114 PROCEEDINGS OF TWENTY-SIXTH FRUIT-GROWERS' CONVENTION. 



Frost, then, is primarily a matter of air drainage. This principle is 

 shown both in the general pressure distribution over the country during 

 frost periods, and on a smaller scale in the motion of the surface air 

 currents in certain small areas. In other words, the usual warm lower 

 air strata are vigorously displaced by cold, dry air; and when the drain- 

 ing ceases and the cold air settles during the period of comparative still- 

 ness, frost forms. What is true on a large scale is true on a smaller 

 scale, and a close agreement exists between frost belts or frost streaks 

 and areas of stagnant, cold air. A careful survey should be made by 

 every ranch owner. The writer has urged this matter, believing that 

 each grower must work out his own salvation, and that besides studying 

 and practicing methods of protection, it is necessary to be familiar with 

 the air drainage of the ranch. On nearly every ranch that the writer 

 has visited there have been certain well-marked cold spots, which, in 

 most cases, were found to correspond with slight depressions in the 

 ground. These three deductions can be drawn from what precedes: 

 (1) Where the air is in brisk motion, damage from frost is generally 

 light; (2) Stagnant air, such as exists in low valleys, basins, and inclosed 

 areas, favors frost; (3) As the coldest layer is generally near the ground, 

 it is sometimes advisable to drain downward upper, warm air, displacing 

 surface layers. 



Methods of Protection. — Every fruit-grower should put himself in 

 communication with the nearest center of distribution of weather fore- 

 casts. If possible, he should be in daily communication with some 

 Weather Bureau office. Whenever frost warnings are issued for his 

 locality he should carefully determine the temperature and dew-point 

 frequently during the late afternoon and night. A good outfit consists 

 of a metallic thermometer, so arranged as to automatically close an 

 electric circuit and ring an alarm whenever the temperature of the air 

 reaches 32°. In addition to a reliable sling psychrometer there should 

 be some small device for testing the motion of the gentle air currents in 

 the orchard. Too much attention can not be given to this question of 

 air motion. Many smudging devices have failed to be effective, because 

 of a slow movement of the smoke away from the orchard. 



It is well known that lowlands are visited with frost while hillsides 

 and hilltops escape. Every fruit-grower should study the topography 

 of his land, and plant accordingly. Windbreaks are, as a rule, con- 

 sidered detrimental. No hard and fast rule, however, can be laid down. 

 By planting a windbreak in the proper place, defects in the topography 

 may be overcome and air currents established where otherwise pools of 

 quiet air would have formed. A windbreak dense enough and so 

 situated as to interfere with any natural circulation and facilitating the 

 formation of still areas or pools would, of course, prove injurious. 



