384 CAUSES OF CHANGE CLIMATE IN SAN FRANCISCO, 



"There is an extensive ocean current of cold water, which com- 

 ing from the northern regions of the Pacific, or, perhaps, from the 

 Arctic, flows along the coast of California. It arrives charged with, 

 and in its progress, emits air, which appears in the form of fog when 

 it comes in contact with a higher temperature of 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 Lieut. 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 Kamptschatka, 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. * * 



"As the summer advances in California, the moisture in the at- 

 mosphere 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 passing 

 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 accom- 

 panies 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 his 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 

 passes, 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 in- 

 creases 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 follow- 

 ing day, when the process re-commences and progresses as before, 



