CHAPTER XXXIV
THE “CHALLENGER” WEATHER REPORTS
The Challenger Expedition, commenced by Sir Wyville Thomson, and after his death continued by Sir John Murray, with an able staff of assistants for the several departments, was one of the splendid exceptions to the ordinary British Government stinginess in the furtherance of science. The results of the Expedition were printed in a great number of very handsome volumes at the expense of the Government.
And the valuable deductions from the Challenger’s Weather Reports by Dr. Alex. Buchan, in his “Atmospheric Circulation,” have thrown considerable light upon oceanic weather phenomena. For some of his matured opinions on these, I am here much indebted to him.
Humboldt, in 1817, published a treatise on “Isothermal Lines,” which initiated a fresh line for the study of atmospheric phenomena. An isotherm is an imaginary line on the earth’s surface, passing through places having a corresponding temperature either throughout the year or at any particular period. An isobar is an imaginary line on the earth’s surface, connecting places at which the mean height of the barometer at sea-level is the same. To isobars, as well as to isotherms, Dr. Buchan has devoted considerable attention. In 1868, he published an important series of charts containing these, with arrows for prevailing winds over the earth for the months of the year. In this way what are called synoptic charts were established.
In the Challenger Report are shown the various movements of the atmosphere, with their corresponding causes. It is thus observed that the prevailing winds are produced by the inequality of the mass of air at different places. The air flows from a region of higher to a region of lower pressure, i.e. from where there is an excessive mass of air to fill up some deficiency. And this is the great principle on which the science of meteorology rests, not only as to winds, but as to weather changes.
Of the sun’s rays which reach the earth, those that fall on the land are absorbed by the surface layer of about 4 feet in thickness. But those that fall on the surface of the ocean penetrate, as shown by the observations of the Challenger Expedition, to a depth of about 500 feet. Hence, in deep waters the temperature of the surface is only partially heated by the direct rays of the sun. In mid-ocean the temperature of the surface scarcely differs 1° Fahr. during the whole day, while the daily variation of the surface layer of land is sometimes 50°. The temperature of the air over the ocean is about three times greater than that of the surface of the open sea over which it lies; but, near land, this increases to five times.
The elastic force of vapour is seen in its simplest form on the open sea, as disclosed by these Reports. It is lowest at 4 A.M. and highest at 2 P.M. The relative humidity is just the reverse. When the temperature is highest, the saturation of the air is lowest, and vice versâ. So on land when the air, by radiation of heat from the earth, is cooled below the dew-point, dew is produced, and, at the freezing-point, hoar-frost.
The Challenger Reports, too, show that the force of the winds on the open sea is subject to no distinct and uniform daily variation, but that on nearing land the force of the wind gives a curve as distinctly marked as the ordinary curve of temperature. That force is lowest from 2 to 4 A.M., and highest from 2 to 4 P.M. Each of the five great oceans gives the same result. At Ben Nevis, on the other hand, these forces are just reversed in strength.