EFFECTS OF FREEZES ON CITRUS IN CALIFORNIA 265 



Protective Hills. The damage done by freezing was closely cor- 

 related with the injury done to the trees by the severe wind. 



It was probably on this account that plantings which were shel- 

 tered on the north by hills passed through the cold wave in much 

 better condition than areas that received the unintercepted blast 

 from the north. It looked in many cases as if the hills were so located 

 that a body .of warm air had been locked in around the trees, and that 

 the cold north wind had slipped over these pocketed areas. 



Again, the presence of a mountain range along the south side of 

 certain sections apparently accounts for the little injury to fruit, 

 probably due to a more gradual thawing of the fruit so situated. In 

 still other sections there appeared to be no frost high-line, and the 

 higher orchards, located along a north slope, appeared to be as severely 

 injured as the lower orchards. 



Influence of Canons. In previous years the territory about the 

 mouths of canons has usually been favored during cold weather. The 

 current of air coming down the canon has kept the air well mixed 

 and prevented the stratification which takes place when the air stag- 

 nates. This year it seems that the whole mass of air, which moved 

 down over the mountains, was below the freezing temperature, and 

 cold air in motion is much more damaging than quiet air of the same 

 temperature, so that many of the canons this year changed their 

 nature. This was not universally true, for there were some canons 

 which gave no evidence of being sources of streams of cold air. The 

 groves nearest to the San Gabriel Canon, for instance, were not dam- 

 aged so much as groves farther out in the valley. The San Antonio 

 Canon, on the other hand, was very cold, judging by the appearance 

 of the groves near its mouth. The San Antonio Canon takes the same 

 general north and south direction as that of the wind which ushered 

 in the cold, while the San Gabriel Canon runs northeast and southwest 

 in its lower portion and nearly east and west in its upper forks. 



Water Protection. The protection afforded by nearness to bodies 

 of water of considerable size, which was shown to be of very great 

 importance in Florida, where there are many lakes and inland bays, 

 was of very little importance in California. In a general way the 

 less injury to trees near the coast may in a measure be due to the 

 modifying influence on the cold, exercised by the nearness of the ocean. 

 Doubtless nearness to the ocean failed to give any very marked 

 indication of water protection, primarily because the wind accom- 

 panying the freeze was from the landward side. 



Protection Afforded Fruit by Location on Tree. The location of 

 fruit on the tree frequently exercised an influence on the damage 



