Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/127

Rh TEMPERATURE.] METEOROLOGY 117 2A.M. -0-24 10A.M. 06 6 P.M. 26 4 ,, -0-33 Noon 24 8 ,, 02 6 ,, -0-29 2P.M. 0-47 10 ,, -0 19 8 ,, -0-12 4 ,, 47 Midnight -0 35 Thus in mid Atlantic, about 30 N. lat., where the sun s heat is strong, and at the time of the year when the sun is north of the equator, the diurnal fluctuation of the tempera ture of the surface is only 80. It is highly probable that nowhere over the ocean does the mean daily fluctuation of the temperature of the surface quite amount to a degree. This small daily fluctuation is a prime factor in meteorology, particularly in discussions relating to atmospheric pressure and winds. Temperature of Air over the Open Sea. The following shows the daily march of the temperature of the air over the North Atlantic on a mean of the same one hundred and twenty-six days for which the temperature of the sea has been given: 2 A.M. -1-13 4 ,, -1-40 6 -1-41 8, -0-21 10A.M. 078 Noon 1 45 2p.M. 1-80 4, 1-56 6 P.M. 073 8 ,, -0-30 10 -0-80 Midnight - 1 02 The amplitude of the daily fluctuation of the air is thus 3 21, or nearly four times greater than that of the sea over which it lies. During the same months the &quot; Challenger &quot; was lying near land on seventy-six days. The observations made on these days show a greater daily range of temperature of the air than occurred out in the open sea. The minimum, - 2 0- 05, occurred at 4 A.M., and the maximum, 2 33, at noon, thus giving a daily range of 4 38. The occurrence of the maximum so early as noon is doubtless occasioned by the greater strength of the sea breeze after this hour, this maintaining a lower tempera ture. Part of the increased range of the temperature of the air as compared with that of the sea was no doubt due to the higher temperature during the day and the lower during the night on the deck of the &quot; Challenger &quot; as compared with that of the air. But, after making allow ance for this disturbing influence, it is certain that the temperature of the air has a considerably larger daily range than that of the sea on which it rests. The point is one of no small interest in atmospheric physics from the im portant bearings of the subject on the relations of the air and its aqueous vapour to solar and terrestrial radiation. The hourly deviations from the mean daily temperature of the air at two places, one near the equator and the other in the north temperate zone, and both near the sea, viz., Batavia (6 8 S. lat., 106 48 E. long., mean temperature 78-7) and Kothesay (55 50 N. lat., 5 4 W. long., mean temperature 47 3), are these : Bataria. Rothesay. Batavia. Rothesay. 1 A.M. -3*2 -17 1 P. M. +57 + 24 2 -3-6 -2-0 2 +5 6 + 27 3 -4-0 -21 3 +5-2 + 2-8 4 -4-3 -2 2 4 +4-3 + 2 6 5 -47 -2 2 5 +3-3 + 21 6 -4-9 -2-0 6 +1-9 + 1-5 7 -4-3 -T5 7 +0-6 + 0-9 8 -2 2 -0-9 8 -0-4 + 0-2 9 -0-5 -0-2 9 -1-2 -0-4 10 +2-8 + 5 10 -1-8 -0-8 11 +4-4 + 1-2 11 -2-3 -1-2 Noon +5 4 + 1-9 Midnight -2 8 -1-5 The times of the four phases of the daily temperature at Batavia are minimum about 5.50 A.M., mean 8.45 A.M., maximum 1.20 P.M., and mean 7.40 P.M. ; while for Rothesay the same times are 4.30 A.M., 9.15 A.M., 3 P.M., and 8.20 P.M. At Batavia, where the days and nights are nearly equal during the year, there is little variation in these times through the months ; but at Rothesay, where the days are much longer in summer than in winter, there is considerable variation in the times of occurrence of these phases. The following table shows the times of the phases for a number of selected places in the northern hemisphere for the two extreme months, January and July : January. July. Min. Mean. Max. Mean. Min. Mean. Max. Mean. Sitka A.M. A.M. P.M. P.M. A M. A.M. P.M. 6.0 6.20 6.50 6.0 5.30 7.20 6.50 6.0 4.30 5.30 6.50 7.10 6.30 6.0 5.40 9.40 10.0 10.0 10.40 10.10 10.10 10.5 10.0 8.25 9.25 9.50 9.50 9.35 9.10 8.0 1.30 1.50 2.40 1.30 2.30 2.0 2.40 2.0 0.55 1.30 2.35 2.25 2.30 2.10 0.40 6.35 9.40 8.45 9.0 8.0 7.0 8.35 8.0 6.45 8.15 7.45 7.50 8.20 8.5 6.45 3.40 3.50 5.0 2.40 3.30 3.40 4.40 3.15 3.0 3.40 4.30 5.0 5.30 5.30 5.0 7.40 8.15 8.40 8.36 9.0 8.45 8.50 8.15 8.10 7.35 8.20 9.5 8.45 9.0 8.45 0.50 3.45 3.10 2.50 3.15 3.10 2.50 2.50 1.20 2.5 2.40 3.10 0.40 1.30 1.25 7.30 8.10 8.0 8.50 8.50 8.25 8.35 8.10 7.50 8.4 8.25 8.15 7.30 6.30 6.50 Toronto Philadelphia.. Havana Archangel Rothesay Oxford Madrid Geneva St Bernard Bogoslovsk Petroalexan- ) drovsk Tiflis Calcutta Bombay Madras During the night in summer the temperature falls con tinuously from the effects of terrestrial radiation till the earliest dawn, when the daily rise in the temperature sets in owing to the heat reflected from the upper strata of the atmosphere, which have begun to be heated and lighted up by the rays of the morning sun. It will be observed that the time of the daily minimum temperature occurs earliest in high latitudes and latest in low latitudes. During winter, on the other hand, the minimum tempera ture takes place in several regions some time before dawn. At this season the two chief causes on which changes of temperature depend are the sun and the passage of cyclones and anticyclones ; and it is probable that those cases where the minimum occurs markedly before the dawn are, where not occasioned by purely local disturbing causes, due to the mean diurnal times of occurrence of the changes of temperature which accompany the great atmospheric dis turbances of cyclones and anticyclones. In July the daily maximum temperature occurs generally from 2 to 4 P.M. At places, however, near the sea, which are within the immediate influence of the sea breeze, and in places at some distance from the sea, such as Calcutta, where the wind, being essentially a sea wind, attains its greatest daily velocity and the sky at the same time is much clouded, the maximum occurs nearly two hours earlier. In high situations, such as the St Bernard hospice, the highest daily temperature also occurs nearly two hours sooner than on the plains below. In the winter months the maximum is about an hour earlier than in the summer. In investigating the daily curves of temperature, Sir David Brewster drew several interesting conclusions from them. By dividing the daily curve of temperature, deduced from the mean of the year, into four portions, at the points representing the two daily means and the two extremes, he showed that the four portions approximate to parabolas, in which the temperatures are the abscissae and the hours the ordinates. The correspondence between the observed and calculated results is so close that the difference did not in any case exceed a quarter of a degree Fahrenheit. This interesting result is true for places at which the horizon is open all round, so that no shadows of hills, trees, or buildings fall on the places where the thermo meters are kept during the day. If a hill rises to the