140 PROCEEDINGS OF THIRTY-SIXTH FRUIT-GROWERS ' CONVENTION. 



plants in no proper condition to withstand the rapid cooling of the 

 qniet night hours following. The plants are also in no condition to 

 withstand the rapid warming after sunrise, when the sun's rays fall 

 through a dry, clear atmosphere. At such times there is nothing to 

 intercept the sun's heat, practically no atmosphere absorption and no 

 gradual heating, to which the plant might accommodate itself. 



Our attention has been drawn to the importance of this drying effect 

 preceding frost by the damage done during the cold spell of Decem- 

 ber 19-20, 1908. Strong desiccating winds blew down the San Joaquin 

 Valley for several days before the freeze. My belief, expressed popu- 

 larly, is that the ground was dried out, the trees in large measure over- 

 stimulated in the effort to meet the intense evaporation at the leaf 

 surfaces. Then when the excessive metabolism could not be longer 

 maintained there was inadequate exudation, and shriveled plant tissue 

 resulted at a time when it was most necessary that the plant should 

 have a good supply of water to enable it to resist rapid temperature 

 change. Again, it must be plain that such dried, chilled tissue is in 

 no condition to withstand rapid heating. There is some evidence to 

 support these views in the well known fact that frosted oranges, for 

 example, are somewhat dried and do not regain their natural juiciness 

 by remaining on the trees. 



If this view be correct, it is plain that in any campaign of frost 

 fighting we must not neglect this antecedent condition of drying and 

 the consequent condition of rapid thawing. 



The general campaign of frost fighting, as thus far laid out, covers 

 three main lines : 



1. Accurate advance information of the occurrence of frost. 



2. Applying preventive measures during critical hours. 



3. Preventing sudden warming of chilled fruit, or a too rapid 



thawing out. 



Under the first of these heads great progress has been made in the 

 past five years. Not only in California, but now throughout the various 

 fruit sections of the United States, Weather Bureau officials recognize 

 the value of special study of types of frost maps. The work first under- 

 taken in California on a large scale, and for a distinct purpose, is now 

 an important feature of the forecaster's work at a large number of 

 places. Frosts are found to occur as a consequence of certain move- 

 ments of low and high pressure areas and the displacement of the 

 lower air incident to the pressure changes. Frost is a matter of air 

 drainage, both on a large scale and on a small scale. In other words, 

 frosts are successfully forecast because we understand the general 

 movements of the lower air and in particular certain local circulations. 

 Types of frost maps, that is, conditions preceding frost twenty-four 

 hours, are shown herewith. 



The forecaster anticipates a condition in which the lower air strata 

 are quiet, dust-free, and very dry. Under such condition there is rapid 

 radiation and loss of heat from plant and ground and a sharp fall in 

 temperature. While the forecaster gives the general conditions favor- 

 able for frost, it must be clearly understood that each individual 

 orchardist must study his own problem of air movement in his own 

 locality. These local circulations should be studied in connection with 

 the daily weather map. It is also advisable to have certain instru- 



