REPORT ON ATMOSPHERIC CIRCULATION. 5 



cannot be considered, as holds good in the case of land, to penetrate by conduction 

 to greater depths, only doing so when increased densities, due to evaporation or 

 other causes, carry the higher temperature to greater depths. The rate of diffusion 

 downwards may be regarded as proportional to the vertical density gradient from the 

 surface layers downward. 



Hence the vital distinction between land and water surfaces in their bearings on 

 climate lies in this, that nearly all the sun's heat falling on land is arrested on the 

 surface, but as regards water it is at once transmitted downwards to great depths. 

 In shallow water, the sun's heat raises the temperature much higher than in deep water, 

 for the obvious reason that nearly all solar radiation falling on the surface raises the 

 temperature of the shallow layer of water ; in other words, the influence of the heat rays 

 of the sun is concentrated on a small depth of water, instead of being diffused through 

 a great depth. On the other hand, water more or less turbid by the presence of 

 organic or inorganic matter, is more highly heated near the surface. 



Surface Temperature of the Sea. — When it is considered that three-fourths of the 

 earth's surface is water, that the temperature of the air resting on its surface is in 

 closest relation to the temperature of the surface, and that the latter has, through the 

 winds, direct and all-important bearings on the temperature of the land surfaces of the 

 globe, it is at once seen to be impossible to overstate the importance of this datum of 

 meteorology. It will also appear further on to have bearings equally important 

 in the elucidation of the fundamental principles of the science. 



Among the first, if not the first, contributions for determining the diurnal march 

 of the temperature of the surface of the sea were those made by Captain Thomas, E.N., 

 during the years. 1859-63, while engaged on the survey of the islands on the north-west 

 of Scotland. During this survey, observations of the temperature of the sea were made 

 every hour of the day at all seasons, and with a frecpiency sufficient for the determina- 

 tion of the diurnal range of the temperature of the surface. From these observations 

 it has been shown 1 that the daily minimum, 0°'17 below the mean, 2 occurred about 

 6 a.m. ; the mean about 11 a.m. ; the maximum, 0°'13 above the mean, between 3 and 

 4 p.m. ; and the mean again shortly before 2 a.m. Hence the daily oscillation of the 

 temperature of the open sea off the north-west of Great Britain is only 0° - 3. 



During the cruise, the temperature of the surface of the sea was observed every 

 two hours as part of the scientific work of the Expedition. From these observations 

 the deviations each two hours of the day from the mean daily temperature of the 

 surface of the sea have been calculated, and the results are given in Table I., App. 

 pp. 1-3. The periods for which the averages have been calculated are about five days, 

 more or less, and in those cases in which the temperature at the beginning of the 



1 Journal of the Scottish Meteorological Society, vol. i. pp. 265-270. 



2 The temperatures iu this Report are Fahrenheit. 



