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ILLUSTRATIONS.

Frontispiece.

FIG. 1. Rainfall at San Francisco, 1849-1902

2. Seasonal rainfall at San Francisco, 1850-1902. 3. Seasonal rainfall at San Francisco, 1850-1902. 4. Seasonal rainfall at Eureka, from 1887 to 1901

5. Sketch map of Humboldt Bay......

6. Monthly curves of hourly wind velocities

7. Hourly wind velocities at San Francisco....

8. Mean relative humidity-upper, 5 a. m.; lower, 5 p. m...

9. Percentage of annual rainfall each month.

From Bulletin D, by Prof. A. J. Henry.

10. Seasonal rainfall at San Diego, from 1850 to 1901 11. Seasonal rainfall at Sacramento, from 1849 to 1901 12. Seasonal rainfall at Fresno, from 1882 to 1901.... 13. Wire baskets in citrus grove..

14. Wire baskets hung from limbs of orange trees.

15. Wire baskets in lemon and orange grove.

Intensity, or amount in inches.
Frequency, or number of days.

16. Mr. Priestly Hall's device for smudging....

17. Eight miner's inches of warm water in orange grove at Meacham ranch.

18. Lath screen at ranch of Mr. A. J. Everest, Riverside (view from above).

19. Lath screen at ranch of Mr. A. J. Everest, Riverside (under view)... ... ... ... ... ....

20. Fog service at San Francisco. Corner of large map standing in main corridor of Ferry Building. By

means of frequent reports from Point Reyes and Mount Tamalpais the extent and character of fog over Drakes Bay, the roadstead, and the Gate itself are known in the city

21. Morning fog over valleys. (Pl. I)..

22. Lifted fog. Height above ground about 500 meters. (Pl. I)..

23. Sea fog pouring over Sansalito Hills and through Golden Gate. (Pl. II).

24. Fog waves. (Pl. II) ....

(Normal, 23 inches).. (Average, 71 days)..

25. Fog lifting. View from United States Weather Bureau, Mount Tamalpais, Cal. (Pl. III). 26. Helmholtzian fog billow.

Cal. (Pl. IV)

View from United States Weather Bureau observatory, Mount Tamalpais,

27. Fog billows. (Pl. V)

28. Fog billows. (Pl. V)..

29. Fog drifting from sea inland.

(Pl. VI)

30. Fog stratum; clear above and cloudy below. (Pl. VI) ..........

31. Probable condition at time of the Rio de Janeiro wreck, February 22, 1901..

PLATES 1 to 9. Pressure, wind, and rainfall during the winter months of January and December, 1899; Decem

ber, 1901; January and February, 1902.....

PLATES 10 to 12. Track of a disturbance across the 'Pacific Ocean from the Ladrone Islands to the coast of California from November 20, 1895, to January 14, 1896...

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ACKNOWLEDGMENTS.

The following-named gentlemen have assisted in the preparation of this volume:

Mr. George H. Willson, local forecaster, and the following members of the office force at San Francisco: Mr. Horace E. Smith, Mr. William Norrington, Mr. Walter H. Scholl, Mr. William J. Reed, Mr. Herbert E. Wilkinson, Mr. Hugo Legler, Mr. Walter J. Bennett, and Mr. William H. Fahlbusch.

Mr. George E. Franklin, local forecaster, Los Angeles, Cal., contributed the article upon the "Climate of Los Angeles;" Mr. James A. Barwick that upon the "Climate of Sacramento;" Mr. Ford A. Carpenter, the "Climate of San Diego;" Mr. Maurice Connell, the "Climate of Red Bluff;" Mr. Aaron H. Bell, the "Climate of Eureka;" Mr. J. P. Bolton, the "Climate of Fresno;" Mr. J. R. Williams, the data for San Luis Obispo, and Mr. J. J. McLean the data for Independence.

The Southern Pacific Company has for many years had its agents keep a daily record of rainfall and temperature, which records have been compiled by the Weather Bureau office at San Francisco. Data from 181 stations in California have been thus collected. Through the courtesy of the Santa Fe System reports from ten stations in the San Joaquin Valley are received.

Due credit must also be given to the voluntary observers of the State; and in particular to Mr. Samuel H. Gerrish, Sacramento; Mr. J. A. Edman, Edmanton; Mr. J. C. Stanton, Rio Vista; Mr. C. W. Hendel, La Porte; and Dr. C. Max Richter, of Santa Barbara. The records, journals, and diaries of Thomas Tennent, Dr. G. H. Gibbons, Dr. T. A. Logan, and Mr. John Pettee have been generously drawn upon for data. To these records we are indebted for all data preceding the period of regular Weather Bureau observations.

Mr. J. B. Lippincott, of Los Angeles, resident hydrographer of the United States Geological Survey, has kindly placed at our disposal the rainfall data at high altitudes used by him in his engineering practice. In California seasonal rainfalls and data showing probable water supply in various drainage basins are of the utmost importance to the engineering profession. In the present paper many fragmentary rainfall records had to be omitted in order to keep the volume within reasonable limits. Many of these can be found in "Irrigation and Water Storage" by Glassford.

The diagrams of rainfall in this memoir show the seasonal amounts, because for engineering and other purposes the seasonal rather than the calendar year totals are desired. In the various tables, however, it was thought best to continue the usual yearly amounts.

The table of elevations has been materially changed and many corrections made by Prof. George Davidson and Prof. Joseph N. Le Conte, both of the University of California.

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CLIMATOLOGY OF CALIFORNIA.

CONTROLLING FACTORS.

