104 ON THE POSSIBILITY OF PREVENTING DAMAGES BY FROST. 



great. Acknowledging the fact that clear air and transparent 

 vapour do not radiate or absorb in a marked degree, the principal 

 radiation from the atmosphere itself falls in the very beginning 

 on these solid particles, and their action becomes that of leading 

 into comlensation of vapour, being first cooled down under the 

 temperature of their surroundings and then attaining the dew 

 point. When condensation has once begun radiation hastens 

 towards the earth as well as into space, because the radiant 

 power of the atmosphere is increased by the condensed vapour, 

 and it is sooj formed into a cloudy veil. 



This veil partly hinders the continued radiation from the 

 earth, and lessens the loss of heat and thereby the fall of tem- 

 perature, which stops at a point or continues to fall, though 

 very slowly. 



The degree of humidity thus determines the fall of temper- 

 ature ; the clearer and drier the air, the more intense the radia- 

 tion and cooling. It is on account of the absence of this 

 qualifying agent that the thermometric range is so enormous in 

 Australia. A clear day and a dry day, however, are very differ- 

 ent things. The atmosphere may possess great visual clearness 

 while it is charged with aqueous vapour, or even water in 

 condensed form, and on such occasions great chilling cannot 

 occur by terrestrial radiation. 



During the first half-hour after sunset the fall of tempera- 

 ture is rapid, but afterwards it becomes slower, for by degrees a 

 cloudy veil, more or less transparent, arises through condensation,, 

 and gives back the greater part of the heat. This veil is such 

 a serious hindrance to the radiation that, when appearing 

 distinctly, the temperature on the surface of the earth will not 

 sink under zero even in places sensitive to the frost. The 

 warmer the summer day the more intense is the evaporation, 

 the greater the amount of vapour in the air, and the thicker 

 the veil of clouds. Considering that vapour in its turning from 

 a gaseous into a liquid state gives out a great quantity of heat, 

 the cause of this great effect will be easily understood. By 

 preventing nocturnal radiation into space the clouds of vapour 

 preserve many a tender plant from being nipped by the frosts. 



II. — The dust particles and the condensed vapour radiate 

 heat, but the air itself only does so very slightly, and thus the 

 cooliiuf of the air rcsxdu principallii from its touching the ground 

 and the plcmts on it. Hence the remarkable fact, that the air is 

 coolest near the surface of the earth, and that its temperature 



