ISOTHERMAL LINES, or IsoutErtms. In meteorology. lines that connect places having the same temperature for a day, month. or for any given interval of time. Systems of isothermal lines for daily maps. as well as for monthly and annual means, present in a very graphic manner the prevailing temperature conditions, and are therefore of universal use in meteorology. They seem to have been first used by Alexander von Humboldt in 1817; but lines of equal departure from normal values were independently used by Brands in his studies of the daily weather map and the storms of Europe. Isothermal lines co incide only accidentally, if at all, with small circles of latitude, owing to the great influence of the distribution of land and water on the temperature of the air. The temperatures ob served at the respective stations insist be cor rected or reduced to the standard level surface of the globe, in order that they may he compar able with each other. This reduction intro duces diserepancies. hut there are many reasons for retaining it. The rate of reduction adopted by Hann and other climatologists is 0.50° C. per 100 meters, or 1° F. for :370 feet. If a chart has b(en constructed using this rate of reduction, and one should desire to know the temperature at the top of a mountain, whose altitude is given, we have hut to read from the chart the tempera ture at sea-level, apply the reduction to the given altitude, and get at once approximately the temperature of the summit. The accuracy of this
method is, however, limiled by the fact that the rate of diminution of temperature varies so much that the adoption of an average rate is liable to lead us astray. Charts of isotherms for each month of the year and for the whole globe have been constructed, first by Buchan, and afterwards by the United States Weather Bureau. and were published in full in 1900 in ele gant style in Bartholomew's Physical Atlas. Iso thermal lines can be calculated and drawn for the upper levels of the atmosphere by using, observa tions on mountain-tops and in balloons and kites. Lines of this character were first drawn by 1lerge sell for Europe for certain days in 1898-1900, on which sufficient data were collected by means of balloons and kites. These lines show that large variations in temperature occur at the highest attainable levels, and that the atmosphere in general is a mixture of masses of descending cold air and ascending warm air. See charts under TEMPERATURE, TERRESTRIAL ; and DIsTRI BUTIoN OF A Nim ALS ; also articles CLIMATE; DISTRIBUTION OF ANIMALS; METEOROLOGY.