10 



Farmers' Bulletin 1096. 



In valleys near the ocean, fog sometimes drifts in from the water 

 toward morning and prevents a damaging frost. On nights with 

 fog the hillsides are practically always colder than the lowlands 

 unless the fog extends high enough to cover both hillsides and val- 

 ley floor. 



INFLUENCE OF SOIL AND VEGETATION ON MINIMUM 

 TEMPERATURE 



In experiments carried on in the cranberry bogs of Wisconsin, 

 Prof. H. J. Cox found differences of from 5 to 10 F. between mini- 

 mum temperatures registered on the surface of level ground at two 

 points within 6 feet of each other. The ground at the warmer sta- 

 tion was bare, while that at the colder station was covered with 



FIG. 2. Average dates of last killing frost in spring. 



spaghnum moss. The soil at both points was peat. At a height of 

 3 feet above the ground this difference in temperature disappeared. 

 Prof. Cox attributes this difference in temperature to unequal 

 warming of the bare and moss-covered soil during the day and un- 

 equal conduction of heat to the surface from below during the 

 night. The soil at the cooler station was shaded by the moss and 

 a large part of the heat received during the day was expended in 

 evaporating water from the plants, while at the warmer station 

 the sun shone directly on the soil, warming it to a greater depth. 

 At night the heat absorbed during the day was slowly conducted to 

 the surface of the bare ground while most of the smaller amount 

 of heat absorbed by the moss-covered ground was prevented from 

 reaching the thermometer because of the intervening moss, which 

 is a poor conductor of heat. 



