On Nifiht-Frosfs 



VII. 



53 



."« ' 



m FORECASTS OF NIGHT-FROST. 



It is, of couise. of tlie greatest iniportauce to interpret correctly the war- 

 ning signs given l\y Natnre before a frosty night, so that tlie préventive mea- 

 sures are not needlessly precipitated. A niglit-frost never comes unawares, and 

 its forewarnings are fortunately sure and easily interpreted, especially tliose 

 of the general, extensive frosts. 



It will be seen from tiie experiments described above, that no damage 

 could liave been caused by frost on tlie nights of the 11"', 12"' and 15"' 

 June, although the temperatiu-e feil considerably below freezing-point. These 

 nights bad been preceded by comparatively wann (16" to 18") and calm days 

 when there was muth evaporation, and. consetiuently, a heavy dewlall at night. 

 Quite otherwise were the circumstances on the 19"", 2U"', 2 T* and even the 

 22'"' June. 



A general falling of température had taken place, continuing throughout 

 these days, aceompanied by a strong wind, wliich, though milder during the 

 night of the 2P', (the température during the day lieing about 14" to 15", 

 and the air at the siuiace of the earth at 11, 3o p m being still + 2", a light 

 breeze still holding), a slight frost injured the beard of the ears of rye and 

 tlie tips of the beards. As the température on the ground was 2" and the 

 breeze still continued, we left the lield at midnight, but an arriving at the 

 house at 12, so p. m., I noticed a slight breeze in the upper layers of air, for 

 I could very well see that the leaves of trees were still in motion. It is, ho- 

 wever, probable that the calm came on earUer in the tields wliere the experi- 

 ments had been made, as they were siUTounded by forest, but certainly not 

 earlier than 12, so a. m. ïhere remained but two hoiu's to sunrise (2,3? a. ni.) 

 tlien, and during that short time the slight injmy had been done. 



Fi'om these facts, and the expérience which I was afterwards able to 

 gather, we maj' infei' that the danger of frost is greatest on nights following upon 

 cold and windy days, i. e., if a calui sets in two or three hours before sunrise. 



Uenerally it happens tlius: a strong wind arises, usually N.W., continuing 

 for two or three days, aceompanied by a general sinking of température and 

 clear air. On the first night there is usually a very strong wind, and often 

 during the second, though less strong, and it is only on the third night at 

 evening that the wind dies away completely — when this happens, all is 

 ready for the night-frost. 



