Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/144

134 METEOROLOGY [TEMPERATURE. carried northwards towards southern Asia, and consequently very high temperatures characterize these seas in summer. It is instruc tive to note the effect on the temperature of the sea resulting from the region of high atmospheric pressure in the North Atlantic at this season. Out of this anticyclonic region the winds blow in all directions, giving rise to surface currents flowing in the same direc tions. Thus to the west of Africa the winds and currents are from north to south ; and hence the temperature of this part of the ocean is abnormally reduced. On the other hand, on the west side of this high pressure area, the prevailing winds ami currents are from south to north, and it will be seen that the temperature of the whole of the region swept by the southerly winds is abnormally raised. On the north side of the area, the winds and currents are westerly as far as about long. 35 W. , and over that space the isothermals follow the parallels of latitude. Farther to eastward and northward the prevailing winds become south-westerly, thus propelling north wards along the western shores of Europe, by oceanic surface drifts, the warmer waters of southern latitudes. Meanwhile the currents of cold water and ice drifts from the Arctic regions keep the tempera ture off America to the north of Newfoundland at a figure con siderably lower than is observed in any other region in the same latitudes. In August similar relations exist as in January between the east and west coasts respectively of South Africa, South America, and Australia, all of which are readily explained by the charts of mean atmospheric pressure and the resulting prevalent winds. One of the most striking facts of ocean temperature is that the temperature of the Southern Ocean from about 50 to 60 S. lat. is practically the same in January and August, a circumstance due chiefly to the magnificent icebergs of that ocean. The Temperature of the Land. In regions where the rainfall is distributed through all the months of the year, and where snow covers the ground for only a small part of the year, the mean temperature of the soil nearly equals that of the air. But when the year is divided into wet and dry seasons, and when snow lies during a considerable portion of the year, the mean annual temperature of the soil may be above or below that of the air. The greatest difference between the temperature of the soil and that of the air occurs where the surface of the ground is covered during several months with snow. Snow is a bad con ductor of heat, and thus obstructs the free propagation of the cold produced by radiation downwards into the soil, and the escape of heat from the soil into the air. In this way, over a considerable portion of the Russian empire, the temperature of the soil is considerably in excess of that of the air. Thus at a place 120 miles south of Archangel the temperature of the soil is 10 higher than that of the air ; and at Semipalatinsk it is 9 higher. The daily changes of temperature only affect the soil to depths of about 4 feet. The precise depth varies with the degree of the sun-heat and with the nature of the soil. Similarly the heat of summer and the cold of winter give rise to a larger annual wave of heat propagated downwards, the amplitude of which diminishes with the depth till it ceases to be perceptible. Principal Forbes showed from observations on the Calton Hill, Edinburgh, that the annual variation is not appreciable lower than 40 feet below the surface, and that under 25 feet the change of temperature through the year is small. The depth at which the annual variation ceases, or where the temperature remains constant, is a variable depending on the conductivity and specific heat of the soil or rock, but particularly on the difference between the summer and winter temperatures. The rate at which the annual wave of temperature is propagated downwards is so slow that at Edinburgh, at a depth of 24 feet, the highest annual temperature does not occur till January 4, and the lowest till about July 13, thus revers ing the seasons at this depth. At Greenwich, at a depth of 25 1 feet, these phases of the annual temperature occur on November 30 and June 1. Professor Everett in the Report of the British Association for 1879 has summarized the results of the observations of underground tem perature. The temperature of the surface of the ground is not sensibly influenced by the flow of heat from below upwards, but is determined by atmospheric and astronomical conditions. The tem perature gradient is defined as the rate of increase of the temperature downwards, and it may be taken as averaging one degree Fahrenheit for every 50 or 60 feet, the exact rate in particular eases being very variable. Thus the temperature gradient of the soil is about five times steeper than the temperature gradient of the air. The temperature gradient is steepest beneath gorges and least steep beneath ridges ; and hence the underground annual isothermals are flatter than the uneven surfaces above them. This is the case even with the uppermost isothermal of the soil, and the flattening increases as we pass downwards until at a considerable depth they become horizontal. Where the surface of the ground and the iso thermal surfaces beneath it are horizontal, the flow of heat is verti cal, and the same quantity of heat flows across all sections which lie in the same vertical. In this case the flow across a horizontal area of unit size is equal to the product of the temperature gradient by the conductivity, if the latter term be used in an extended sense, so that it includes convection by the percolation of water, as welJ as conduction proper ; and hence, in comparing different strata in the same vertical, the gradient varies in the inverse ratio of the conductivity. Since the effects of the cold generated by nocturnal radiation mostly accumulate on the surface of the earth, but the effects of solar radiation are spread to some height by ascending currents from the heated ground, it might be expected that the annual tempera ture of the surface layer of the soil would be lower than that of the air resting over them. Observations prove that such is the case. Springs which have their sources at greater depths than that to which the annual variation penetrates have a constant temperature throughout the year, and if they do come from a depth considerably greater than this they may be regarded as giving a very close approximation to the mean annual temperature of the place. The temperature of cellars is also very near the mean annual temperature of the locality; at any rate this temperature may be secured for cellars anywhere. Distribution of Temperature in the Atmosphere. Of the larger problems of meteorology, the distribution of tempera ture in the atmosphere over the land surfaces of the globe was the first that received an approximate solution (by Humboldt). But as regards the ocean, which comprises three-fourths of the earth s surface, the question of the monthly and annual distribution of temperature in the atmosphere over it can scarcely yet be said to have been seriously looked at. The isothermals of the temperature of the atmosphere which cross the oceans continue still to be drawn essentially from observations made on the islands and along the coasts of these oceans. The first step towards the solution of this vital problem in climatology and other branches of meteorology is the construction of charts of mean monthly temperature of the surface water of the sea over all parts of the ocean from which observa tions for the purpose are available. In prosecuting this line of inquiry, excellent work has been done by the Meteorological Office as regards parts of the Atlantic between the tropics and the ocean to the south of Africa, and also by the Dutch, French, and German meteorologists. With such charts it would not be difficult, by a careful comparison during the same intervals of time between the temperature of the surface of the sea and that of the air resting over it, to construct monthly charts of the tempera ture of the atmosphere over the oceans of the globe. In this connexion the whole of the observations of the tempera tures of the air and sea made on board the " Challenger " have been examined, and sorted into one hundred and seventy-four groups according to geographical position, and the differences entered on a chart of the route of the expedition. In the Southern Ocean between latitudes 45 and 60 the temperature of the sea was lower than that of the air, the mean difference being I 4. The temperature of the air is here higher owing to the prevailing AV.N.W. winds, and that of the sea lower owing to the numerous icebergs. To south of lat. 60 S. the sea was nearly 2 warmer than the air, the result in this case being due to the open sea, which keeps up a higher surface temperature, and to an increased prevalence in these higher latitudes of southerly winds, thus lowering the temperature of the air. The period during which the temperature of the sea exceeded that of the air was from June 1874 to March 1875, or during that part of the cruise from Sydney to New Zealand, and through the East India Islands to Hong Kong and thence to the Admiralty Islands. During the whole of this time, except when passing the

north of Australia, the sea was much warmer than the air, the