500 m. farther south; Cape Flattery, exposed the year round to cold ocean fogs, receives more heat than Eastport, Maine, which is 3° farther south and has a warmer summer. In the second place, the means of winter and summer are much nearer the mean of the year in California than in the east. This condition of things is not so marked as one goes inward from the coast; yet everywhere save in the high mountains the winters are comparatively mild. In the third place, the division of the year into two seasons—a wet one and a dry (and extremely dusty) one—marks this portion of the Pacific Coast in the most decided manner, and this natural climatic area coincides almost exactly in its extension with that of California; being truly characteristic neither of Lower California nor of the greater part of Oregon, though more so of Nevada and Arizona. And finally, in the fourth place, except on the coast the disagreeableness of the heat of summer is greatly lessened by the dryness of the air and the consequent rapidity of evaporation. Among the peculiarities of Californian climate it is not one of the least striking that as one leaves the Sacramento or San Joaquin plains and travels into the mountains it becomes warmer, at least for the first 2000 or 3000 ft. of ascent.
Along both the Coast Range and the Sierra considerable rainfall is certain, although, owing to the slight snow accumulations of the former, its streams are decidedly variable. A heavy rain-belt, with a normal fall of more than 40 in., covers all the northern half of the Sierra and the north-west counties; shading off from this is the region of 10-20 in. fall, which covers all the rest of the state save Inyo, Kern and San Bernardino counties, Imperial county and the eastern portion of Riverside county; the precipitation of this belt is from 0 to 10 in. In excessively dry years the limits of this last division may include all of the state below Fresno and the entire Central Valley as well. In the mountains the precipitation increases with the altitude; above 6000 or 7000 ft. it is almost wholly in the form of snow; and this snow, melting in summer, is of immense importance to the state, supplying water once for placer mining and now for irrigation. The north-west counties are extremely wet; many localities here have normal rainfalls of 60-70 in. and even higher annually, while in extreme seasons as much as 125 in. falls. Along the entire Pacific Coast, but particularly N. of San Francisco, there is a night fog from May to September. It extends but a few miles inland, but within this belt is virtually a prolongation of the rainy season and has a marked effect on vegetation. Below San Francisco the precipitation decreases along the coast, until at San Diego it is only about 10 in. The south-east counties are the driest portions of the United States. At Ogilby, Volcano, Indio and other stations on the Southern Pacific line the normal annual precipitation is from 1.5 to 2.5 in.; and there are localities near Owen’s lake, even on its very edge, that are almost dry. For days in succession when it storms along the Southern California coasts and dense rain clouds blow landwards to the mountains, leaving snow or rain on their summits, it has been observed that within a few miles beyond the ridge the contact of the desert air dissipates the remaining moisture of the clouds into light misty masses, like a steam escape in cold air. The extreme heat of the south-east is tempered by the extremely low humidity characteristic of the Great Basin, which in the interior of the two southernmost counties is very low. The humidity of places such as Fresno, Sacramento and Red Bluff in the valley varies from 48 to 58. Many places in northern, southern, central, mountain and southern coastal California normally have more than 200 perfectly clear days in a year; and many in the mountains and in the south, even on the coast, have more than 250. The extreme variability in the amount of rainfall is remarkable.[1] The effects of a season of drought on the dry portions of the state need not be adverted to; and as there is no rain or snow of any consequence on the mountains during summer, a succession of dry seasons may almost bare the ranges of the accumulated stock of previous winter snows, thus making worse what is already bad.
The Colorado desert (together with the lower Gila Valley of Arizona) is the hottest part of the United States. Along the line of the Southern Pacific the yearly extreme is frequently from 124° to 129° F. (i.e. in the shade, which is almost if not quite the greatest heat ever actually recorded in any part of the world). At the other extreme, temperatures of -20° to -36° are recorded yearly on the Central (Southern) Pacific line near Lake Tahoe. The normal annual means of the coldest localities of the state are from 37° to 44° F.; the monthly means from 20° to 65° F. The normal annual means on Indio, Mammoth Tanks, Salton and Volcano Springs are from 73.9° to 78.4 F.; the monthly means from 52.8° to 101.3° (frequently 95° to 98°). The normal trend of the annual isotherms of the state is very simple: a low line of about 40° circles the angle in the Nevada boundary line; 50° normally follows the northern Sierra across the Oregon border; lines of higher temperature enclose the Great Valley; and lines of still higher temperature—usually 60° to 70°, in hotter years 60° to 75°—run transversely across the southern quarter of the state.
Another weather factor is the winds, which are extremely regular in their movements. There are brisk diurnal sea-breezes, and seasonal trades and counter-trades. Along the coast an on-shore breeze blows every summer day; in the evening it is replaced by a night-fog, and the cooler air draws down the mountain sides in opposition to its movement during the day. In the upper air a dry off-shore wind from the Rocky Mountain plateau prevails throughout the summer; and in winter an on-shore rain wind. The last is the counter-trade, the all-year wind of Alaska and Oregon; it prevails in winter even off Southern California.
There is the widest and most startling variety of local climates. At Truckee, for example, lying about 5800 ft. above the sea near Lake Tahoe, the lowest temperature of the year may be -25° F. or colder, when 70 m. westward at Rocklin, which lies in the foothills about 250 ft. above the sea, the mercury does not fall below 28°. Snow never falls at Rocklin, but falls in large quantity at Truckee; ice is the crop of the one, oranges of the other, at the same time. There are points in Southern California where one may actually look from sea to desert and from snow to orange groves. Distance from the ocean, situation with reference to the mountain ranges, and altitude are all important determinants of these climatic differences; but of these the last seems to be most important. At any rate it may be said that generally speaking the maximum, minimum and mean temperatures of points of approximately equal altitude are respectively but slightly different in northern or southern California.[2]
Death Valley surpasses for combined heat and aridity any meteorological stations on earth where regular observations are taken, although for extremes of heat it is exceeded by places in the Colorado desert. The minimum daily temperature in summer is rarely below 70° F. and often above 90° F. (in the shade), while the maximum may for days in succession be as high as 120° F. A record of 6 months (1891) showed an average daily relative humidity of 30.6 in the morning and 15.6 in the evening, and the humidity sometimes falls to 5. Yet the surrounding country is not devoid of vegetation. The hills are very fertile when irrigated, and the wet season develops a variety of perennial herbs, shrubs and annuals.
Fauna.—California embraces areas of every life-zone of North America: of the boreal, the Hudsonian and Canadian subzones; of the transition, the humid Pacific subzone; of the upper austral, the arid or upper Sonoran subzone; of the lower austral, the arid or lower Sonoran; of the tropical, the “dilute arid” subzone. As will be inferred from the above
- ↑ During the interval from 1850 to 1872 the yearly rainfall at San Francisco ranged from 11.37 to 49.27 in.; from 1850 to 1904 the average was 22.74, and the probable annual variation 4 in.
- ↑ The means for Los Angeles and Red Bluff, of Redding and Fresno, of San Diego and Sacramento, of San Francisco or Monterey and Independence, are respectively about the same; and all of them lie between 56° and 63° F. The places mentioned are scattered over 312° of longitude and 612° of latitude.
