WEATHER IN CRANBERRY CULTURE 43 



Frost usually forms on the vines when low temperatures harm cranberry bogs, 

 but sometimes damage is done without it when the dew point is lower than the 

 minimum temperature reached.''' This is called a "dry freeze" or "black frost" 

 by growers. Hardly one bog frost in sixty on Cape cod is a "black frost," the 

 nights in which they occur being nearly- always windy. One never occurs there 

 from late May to the tenth of October. 



CONDITIONS RELATED TO FROST OCCURRENCE ON CRANBERRY 

 BOGS AND INJURY THEREFROM 



Pressure 



Damaging cranberry frosts occur more or less during the spring frost season 

 and in October with the barometer below normal; but dangerous temperatures 

 never come on Massachusetts or New Jersey bogs during August or the first three 

 weeks of September, or on Wisconsin bogs in July or August or the first decade 

 of September, unless the pressure is above normal (30.00 inches).^** 



Cloudiness 



It has been said*^ that the frost danger is increased if the sky is overcast during 

 the day, so preventing the sun from warming the ground, and then clears in the 

 evening. Cranberry frost records fail to show that this is a considerable factor, 

 for three fourths of the widely harmful frosts have been preceded by days that 

 were very largely or entirely clear. Heavy low clouds in the night radiate much 

 heat to the ground and so prevent frost. Some protective effect of this radiation 

 continues after they are gone if they remain till late in the night. Also, much 

 moisture often persists in place of the clouds for some time after they vanish and 

 this radiates some heat to the ground. Persistent low cloudiness till after mid- 

 night, therefore, reduces the frost hazard. 



Cloudiness in the evening can be relied on to continue till well toward morning 

 if it is general over southern New England with rain in some places, if there is 

 little difference everywhere between the dry-bulb and wet-bulb temperature, 

 and especially if the barometer is not rising. 



High clouds, cirrus or cirro-stratus, consist of small particles of ice and radiate 

 heat to the ground only feebly, so the net loss of heat from the ground with a sky 

 covered with them is almost as great as with a clear sky.^^ 



Wind 



A cold dry wind during the day promotes evaporation and keeps soil tempera- 

 tures from rising normally, so it may materially increase the frost danger. ^^ Wind 

 at night usually curtails the accumulation of cold air near the ground by mixing 



^'Cox, op. cit., p. 95. 



^"Cox, op. cit., p. 97. 



''IWeather forecasting in the United States, W. B. 583, 1916, pp. 187, 201, and 206. (Copy in 

 the Middleboro library.) 



^^Brunt, David, Physical and dynamical meteorology, 1939, p. 144; Young, op. cit., p. 8. 



■^^Owing partly to this and partly to the usual timing of wind and pressure changes, damaging 

 cranberry frosts occur more commonly after windy days than after calmer ones. The lowest bog 

 temperatures in a cold spell in the spring or early fall usually follow closely the calming of the cool 

 dry wind that blows with the inrush of the anticyclone unless a second cold front appears. The 

 loss of heat by radiation and otherwise during the rest of the cold period, after the pressure ha3 

 risen and steadied, is fully offset by the warming from insolation and other modification of the cold 

 air mass. Here lies also the relation between north and northwest winds and most of the more 

 positive frosts (p. 41). 



