Part 7 (2/2)
”It is a well-established theory, that the currents of air under which the earth pa.s.ses in its diurnal revolutions, follow the line of the sun's greatest attraction. These currents of air are drawn towards this line from great distances on each side of it; and, as the earth revolves from west to east, they blow from north-east and south-east, meeting, and, of course, causing a calm, on the line.
”Thus, when the sun is directly, in common parlance, over the equator, in the month of March, these currents of air blow from some distance north of the Tropic of Cancer, and south of the Tropic of Capricorn, in an oblique direction towards this line of the sun's greatest attraction, and forming what are known as the north-east and south-east trade winds.
”As the earth, in its path round the sun, gradually brings the _line_ of attraction north, in summer, these currents of air are carried _with_ it; so that about the middle of May the current from the north-east has extended as far as the 38th or 39th degree of north lat.i.tude, and by the twentieth of June, the period of the sun's greatest northern inclination, to the northern portions of California and the southern section of Oregon.
”These north-east winds, in their progress across the continent, towards the Pacific Ocean, pa.s.s over the snow-capped ridges of the Rocky Mountains and the Sierra Nevada, and are, of course, deprived of all the moisture which can be extracted from them by the low temperature of those regions of eternal snow, and consequently no moisture can be precipitated from them, in the form of dew or rain, in a higher temperature than that to which they have been subjected.
They, therefore, pa.s.s over the hills and plains of California, where the temperature is very high in summer, in a very dry state; and, so far from being charged with moisture, they absorb, like a sponge, all that the atmosphere and surface of the earth can yield, until both become, apparently, perfectly dry.
”This process commences, as I have said, when the line of the sun's greatest attraction comes north in summer, bringing with it these vast atmospheric movements, and, on their approach, produce the dry season in California; which, governed by these laws, continues until some time after the sun repa.s.ses the Equator in September, when, about the middle of November, the climate being relieved from these north-east currents of air, the south-west winds set in from the ocean charged with moisture--the rains commence and continue to fall, not constantly, as some persons have represented, but with sufficient frequency to designate the period of their continuance, from about the middle of November until the middle of May, in the lat.i.tude of San Francisco, as the _wet season_.
”It follows, as a matter of course, that the _dry season_ commences first, and continues longest in the southern portions of the territory, and that the climate of the northern part is influenced in a much less degree, by the causes which I have mentioned, than any other section of the country. Consequently, we find that, as low down as lat.i.tude 39, rains are sufficiently frequent in summer to render irrigation quite unnecessary to the perfect maturity of any crop which is suited to the soil and climate.
”There is an extensive ocean current of cold water, which comes from the northern regions of the Pacific, or, perhaps, from the Arctic, and flows along the coast of California. It comes charged with, and emits in its progress, cold air, which appears in the form of fog when it comes in contact with a higher temperature on the American coast, as the gulf-stream of the Atlantic exhales vapor when it meets, in any part of its progress, a lower temperature. This current has not been surveyed, and, therefore, its source, temperature, velocity, width, and course, have not been accurately ascertained.
”It is believed, by Lieutenant Maury, on what he considers sufficient evidence--and no higher authority can be cited--that this current comes from the coasts of China and j.a.pan, flows northwardly to the peninsula of Kamtschatka, and, making a circuit to the eastward, strikes the American coast in about lat.i.tude 41 or 42. It pa.s.ses thence southwardly, and finally loses itself in the tropics.
”Below lat.i.tude thirty-nine, and west of the foot-hills of the Sierra Nevada, the forests of California are limited to some scattering groves of oak in the valleys and along the borders of the streams, and of red wood on the ridges and in the gorges of the hills--sometimes extending into the plains. Some of the hills are covered with dwarf shrubs, which may be used as fuel. With these exceptions, the whole territory presents a surface without trees or shrubbery. It is covered, however, with various species of gra.s.s, and, for many miles from the coast, with wild oats, which, in the valleys, grow most luxuriantly. These gra.s.ses and oats mature and ripen early in the dry season, and soon cease to protect the soil from the scorching rays of the sun. As the summer advances, the moisture in the atmosphere and the earth to a considerable depth, soon becomes exhausted; and the radiation of heat, from the extensive naked plains and hill-sides, is very great.
