s Selim Lemströji. 



That veil pailly hinders the coiitinuecl radiation from the eartli and less- 

 ens the loss of heat and thereby the fall of température, which stops at a 

 point or continues to fall, though very slowly now. 



The degree of humidity thus détermines the fall of température; the rlrn- 

 rrr and drier the air, the more intense the radiation and cooling. 



During- the first half-hour after sunset the fall of température is rapid, 

 hut afterwards it becomes slower, for by degrees a cloudy veil, more or less 

 transparent, arises through condensation and gives back the greatest part of 

 the heat. This veil, which is announced by the light-gray-hlue colour of 

 the sky and through which the bigger stars can be seen, though with diffi- 

 culty, is such a serions hindrance to the radiation, that, when appearing 

 distinctly, the température on the surface of the earth will not sink under 

 zéro, even in places sensitive to frost. The warmer the summer-day, the more 

 intense is the evaporation, and the greater the amount of vapour in the air, 

 and the thicker the veil ot clouds. Considering that vapour, in its passage from 

 gaseous to a liquid state, gives ont a great quantity of heat, the cause of this 

 great eftect will be easily understood. 



I have called the colour of the sky light-gray-blue. In that parti- 

 cular mixture of colours the blue tint plays the principal part, and the more 

 distinctly it appears, the more intense is the radiation and the lower mil the 

 température of the plants sink. Though that colour of the sky can pass through 

 a great number of shades, it can however in any case be referred to one of 

 three or four principal types, thus being a good means for deterraining the 

 greatness of the impending danger of frost. In judging this danger it will be 

 seen, from the account above what great importance must be ascribed to the 

 degree of humidity in the air and to the dew-point. 



2:o. The dust-particles and the Condensed vapour radiate heat, hut the 

 air itself only does so very slightly, and thus the COOling of the air 



results principally from its touching the ground and the plants on it. 



Hence arises the remarkable fact, that the air is coolest near the surface 

 of the earth and that its température increases with the height, and at 0,5 m ' 

 or 0,7 m above the ground it is already ' 3° or 5" higher. 



' All measures are given in the nietric system and tlie nsiial alircviations are employed as: m = 

 meter, cm = centimeter, mm = millimeter m' = cub. meter, m' square meter, kg = kilogramm, gr = 

 gramm, etc. 



- Celsius degrees are used in this jjaper. 



