APPENDIX. 265 



to raise moi-e than five pints of water from the freezing to the boiling point. 

 All this heat must be lost by radiation in order that the formation of dew 

 may proceed or the temperature fall. It is, therefore, evident that when 

 condensation begins, the heat evolved by this means practically prevents 

 further cooling. 



The temperature at which condensation begins is called the "' dewpoint, *' 

 and varies with the amount of moisture in the air, being higher the greater 

 the amount of moisture present. It is always constant for the same amount 

 of vapor. 



Radiation takes place most rapidly when there is nothing to obscure the 

 sky. Clouds or any other obstruction act as a screen in retarding it. Even 

 water vapor, while invisable, has a very appreciable effect in retarding it. 

 It takes place more rapidly from the surface of plants than it does from the 

 air about them, so that on still nights these surfaces are frequently cooled 

 several degrees below the temperature of the surrounding air. 



One more principle should be considered in the study of conditions under 

 w hich frost forms, and that is the increased density of the air as its temper- 

 ature is lowered. Owing to this principle, the air, on calm nights, arranges 

 itself in accordance with its density. The heavier cold air rests on the sur- 

 face and surrounds the plants and trees, thus increasing their liability to 

 injury. On still nights this fact is often very manifest. Frequently a ther- 

 mometer close to the ground will read 5° or 10° lower than one eight or ten 

 feet higher. This principle causes the air on slopes, as it becomes chilled 

 by radiation, to flow down into the valleys, where it accumulates, thus fre- 

 quently causing severe frosts in the lowlands, while the hillsides remain un- 

 injured. It is for this reason that frost does not so readily occur on windy 

 nights, since the wind mixes the air to a more uniform temperature through- 

 out and causes that near the ground to be warmer than it would be other- 

 wise. 



Therefore, the conditions favorable to frost formation are : (1) Clear 

 sky, because radiation of heat is rapid under these conditions ; (2) dry air, 

 because with dry air, cooling by radiation will continue to a lower temper- 

 ature before it is checked bj' the heat given off by condensation ; (3) still 

 nights, because under these circumstances the air arranges itself in layers 

 according to its density, and the colder, denser air collects near the surface. 



METHODS OF FROST PREVENTION. 



From a study of the foregoing principles vinder which frost forms, it 

 would seem that there would be the greatest probability of success in pre- 

 venting frost or diminishing its severity by working along the following 

 lines : 



First — Diminishing the radiation of heat. 



Second — Raising the dewpoint by adding moisture to the air, and thus 

 making sensible the latent heat of condensation at a temperature above the 

 danger point. 



Third — Adding heat to the air. 



Fourth — Draining the cold air away from the section which needs pro- 

 tection. 



