WARMAN, P. C. _Bibliography and Index to the Publications of the United States Geological Survey._ United States Geological Survey, Bulletin No. 100, Washington, 1893. (Relates to publications of the United States Geological Survey issued previous to 1892; continued in Bulletin No. 177 by the same author.) WARMAN, P. C. _Catalogue and Index of the Publications of the United States Geological Survey, 1880 to 1901._ United States Geological Survey, Bulletin No. 177, Washington, 1901. WEEKS, F. B. _Bibliography of North American Geology, Paleontology, Petrology, and Mineralogy for the Years 1892-1900, inclusive._ United States Geological Survey, Bulletins No. 188 and 189, Washington, 1902. WHITNEY, J. D. _The United States._ 2 vols. Little, Brown & Co., Boston, 1889 and 1894. CHAPTER III CLIMATE _The Elements of Climate_ North America, embracing as it does essentially a quadrant of the earth's surface, presents a variety of climatic conditions ranging from those characteristic of the equatorial belt to those normal to polar regions, as well as every gradation due to variations in elevation from sea-level and even below that horizon in Death Valley, California, to the summits of high plateaus and lofty mountains. [Illustration: PLATE II.--Mean Annual Rainfall & Temperature.] The principal elements of the weather which go to make up the conditions of the atmosphere embraced in the broader term _climate_ are temperature, precipitation, and the winds. On the accompanying map, Plate II, the mean annual temperature of the continent is represented by _isotherms_, or lines drawn through localities having the same average temperature for the year. On the same map is also shown in blue the average depth of precipitation, including both rains and melted snow. On Fig. 24 lines are drawn through points having the same average barometrical pressure (_isobars_) for the months of January and July, together with arrows indicating the general direction of the surface winds during each of these months, which may be considered as representative of the summer and winter seasons. The data shown on these maps have been compiled mainly from the reports of the weather bureaus of Canada, the United States, and Mexico, and indicate, at least in a general way, a summary of what is known concerning the main meteorological elements which determine the climatic conditions in North America. An examination of these maps will suggest certain general conclusions in reference to the leading characteristics of the climate in various portions of the continent and the changes they undergo from season to season. [Illustration: FIG. 24.--Average barometric pressure and direction of wind for January and July.]