The general climatic conditions of the Pacific coast, and particularly the climate of California, may be said to be controlled by four great factors. These are

1. The movements of the great continental and oceanic pressure areas the so-called permanent "highs" and "lows." Under this head we include also the most active factor in climatic development, namely, the movements of individual pressure areas, since there is now good ground for believing that the paths of these individual disturbances-large-sized whirls and counter whirls are largely determined by the general relations of the permanent pressure areas;

2. The prevailing drift of the atmosphere in temperate latitudes from west to east;

3. The proximity of the Pacific Ocean with a mean annual temperature near the coast line of about 13° C. (55° F.), a great natural conservator of heat, and to which is chiefly due the moderate range of temperature along the coast from San Diego even to Tatoosh Island; and

4. The exceedingly diversified topography of the country for a distance of 200 miles from the coast inland.

PRESSURE DISTRIBUTION.

It was early shown by Hoffmeyer that the distribution of the great so-called permanentpressure areas over the North Atlantic Ocean determined largely the character of the seasons in northern Europe. Teisserenc de Bort, discussing the causes of an abnormal winter in central Europe, called attention to the fact that during this period the great high-pressure area ordinarily overlying the Atlantic Ocean between latitudes 20° and 40° north had moved somewhat from its normal position. Eliot, Blanford, and others have shown that the great atmospheric movements over India are more or less connected with the chief features of the weather there, particularly with respect to the monsoons and rainfalls. Fassig has recently shown that the weather conditions prevailing in the United States east of the Rocky Mountains during March may be connected with the movement of the large pressure areas; and, in brief, that the weather of the Middle Atlantic States "is cold when the continental high controls, warm when the Atlantic high extends its influence westward beyond the coast, and normal when there is a fairly equal development in strength and extent of the two high areas, in which case now one, then the other, is in control of the wind direction, bringing alternately cold and warm air to the region. The paths of storms lie within the trough between the two high areas; when the trough is wide the storm paths are widely scattered; as the high areas approach one another the storm paths are contracted within narrower limits."

a

Over the North Pacific Ocean in winter there exists an area of low barometer covering the region between the latitudes of 40° and 60° north and 130° west to 140° east longitude. An area of high pressure overlies the greater part of North America with a southwest extension to the Tropics and west to the one hundred and sixtieth meridian. We shall find that typical wet winters on the California coast occur when this great North Pacific low extends well eastward overlying the continent west of a line drawn from San Francisco to Calgary. At the same time the great continental high area apparently recedes to the southeast. On the other hand, the a Am. Jour. of Science, Vol. VII, Nov., 1899.

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pressure distribution characteristic of a dry winter on the California coast is marked by the prevalence of the continental high over the entire country west of the Rocky Mountains. This relation is very clearly shown in the series of four charts following taken from Prof. A. J. Henry's "Rainfall of the United States." Professor Henry states "The prevailing winds and the pressure distribution shown on the chart for December, 1889, are favorable for heavy precipitation in California and the plateau region. The pressure distribution is abnormal, as is also the rainfall; * * * the chart represents an extreme condition, viz, a transfer of the usual track of lowpressure areas of the northern boundary southward to central California and the plateau region of Nevada and Arizona, * * this region being ordinarily covered by an area of high pressure."

*

This relation of permanent pressure distribution and rainfall is more plainly shown on the series of charts for December, 1901, January and February, 1902. December and January were months of marked deficiency in rainfall, and it will be noted that during this period the continental "high" overspread nearly the entire country west of the Rocky Mountains. The prevailing air movement in California under such conditions is from the north or northeast and, as might naturally be inferred, such a circulation is accompanied by scanty precipitation. During February, however, there is a noticeable change in pressure distribution. The continental "high" is now well to the east of the Rocky Mountains, while the North Pacific "low" has apparently moved well in over Vancouver Island and Washington. The general air movement is now from the south and southwest and the rainfall very heavy, especially in the northern coast counties of California. At some places in the redwood section the rainfall amounted to as much as 40 inches during the month.

No one who has not lived in California can realize the relief which this generous rainfall of February brought to an anxious community.

The extensive record of rainfall at San Francisco, covering a period of fifty-three years, may be studied to advantage in connection with the pressure distribution. There were six Decembers during which the rainfall exceeded 10 inches, the normal December rainfall being 5 inches. These months were in 1852, 1866, 1867, 1871, 1880, and 1889. The pressure distributions for the earlier years are not available, but the conditions for the month of December, 1889, are characteristic and are shown in detail in the charts here given. The mean rainfall for December at this station is about 5 inches; the greatest rainfall was in 1866, when 15 inches fell. In 1871 14 inches fell. The driest December on record was that of 1876, when no rain fell during the entire month. Pressure charts are not available, but the probability is that the chart would greatly resemble that given for December, 1901. There were seven Decembers in this period in which the total monthly precipitation did not exceed 1 inch.

For January a similar relationship is found to exist. In 1862 the rainfall exceeded 24 inches, or nearly five times the normal amount. In 1866 over 10 inches fell; in 1878 nearly 12 inches, and in 1890 over 9 inches. The driest January of recent years was in 1891, when less than 1 inch fell. The pressure distribution is that of the type shown by 1892 and 1889. For February the mean rainfall is about 3 inches. In 1878 over 12 inches fell; in 1887 over 9 inches. In 1891 the February rainfall was 7.26 inches and in 1902, 7.27 inches. It is interesting to note that the pressure charts for these years closely resemble each other over the country west of the Rocky Mountains. On the Atlantic seaboard the pressure distributions are not alike.

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