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1. TABLE OF MONTHLY RAINS,
Showing the Amount of Sain, in inches, in each Month, from the Autumn of 1850 to tht. Summer o/1858; also, the mean quantity for each Month of the Year in that period.
V.—CONTRIBUTIONS TO THE CLIMATOLOGY OF CALIFORNIA.
BASED UPON FIVE YEARS' METEOROLOGICAL OBSERVATIONS AT SACRAMENTO. [By Thos. M. Logan, M. D.]
The physical features of California present every diversity of character, and this difference of conformation is a powerful cause of the variety of climate met with within its area. As a whole we cannot speak intelligibly of its climate, as we do of that of Cuba, or of our Southern, Middle, or the New England States, for it has many climates, anomalously distributed in abrupt contrast, and setting at naught our accepted notions of the effects of latitude and altitude.
In no other portion of the globe, therefore, are the requisite facts and statistics of meteorology more absolutely necessary than here for a correct understanding of the climatology. "When the measures of heat, rain, wind and other conditions necessary to make up our knowledge of what is understood by the word "climate," are determined in a sufficient number of stations, we may then, by a comparison of these facts, and from the relation they bear to each other, reach inductively the processes by which the combinations and varieties are formed. As subservient to this end, the accompanying results of observations made at Sacramento have been prepared. The statistics of greatest value in climatology are the monthly summaries of the measurements of the conditions just referred to, giving the averages, or the fixed quantities, and these we have compressed into the smallest possible compass, in tabular form, purposing, when treating on any special subject to which they appertain, to comment thereon textually.
The results are most of them calculated from five years' observations made in accordance with the uniform system adopted by the Smithsonian Institution, at 7 A. M., 2 P. M., and 9 p. M.; and although the period is too short to meet the rigid exactions of meteorological science, in deducing positive conclusions, still an approximation may be arrived at that will be sufficiently near to afford a very just appreciation of the climatic features of that great valley, which constitutes, what may be topographically considered, the middle portion of California, and of which the city of Sacramento is the center.
Viewed in connection with the reports furnished for this annual by Dr. Henry Gibbons, of San Francisco, we have now the types of two of the grand divisions of California. As soon as results are obtained from that third general division of the State—the slope of the Sierra chain, which binds our great valley on the east—we may then be enabled, by comparison and contrast, to form some idea of the subordinate local differences of climate, than which, owing to peculiar local configurations, a more confused variety ia hardly to be imagined.
Considered in a general manner, we would pronounce the climate Asiatic; and this expressive designation, which Fremont has applied also to the great basin and arid plains west of the 100th meridian, is particularly applicable during the summer months, when the number of hours of increasing heat considerably exceed those of radiation.
At this season the surface humidity is too little to retard the accumulation of heat from the sun's rays; consequently the afternoon temperature frequently becomes, for a short time during the day, as great as that of districts ten or twelve degrees further south on the Atlantic side—the absence of clouds, owing to the same cause, which creates the cessation of rain, (deficiency of moisture and insufficient coldness of the earth to condense what little there is,) facilitating both processes.
The temperature of evaporation gives the instrumental proof of this great measure of dryness. The difference between the temperature of the wetbulb thermometer and that of the air is often 20° during the hottest part of the day for several successive days, and not unfrequently reaches 25° and 3.0°.
The most striking Asiatic feature, however, is found in the great reduction of temperature after the maximum is reached. However high the wave of temperature towers up under the influence of a vertical sun and cloudless sky, it sinks proportionately low during the night, rendering it cool and chilly. Upon examination of the record of our thermometrograph, the variation between the maximum and minimum observation is frequently found to be as much as 25° and 30°; and the mean daily range for the five warmer months, May to September, during the last two years, amounted to 19° 68.
The following table exhibits the mean daily range for each month during the years 1856 and 1857:
As far as our information extends, this remarkable diumal range of temperature appears to be a characteristic feature of all the great valley regions and plains of the State; in fact, we may say of all the Pacific and interior arid climates, irrespective of altitude, with the exception of the immediate coast district. All the officers of the TJ. S. Surveys have remarked it. At Fort Yuma, on the west bank of the Colorado, lat. 32° 43' 00", and 120 feet above sea-level, it is, according to the U. S. Army Meteorological register, from 20° to 30° for the summer months. At parts of the greatest elevation, as Fort Defiance, altitude 1,200 feet, and Fort Union, altitude 6,670 feet, the range is still greater, sometimes attaining 35°. It is still great at Fort Snelling, and at all points on the dry plains, while at San Francisco, the representative point of the Pacific coast climate, it is about 10° only, less even than at New York city on the Atlantic
In reference to the adaptation of cultivated plants and agricultural staples to this climate, we would here apply the remarks of Blodgett, viz.: that the effect of extremes in the growth of vegetation is apparently similar to that on animal life—to produce the maximum activity. Central Asia has been regarded as the point of departure for most of the cultivated fruits and grains, domestic animals, the race of man, and the germ of civilization and letters. Valleys, with arid districts and plains intervening, and a climate possessing the greatest curve of changes, both of a constant and of a non-periodic character, make up as large a proportion of the surface of California and Western America as of Asia; and we accordingly find the fruits and plants of the zones of transition from tropical to temperate climates eminently vigorous in the transition district here. Murray mentions the stone pine, and a "cypress of prodigious hight," as characteristic forest trees of Upper Persia, and these are characteristic of California also; whether or not specifically the same with our mammoth cypress, the Sequoia Gigantea, or the large coned pines of our valleys, is unimportant to the analogy, or to the similarity of climate they establish.
