30 years ago, and has found that the 1 1 -year periodicity appears to be somewhat less marked but more regular than he at first thought it to be. 1 The increase of temperature within the tropics at times of sunspot minima is about IF. higher than in years of sunspot max- ima, and becomes less and less apparent outside the tropics. The general conclusions reached by Dr. Gilbert T. Walker do not appear to indicate any marked influence of variations in sunspot activity upon atmospheric conditions. 2 The correlation coefficient in the case of rainfall, e.g., is not, in general, shown to be much larger than would result from chance.
A. E. Douglass has for some 20 years been studying the evidence of climatic fluctuations given by tree rings in California and else- where. Some of his conclusions have been used by Huntington and others in their investigation of climatic fluctuations. In a recent volume, which also summarizes his earlier work, Douglass indicates that a close relation exists between the thickness of tree-rings and climatic conditions; sees an agreement between the tree-ring records and the results of meteorological observations during recent years, and finds evidence of periodicity, over large areas, in agreement with the sunspot cycle or multiples of it. 3 There is not as yet any agree- ment as to the causes of such climatic fluctuations. Very small irregular variations in the intensity of solar radiation are known to exist. There is also the sunspot period, and longer periods may later be established. A distinct inclination at present exists among meteorologists to seek the cause of climatic variations in changes in the general atmospheric circulation resulting from fluctuations in the sun's activity. There has been much discussion, but there is no unanimity of opinion, as to just how such variations in the amount of solar radiation will affect conditions on the earth's sur- face. A highly complex train of effects must obviously result, in which temperature, pressure, evaporation, cloudiness, and rainfall are all concerned, and in which readjustments in the general cir- culation of the atmosphere play an important part. The varying strength of the atmospheric and oceanic circulations and the re- sulting effects upon the development and location of the great " centres of action," and of the wind and rain belts, seem to many writers competent to account for any climatic variations which may have taken place in historical times. Thus, in one of the outstanding publications of the past decade, Helland-Hansen and Nansen, in their study of North Atlantic temperatures, conclude that variations in the supply of solar energy, acting through the atmospheric cir- culation, are the initial cause of temperature changes on the earth's surface.*
So far as the effect of a variation of short period like that of the sunspots is concerned, it seems highly probable that the effects are so many, so complex, and so mutually interdependent, that the periodic cause undergoes its next change before its effects are every- where fully established. This point is emphasized by C. E. P. Brooks in a significant study of the secular variations in climate. 6 The sunspot period being so short, the " repercussions " do not " die down sufficiently to allow a clear vision of the relation between the solar causeand the terrestrial effect." By means of a new method of analyzing meteorological data with reference to secular variation, it appears that opposite kinds of changes in temperature, pressure and rainfall are taking place in different parts of the world in rela- tion to a long period of sunspot numbers which shows a general decline since 1870.
The effect of volcanic dust veils in diminishing atmospheric transparency and thus affecting terrestrial temperatures has been brought forward by several writers as a possible contributing cause in climatic variations, in historical and in geological time.'
So far as changes of climate during the geological past are con- cerned, there has been a decided tendency towards seeking an explanation in factors which are recognized as being effectively at work in determining present-day climates, and a lessened appeal to purely astronomical causes, which in the past were most widely advocated.
The time has clearly not yet come when a general agreement fs to be expected on a subject as highly complex as that of climatic
1 W. Koppen, " Lufttemperatur, Sonnenflecken und Vulkan- ausbriiche,' Met. Zeitschr., vol. xxxi., 1914, pp. 305-328.
2 G. T. Walker, " Sunspots and Temperatures," Mem. Indian Met. Dept., voLxxi., Pt. n, Simla, 1915, pp. 61-90; "Sunspotsand Rainfall," ibid., vol. xxi., Pt. 10, 1915, pp. 17-59.
8 A. E. Douglass, " Climatic Cycles and Tree Growth: A Study of the Annual Rings of Trees in Relation to Climate and Solar Activity," Carnegie Inst. Publ. No. 289, Washington, D.C., 1919.
4 B. Helland-Hansen and F. Nansen, " Temperature Variations in the North Atlantic Ocean and in the Atmosphere: Introductory Studies on the Cause of Climatological Variations," Smithson. Misc. Coll., vol. Ixx., No. 4, Publ. 2537, Washington, D.C., 1920.
5 C. E. P. Brooks, " The Secular Variation of Climate," Geog. Rev., vol. xi., 1921, pp. 120-35.
See, e.g., the following: C. G. Abbot and F. E. Fowle, " Vol- canoes and Climate," Smithson. Misc. Coll., vol. lx., No. 29, 1913; Charles Schuchert, " Climates of Geologic Times," Carnegie Inst. Publ. No. 102, Washington, D.C., 1914; W. J. Humphreys, Physics of the Air, Philadelphia, 1920, pt. iv.
fluctuations. The facts which demand explanation are not yet sufficiently well determined or correlated, and the processes which are at work are still too imperfectly understood.
