History of the State of California / From the Period of the Conquest by Spain to Her Occupation by the United States of America

by John Frost

"I come now to consider the climate. The climate of California is so remarkable in its periodical changes, and for the long continuance of the wet and dry seasons, dividing, as they do, the year into about two equal parts, which have a most peculiar influence on the labor applied to agriculture and the products of the soil, and, in fact, connect themselves so inseparably with all the interests of the country, that I deem it proper briefly to mention the causes which produce these changes, and which, it will be seen, as this report proceeds, must exercise a controlling influence on the commercial prosperity and resources of the country.

"It is a well-established theory, that the currents of air under which the earth passes 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 latitude, 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, pass 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, pass 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 repasses 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 latitude 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 latitude 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 Japan, flows northwardly to the peninsula of Kamtschatka, and, making a circuit to the eastward, strikes the American coast in about latitude 41° or 42°. It passes thence southwardly, and finally loses itself in the tropics.

"Below latitude 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 grass, and, for many miles from the coast, with wild oats, which, in the valleys, grow most luxuriantly. These grasses 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.