112 PROCEEDINGS OF THE CALIFORNIA 



they have been but very imperfectly studied. All I wish now to point out is 

 that the edges of these currents, or where they meet, is frequently the seat of vio- 

 lent atmospheric disturbances, either in the form of cyclones or from large masses 

 of the warmer and colder currents breaking into each other sideways and thus 

 interpenetrating for considerable distances, causing storms of wind and rain. The 

 direction of the mountain ranges in a country in relation to these currents also 

 exerts a marked influence on the general character of the climate. On this con- 

 tinent, where the direction of our mountain ranges being more nearly parallel to 

 these currents, they are subject to less disturbance than in the old continent, 

 where the direction of the mountain chains is more directly opposed to the 

 course of the air current. I will now endeavor to point out how the character of 

 our seasons, as regards the rainfall, is dependent on these air currents. In ordin- 

 ary seasons there can be no doubt but that the rainfall is determined to a great 

 extent by the vertical mixing, so to speak, of these currents— the upper Equa- 

 torial current reaching the surface and breaking into the northerly current at 

 different places and at short intervals. But seasons occasionally occur in which 

 these currents, instead of frequently intermingling, pursue their course in broad 

 continuous belts, extending sometimes for thousands of miles, and flow in these 

 broad belts for months together without much lateral or vertical displacement. 

 It is such a condition of the atmosphere that gives rise to extreme seasons, par- 

 ticularly in those countries which usually have winter rains and which happen 

 to be within a Polar current at this season of the year. 



We will now consider the connection of these currents with the climate of our 

 State. During the summer months the whole of the State is generally within 

 the region of the Northeast Trade, which, however, does not prevail as such on 

 account of the barrier offered by the Sierra to the East and to the other ranges 

 of mountains running southeast and northwest, so that when we have a north 

 wind it is usually deflected in a westerly direction. These winds prevail to a 

 much greater extent in the interior valleys than in the western border of the 

 State, although at a short distance from the shore northerly currents prevail 

 during the whole summer. [Our local westerly and southwesterly winds in 

 summer are undoubtedly a sort of monsoon, caused by the rarefaction of the air 

 in the interior. Their low temperature is owing, in part, to their being north- 

 erly winds that have not been heated by their passage over any great extent of 

 heated land, and also to the cold sea current that flows along our coast.] Dur- 

 ing the winter the Equatorial current reaches the surface about in the latitude 

 of our State and mixing with the colder Polar current, causes our winter rains. 

 We then have frequent storms with strong southeast winds, for although the 

 Equatorial current is a southwest current, and shows itself as such in Europe— 

 and even in Nevada, directly over the Sierra— yet, here it is deflected to a 

 southeast current by the mountain barrier of the Sierra.* Such are the gen- 



» It is uudoubtedly owing to tliis cause that thunder storms are so rare in our State, the 

 Sierras opposing a barrier to the sudden lateral intrusion of masses of cold, dry air into the 

 Eiiuatorial current. The same cause also probably accounts for the slight fluctuations of 

 the barometer which are observed in our southerly storms, the air being heaped up against 

 tlie mountain barrier, and thus causing an increased pressure.or at least counterbalancing, to 

 some extent, the fail that would otherwise take place. 