Local Climatology. Two countries, the United States and Aus- tralia, stand out by reason of the progress which has been made, during the past decade, in the scientific investigation of their climates. Mention is here made only of general studies dealing with these areas as a whole. In the United States, the preparation of a section on climate for a new " Atlas of American Agriculture " marks an important advance in the accurate charting and discussion of many of the essential features of the climatic conditions of this large area. This atlas will, for many years to come, be the standard authority on all the subjects with which it deals. At the beginning of 1921 only one part of the climatic section, that on frost, had been issued in its final form. Advance publication had, however, been made of the new maps of mean annual, 7 monthly and seasonal rainfall 8 and of the new maps of sunshine. 9
The new rainfall maps are based on the records from about 3,600 stations, all covering or reduced to the uniform period of 20 years (1895-1914). The base maps show the main features of the topog- raphy, reasonable account of which is taken in locating the iso- hyetal lines. In the new series of sunshine maps the same basic period is used. Many details of rainfall and sunshine are further set forth by means of special diagrams and graphs. The whole subject of frost has been presented with a detail not hitherto at- tained in any other area of equal size in the world. 10 The average dates of first and last killing frost are charted (2O-year period, 1895-1914), as well as the variations in the dates of spring and autumn frosts; the length of the growing season, etc.
Two new maps of average annual snowfall of the United States have been prepared. The first of these is based on observations made at about 2,000 stations during the 15 winters from 1895 to 1910." In earlier maps, the observations came mostly from near sea- level, and hence the heavy snowfalls on the mountains were not indicated. On this new map, observations made at higher altitudes were also used and topographic effects were taken account of. A later map bears the date igig. 12 This was prepared from all available records in the western mountains, and from the complete records E. of the Rockies for the period 1895 to 1914, and revised somewhat in order to bring it into conformity with certain obvious topo- graphic influences. The first-named map brings out more clearly the heavier snowfalls on the mountains; the second adheres more rigidly to the actual observations. Investigations of relative humidities and of vapour pressures, and of the wind records for the 2O-year period 1891 to 1910, have added to the more accurate knowledge of United States climates. 13
The United States Weather Bureau has done useful work in summarizing the essential climatological data of the country by sections. 14 This publication includes the information usually desired regarding the climates of different parts of the country, brought together in convenient form for ready reference.
Australia is the second large area a knowledge of whose climatology has advanced very rapidly in the past decade. The Australian Com- monwealth Bureau of Meteorology has issued an unusually valuable series of reports, dealing especially with rainfall, but also presenting many other essential facts concerning the general climatic charac- teristics of the country. These studies are notable because of their clear and concise method of treatment, and the emphasis which is laid on the practical economic aspects of the subject. A report on the climate and weather of Australia is one of the best available discussions of the meteorological and climatic conditions of any part of the globe. 16 Australian weather and climate have been dis- cussed in publications by Dr. Griffith Taylor. 16
7 R. de C. Ward, " Mean Annual Rainfall of the United States, with Notes on the New Chart of Average Annual Precipitation from the ' Atlas of American Agriculture,' " Mo. Weather Rev. (Washington, D.C.), vol. xlv., 1917, pp. 338-345.
- I. B. Kincer, " The Seasonal Distribution of Precipitation and
its Frequency in the U. S.," ibid., vol. xlvii., 1919, pp. 624-631.
9 Idem, "Sunshine in the U. S.," ibid.,vo\. xlviii., 1920, pp. 12-17.
10 W. G. Reed, " Frost and the Growing Season," Atlas of Ameri- can Agriculture, pt. ii., Climate, Sec. I., Washington, D.C., 1918.
"Charles E. P. Brooks, "The Snowfall of the United States," G. J. Met. Soc., vol. xxxix., 1913, pp. 81-84.
12 Mo. .Weather Rev., vol. xlvii., 1919, Chart 151.
13 P. C. Day, "Relative Humidities and Vapor Pressures over the United States, including a Discussion of Data from Self-Record- ing Hygrometers," Mo. Weather Rev., Suppl. No. 6, Washington, D.C., 1917; "The Winds of the United States and Their Economic Uses," Yearbook, Dept. of Agric. for 1911, Washing- ton, D.C., 1912, pp. 337-350.
4 " Summaries of Climatological Data by Sections, Bulletin W. U.S. Weather Bureau, Washington, D.C., 1912 (and later).
16 H. A. Hunt, G. Taylor and E. T. Quayle, " The Climate and Weather of Australia," Common. Bur. of Met., 1913.
"See, e.g., Griffith Taylor, " The Australian Environment, espe- cially as Controlled by Rainfall," Federal Advisory Council of Science and Industry, Mem. No. i, Melbourne, 1918.