”The cold, dry currents of air from the north-east, after pa.s.sing the Rocky Mountains and the Sierra Nevada, descend to the Pacific, and absorb the moisture of the atmosphere, to a great distance from the land. The cold air from the mountains, and that which accompanies the great ocean current from the north-west, thus become united; and vast banks of fog are generated, which, when driven by the wind has a penetrating, or _cutting_, effect on the human skin, much more uncomfortable than would be felt in the humid atmosphere of the Atlantic, at a much lower temperature.
”As the sun rises from day to day, week after week, and month after month, in unclouded brightness during the dry season, and pours down its unbroken rays on the dry, unprotected surface of the country, the heat becomes so much greater inland than it is on the ocean, that an under-current of cold air, bringing the fog with it, rushes over the coast range of hills, and through their numerous pa.s.ses, towards the interior.
”Every day, as the heat, inland, attains a sufficient temperature, the cold, dry wind from the ocean commences to blow. This is usually from eleven to one o'clock: and, as the day advances, the wind increases and continues to blow till late at night. When the vacuum is filled, or the equilibrium of the atmosphere restored, the wind ceases; a perfect calm prevails until about the same hour the following day, when the same process commences and progresses as before; and these phenomena are of daily occurrence, with few exceptions, throughout the dry season.
”These cold winds and fogs render the climate at San Francisco, and all along the coast of California, except the extreme southern portion of it, probably more uncomfortable, to those not accustomed to it, in summer than in winter.
”A few miles inland, where the heat of the sun modifies and softens the wind from the ocean, the climate is moderate and delightful. The heat, in the middle of the day, is not so great as to r.e.t.a.r.d labor or render exercise in the open air uncomfortable. The nights are cool and pleasant. This description of climate prevails in all the valleys along the coast range, and extends throughout the country, north and south, as far eastward as the valley of the Sacramento and San Joaquin. In this vast plain, the sea-breeze loses its influence, and the degree of heat in the middle of the day, during the summer months, is much greater than is known on the Atlantic coast in the same lat.i.tudes. It is dry, however, and probably not more oppressive. On the foot-hills of the Sierra Nevada, and especially in the deep ravines of the streams, the thermometer frequently ranges from 110 to 115 in the shade, during three or four hours of the day, say from eleven until three o'clock. In the evening, as the sun declines, the radiation of heat ceases. The cool, dry atmosphere from the mountains spreads over the whole country, and renders the nights cool and invigorating.
”I have been kindly furnished, by Surgeon-General Lawson, U.S. Army, with thermometrical observations, taken at the following places in California, viz: At San Francisco, by a.s.sistant-Surgeon W. C. Parker, for six months, embracing the last quarter of 1847 and the first quarter of 1848. The monthly mean temperature was as follows: October, 57; November, 49; December, 50; January, 49; February, 50; March, 51.
”At Monterey, in lat.i.tude 36 38' north and longitude 121 west, on the coast, about one degree and a half south of San Francisco, by a.s.sistant-Surgeon W. S. King, for seven months, from May to November inclusive. The monthly mean temperature was: May, 56; June, 59; July, 62; August, 59; September, 58; October, 60; November, 56.
”At Los Angeles, lat.i.tude 34 7', longitude west 118 7', by a.s.sistant-Surgeon John S. Griffin, for ten months, from June, 1847, to March, 1848, inclusive. The monthly mean temperature was: June, 73; July, 74; August, 75; September, 75; October, 69; November, 59; December, 60; January, 58; February, 55; March, 58. This place is about forty miles from the coast.
”At San Diego, lat.i.tude 32 45', longitude west 117 11', by a.s.sistant-Surgeon J. D. Summers, for the following three months of 1849, viz: July, monthly mean temperature, 73; August 75; September, 70.
”At Suttersville, on the Sacramento River, lat.i.tude 38 32' north, longitude west 121 34', by a.s.sistant-Surgeon R. Murray, for the following months of 1849: July, monthly mean temperature, 73; August, 70; September, 65; October, 65.
”These observations show a remarkably high temperature at San Francisco during the six months from October to March inclusive; a variation of only eight degrees in the monthly mean, and a mean temperature for the six months of 51 degrees.
”At Monterey, we find the mean monthly temperature of the seven months to have been 58. If we take the three summer months, the mean heat was 60. The mean of the three winter months was a little over 49; showing a mean difference, on that part of the coast, of only 11 between summer and winter.
”The mean temperature of San Francisco, for the three winter months, was precisely the same as at Monterey--a little over 49.
”As these cities are only one degree and a half distant from each other, and both situated near the ocean, the temperature at both, in summer, may very reasonably be supposed to be as nearly similar as the thermometer shows it to be in winter.
<script>