294 METEOROLOGY. 



Hygrometry , Wind, Hail, Snow, Electrical and other Phenomena. 



An examination of the results of the psychrometer will reveal the 

 peculiar state of the atmosphere during the summer months. So great 

 is the apparent aridity at times that the lower asrial strata are fre- 

 quently found to contain during the hottest part of the day not more 

 than 15 to 20 per centage of their capacity for moisture. As an iso- 

 lated and extreme case, on the 10th July, 1856, at 2 p. m., wind 

 N. and light, and temperature 100°, the dew-point was found at 

 22°. This, we believe, is the greatest dryness that has yet been ob- 

 served on the surface of the globe on low lands. Humboldt, in his 

 Cosmos, states that the greatest dryness he has observed was in the 

 steppe of Platowskaja, after a SW. wind had blown for a long time 

 from the interior of the continent. With a temperature of 74°. 07 he 

 found the dew-point at 24°, the air containing yVo^^^s of aqueous 

 vapor. 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, on account of the distance of the sun, 

 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 southeast 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 con- 

 tain 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 southeast. 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 estab- 

 lished by Lieut. Maury. This wind, rushing into the heated valleys 

 through the gap at San Francisco and Benicia, reaches us at Sacra- 

 mento and the northern part of the valley as a southwest 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 just seen 

 with respect to 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 our 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 



