Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/138

Rh 128 point to the fact that the current ascending from the surface is broken up and thereafter grouped into separate well-defined ascending currents, which are marked out and overtopped by these cumuli ; and further it is probable, from their well-defined position, that the air composing the ascending currents is not only warmer but also more humid than the air in the clear interspaces at the same heights. It may also be regarded as highly probable that it is down through these clear interspaces that the descend ing air-filaments shape their course in their way to take the place of the air-molecules that ascend from the heated surface of the earth. The secondary minimum occurs from about sunset onwards during the time of the P.M. maximum of atmo spheric pressure. In a highly saturated atmosphere, which is so characteristic a feature of many tropical climates at certain seasons, this time of the day is remarkable for the amount of cloud ; and it is during those seasons and hours that heat-lightning, or lightning without thunder, attains its annual and diurnal maximum period, which is from six to eight hours later than that of thunderstorms. The morning maximum, shortly before and after sunrise, has two quite distinct and characteristic clouds accompanying it. One of these is the cumulo-stratus, which is a con sequence of the cooling of the atmosphere through all its height by nocturnal radiation. As the colouring of the cloudlets is often singularly fine, it has been a favourite theme with poets of all ages and climes. The other, which results from the cooling of the surface of the earth by terrestrial radiation and thence of the lower stratum of the atmosphere, is quite different, being a low creeping mist, appearing first in low-lying situations, and gradually extending upwards as the temperature falls. Fog is a well-known form of this cloud, having its diurnal maximum in the morning and early part of the day. Diurnal Variation in the Amount of the Rainfall. From the sixteen years during which hourly observations of the rainfall were made at Batavia Dr Bergsma has given a table showing the diurnal variation, of which the follow ing shows how much per cent, of the total daily amount fell every two hours : Midt. to 2 A.M. 87 8 A.M.tolO A.M. 5 5 2A.M. ,,4 ,, 6-4 10 ,, ,, Noon 6 3 4 ,, ,,6 ,, 6-1 ! Noon ,, 2 P.M. 9 5 6 ,, 8 5-2 i 2 P.M.,, 4 ,, 12-2 The diurnal curve of rainfall is thus very distinctly marked at Batavia. The minimum is from 6 to 10 A.M. and the maximum from 2 to 6 P.M., 10 7 per cent, falling during the four hours ending 10 A.M., but 25 7 per cent, in the four hours ending 6 P.M. The observations were arranged and averaged by Dr Bergsma with the view of seeing how far the phases of the moon influence the rainfall. The results for the eight phases of the moon, beginning with new moon, showing the mean amount of rain in twenty-four hours during the seventeen years ending 1880, are these : 0-243, 0&quot;236, 0193, 0181, 212, 183, 0189, and 203, and daily mean 205 inch. The influence of the moon s phases on the rainfall at Batavia is thus quite decided from these seventeen years; for, while the mean daily rainfall is 205 inch, it rises at full moon to 243 inch, from which time it gradually falls to 0181 inch at the third octant, rises to 212 inch at the fourth octant, falls again to 0183 inch at the fifth octant, and finally rises to the maximum at the time of new moon. At Coimbra, where the rainfall has been observed every two hours for the six years ending 1881, the means show a minimum of 3 08 inches from midnight to 2 A.M., a maximum of 4 03 inches from 2 to 4 A.M., a second minimum of 3 20 inches from 10 A.M. to noon, and a second maximum from 2 to 4 P.M. These four phases of the rainfall are pretty nearly accordant with the four phases of the barometric pressure, the maximum periods being near the times of minimum pressure, and the minimum periods near the times of maximum pressure. The rainfall at Philadelphia shows a decided maximum at 6 P.M. and minimum at 3 A.M. At Vienna during the summer half of the year there are three maxima and three minima in the curve of the daily rainfall, the chief 4 P.M. to6 P.M. 13 5 6 8 10-5 8 ,, 10 ,, 7 4 10, ,, Midt. 87 [DIURNAL RAINFALL, maximum, which is nearly double of each of the other two, occurring from 2 to 4 P.M., and the chief minimum from 3 to 6 A.M. At this place the number of hours of rain has been recorded, showing two maximum periods, the first from 2 to 8 A.M. and the second from 4 to 7 P.M. Since the time of maximum amount indicates a number of hours for the fall under the mean of the day, it follows that the showers from 2 to 4 P.M. are often very heavy. At Prague the chief maximum is from 2 to 6 P.M. and the minimum from 1 to 6 A.M. At Zechen the maximum is from 2 to 7 P.M., with a secondary maximum from 5 to 8 A.M., and the minimum from midnight to 4 A.M. At Bern the maximum fall is from 7 P.M. to midnight. Twenty-one years observations at Calcutta show the maximum occurrence of rainfall during the rainy season from June to October to be from 11 A.M. to 6 P.M., the absolute maximum being from 2 to 3 P.M. and the minimum from 9 P.M. to 1 A.M.; and during the hot dry season from March to May the maximum is from 5 to 9 P.M., and the minimum from midnight to 9 A.M. The data already collected show the general occurrence of a diurnal maximum from about 11 A.M. to 6 P.M., and this feature of the curve is particularly well seen in the rainfall of continental climates during the summer half of the year. A marked diminution of the rainfall is very generally observed from about sunset to midnight, when the diurnal amount of cloud in many climates falls to the minimum, and the evening maximum of atmospheric pressure takes place. The time of the morning minimum pressure from about 2 to 6 A.M. is, curiously, strongly marked in some places with an increase, while in others it is as strongly marked with a diminution, in the rainfall. The &quot; Challenger&quot; observations show that the occurrence of rain on the open sea is inversely as the temperature, 684 days observations giving 96 cases from 9 to -i P.M., and 135 from midnight to 2 A.M., these being the minimum and maximum periods. Diurnal Variation of Thunderstorms. It has just been remarked that a prominent feature of the diurnal rainfall in continental climates during the summer months is the increased rainfall from about 11 A.M. to 6 P.M., and the Vienna observations point to unusually heavy falls occur ring at these hours. This is caused by the rains which accompany the thunderstorms of these regions. The following table gives for the hours of the day the times of occurrence of thunderstorms at Ekaterinburg in the Ural Mountains during the fourteen years ending 1872 : April. May. June. July. Aug. Sept. Year. Midnight to 1 A.M 5 5 5 15 1 A.M. 2 1 5 2 8 2 3 1 4 1 G 3 4 1 2 3 6 4 5 2 1 1 4 5 6 o 3 6 6 7 2 3 5 7 8 1 5 2 o 10 8 9 1 4 4 1 10 9 10 1 6 7 5 19 10 11 1 8 12 4 25 11 Noon 1 2 14 30 5 1 53 Noon 1 P M 3 4 19 25 5 4 60 1 P M. 2 2 8 21 29 12 2 74 2 3 3 10 22 35 15 4 89 3 4 3 6 26 45 20 1 101 4 5 2 5 24 33 9 73 5 6 1 6 25 30 11 2 75 6 7 2 7 15 20 10 54 7 8 2 5 16 20 9 52 8 9 2 6 14 14 6 42 9 10 2 3 8 10 5 1 29 10 11 1 6 6 5 18 11 Midnight... 1 7 6 8 22 Sums 27 64 255 351 144 15 856 Hence the thunderstorms at this place have a diurnal period as strongly marked as any other meteorological phenomena, and in this respect Ekaterinburg is fairly representative of extratropical continental climates during summer. For the fourteen years no thunder occurred