NO. 3 DUNKLE : PLANT ECOLOGY, CHANNEL ISLANDS 253 



Precipitation. The summer months are generally dry with the only 

 effective precipitation during the winter. Light rains may start in the 

 latter part of September and gradually increase in intensity until early 

 February, after which they show a rapid decrease until the last of 

 April. The regime of the insular precipitation is shown in table 1. 



There is slightly more rain during the summer months on the 

 westernmost and the higher of the islands than on the southern islands 

 and the lower islands to the east. Thus San Miguel, Santa Rosa, San 

 Nicolas, and Santa Catalina each have some light rain during the 

 summer, while practically no summer rain is reported from Anacapa, 

 Santa Barbara, and San Clemente. More rain is reported from San 

 Clemente, Santa Catalina, and San Nicolas for October and the latter 

 part of September than for November. This undoubtedly is owing to 

 the tropical storms that come up along the coast of Mexico in the early 

 fall and tend to turn eastward or die out north of latitude 30° N. 

 The area of precipitation from these storms may occasionally reach 

 the southern islands. 



The hydrotherm shown in figure 6 summarizes the monthly pre- 

 cipitation and temperature for the Channel Islands, and is almost 

 identical with those of some of the islands of the Mediterranean Sea, 

 as shown by Raunkiaer (1934). The break in regularity of the pre- 

 cipitation graph for November shows the effect of the tropical storms. 



The summer aridity of the islands is reflected in the abundance of 

 many annuals and the marked abundance of suffrutescence of the island 

 vegetation. The general dominance of grasslands and the presence of 

 chaparral in certain areas of Santa Cruz, Santa Rosa, and Santa Cata- 

 lina, and formerly of San Miguel, is typical of the Mediterranean 

 climates in similar latitudes in corresponding sections of continental 

 masses; i.e., the Mediterranean "maqui," the South African 'Veldt," 

 the Australian ''bush," and the South American "llanos." 



In addition to the records obtained from the governmental meteoro- 

 logical stations it was deemed desirable to obtain further information 

 in regard to temperature and humidity ranges, and precise measurement 

 of the evaporation rates in different exposures. This was done by means 

 of specially designed, recording instruments. 



Temperature and humidity. In order to obtain records of the temper- 

 ature and humidity ranges, thermographs and hygrographs were estab- 

 lished at two stations on Santa Barbara Island. Because the weather was 

 such as to make visits to this island impracticable for a small boat during 

 the months of January, February, and March of 1942, no records are 

 available for that period. Consequently the records for the first week 



