simplest form as found in the Pacific, or in the midst of the largest water
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