Besides the effect of the extremes of heat and cold which have thus been considered in a general manner in reference to the climatological distinctions of California, a knowledge of the mean annual temperature, and especially of the distribution of temperature among the different seasons of the year, more particularly with regard to the heat and duration of the summer months, is of the utmost practical importance in an agricultural point of view. Turning to our table of monthly and annual means, we find the respective mean temperature of the seasons to be as follows: for the spring months, mean, 55° 31, the mean maximum being 71° 20, and the mean minimum 42° 13; for the summer, mean 70° 19, and the mean maximum and minimum 92° SO and 55° 11 respectively; for the autumn, mean 58° 47, and mean maximum and minimum 78° 20 and 44° 00 respectively. In the two wiuter months the mean is 45° 94, the mean maximum 60° 90, and the mean minimum 29° 70. Thus it is demonstrated that there is a mean difference between winter and spring of 9° 35, between spring and summer, of 14° 88, between summer and autumn, of 11° 72, and between autumn and winter of 12° 53. The difference of the means of the hottest and coldest months, between summer and winter, is also shown to be 24° 25, and the extreme variation, or the difference between the mean maxima of the former and the mean minima of the latter, 41° 50.
It will be noticed that in our division of the seasons we have, in accordance with the phenomena observed, defined February as the first of the three spring months, and appropriated five months to summer and only two to autumn and two to winter. Indeed, the dormant season is of so short duration, that the tropical division into the wet and dry seasons would perhaps be more appropriate. The whole period of sensible winter is far from being a complete season of suspension of vegetation. During the period we have assigned to it, many forms of vegetable life are still active, particularly the roots of grasses and winter grains. The lowest mean daily temperature belonging to this period is seldom below 40°. Although the thermometer has been known to fall as low as 33° as late as the 10th February, still the leafing process generally commences during the first week of February, and is completed at a temperature not much exceeding that of the mean annual. The measure of heat increases very gradually from month to month; indeed the same uniformity of temperature obtains throughout all the meteorological seasons. In summer the greatest vicissitudes of temperature are found to occur. The commencement of autumn is quite similar to the beginning of spring in its mean of daily temperature. The earth remaining warmer than the atmosphere under the decline of temperature, activity is partially renewed, after the drought of summer, by the influence of the light early showers of October. The first frosts occur about the middle of November, and the decline into winter is prolonged until the latter part of December. Ice is seldom formed before the beginning of January, and then rarely remains unthawed for twenty-four consecutive hours.
We have already alluded to the arid state of the atmosphere, during the summer months, revealed by the psychrometer. The principal agent in this hygrometric peculiarity of the climate, is to be found in the direct effect of northerly winds. In the winter and spring the north winds are the coldest, and serve, as the land is then cooler than the sea, to condense the moisture wafted with the atmospherical current from the southern hemisphere, and to precipitate it in the form of rain. During this season the south-east trades, charged to their utmost capacity with moisture, commence descending as their temperature decreases, and precipitate more and more rain as they become chilled by the north winds.
During the summer, owing to the fact of these northerly winds passing over a highly heated and arid surface, their temperature is raised, thereby increasing their capacity for moisture, which not being able to obtain from the surface passed over, they appear as dry winds, reminding one of the reputed sirocco of Italy. Nevertheless, dry as these winds apparently are, on coming in contact with the westerly winds, chilled by the oceanic polar current along the coast, and their temperature being again reduced, the vapor they contain is rapidly condensed; hence the heavy mists that are precipitated during the afternoon at San Francisco, and at the gaps along the coast. In the valley, as a general rule, the direction of the wind is from north by west to south-east. It seldom blows from the east or north-east with any appreciable force. Doubtless the prevailing wind off the coast, where no causes of local deflection exist, is west, as stated by Lieut Maury. This wind, rushing into the heated valleys through the gap at San Francisco and Benicia, reaches us at Sacramento and the northern part of the valley as a south-west wind, while at Stockton and the San Joaquin valley it is a westerly and north-westerly wind. To this wind, together with that descending from the slopes of the Sierras, may be attributed our cool summer nights. The influence of the winds on the temperature, as we have seen respecting the hygrometric condition of the air, varies according to the season of the year. It is during the occurrence of northerly winds in the summer that we experience the hottest weather, which seldom lasts long, however, before the temperature becomes equalized by a change of wind to the southward. Upon an examination of our daily and hourly records we find it to be a common occurrence during the summer months for the wind to commence blowing from the north at or shortly after the morning observation, and to remain in this quarter until afternoon, when it would change round to the south, freighted with moisture and invigorating freshness. It is the prevalence of these cool winds which temper our summer climate so delightfully, the greater or less predominance of which renders the mean temperature plus or minus. As regards the force of the wind, it is generally but slight. The observations in this respect having been registered for the last two years only, it is impossible to make full deductions therefrom with any degree of completeness. As a general rule it very rarely rains with the wind from the northern half of the octant, which may be attributed to its coming to a warmer from a colder region. As is well known, the rains of California are confined to a particular period; that period being the summer of the southern hemisphere, when the greatest amount of vapor is being taken up and borne by the south-east trades to form the rains of the northern hemisphere. The south-east trades, on approaching the latitude of California, descend to the surface, and meeting the colder northerly winds, precipitate during the wet season almost the whole rain we experience. In the more elevated regions a greater amount of precipitation occurs, while towards the center of the valley the rain-fall is least. At Stockton, according to a register kept during 1853, '54, '55 and '56, by Dr. R. K. Beid, it is only 15 inches, while the average of the same years at Sacramento gives 18.163 inches.