NO. 3 DUNKLE : PLANT ECOLOGY, CHANNEL ISLANDS 255 



of each of several consecutive months have been arbitrarily chosen as a 

 fair example. It is worth noting, however, that the maximum-minimum 

 thermometer readings showed no lower temperatures than are indicated 

 on figs. 5, 6. (pp. 350-2). Consequently these records do well illustrate 

 the remarkably small range of temperature throughout the year. The 

 recordings were taken from instruments at the station situated on the 

 main ridge about the center of the island. The low range of temperature 

 is clearly shown especially for the fall months, as well as the uniformly 

 high humidity. For weeks at a time, during the summer, the relative 

 humidity ran at 100 per cent for from sixteen to eighteen hours of the 

 day. Observation has shown that this is similar to conditions on all 

 of the islands, with the exception of the eastern part of Santa Catalina. 

 Here there is less fog than on the other islands or on the adjacent main- 

 land coast. 



The hygro-thermogram for May 4 to 11, 1941, indicates a most 

 remarkable deviation from the normal uniformity. Examination of 

 the Weather Bureau records for southern California for the period in 

 question shows that exceptional temperatures then prevailed on the 

 mainland. Periods of high pressure over the southern part of the Great 

 Basin cause a pressure gradient, bringing dry, hot wind to the coastal 

 area. The inland high pressure area is related to the extensive sub- 

 tropical anticyclone over the ''horse latitudes" of the Pacific, which 

 is characterized by settling air. The adiabatic warming of this air causes 

 extremely low relative humidity. 



The extreme fluctuations of relative humidity indicated by this 

 May hygro-thermogram, sometimes as much as 50 per cent within a 

 period of minutes, could only be caused by waves of superior air alter- 

 nating with the horizontal movement of Pacific maritime air. Only 

 tropical air from the upper troposphere would be capable of producing 

 such exceptionally low relative humidities in an area so far removed 

 from the mainland shore. Dry periods similar to that of this May week 

 have been experienced on several of the other islands and doubtless occur 

 several times a year. Only rarely, however, would conditions probably 

 be such as to produce a relative humidity as low as 2 per cent. The 

 normal offshore breeze usually reaches Santa Barbara Island late at night 

 and persists several hours after sunrise. This normal land breeze, how- 

 ever, does not noticeably affect the relative humidity. 



Wind. There are few records of wind velocity but these are fairly 

 consistent. San Miguel is the most windy of all the islands, with San 

 Nicolas a close second. Plate 2, p. 368, in the next sub-section shows 

 clearly the effect of the northwest wind on San Miguel. There it has 



