350 



FUNDAMENTALS OF FRUIT PRODUCTION 



thermometers exposed at a height of 5.5 feet, the lower at 0.5 feet, show 

 differences tabulated in Table 57, from which it appears that a strawberry 

 plant may be exposed to considerably lower temperatures on a frosty 

 night than the trees above it or than the thermometer in the ordinary 

 shelter. Milham points out that the differences are greatest at the 

 time of the minimum temperature and at the coldest station, in other 

 words when conditions for frost are most favorable. Strawberry growers 

 should bear this in mind in interpreting for their own use forecasts issued 

 by the Weather Bureau. 



Table 57. — Temperature Differences with Height"'' 

 (Degrees Fahrenheit) 



Station 8 

 (8 p.m.) 



station 7 Station 1 (St-,f°"J, Station 7 



Average difference . 

 Largest difference. 



0.5 

 2.0 



1.5 

 4.0 



2.1 



5.0 



0.5 

 2.0 



2.0 

 4.0 



2.9 

 5.0 



On the other hand Cox^^ found temperatures at 5 inches above the 

 soil lower than those at the surface, particularly on nights with good 

 radiation conditions. 



"The average depression of temperature," he writes, "at the 5-inch height 

 below that at the surface for the season of 1907 (May to October inclusive) was 

 1°[F.]. The average depression on clear cool nights probably reached 4°. There 

 were several instances of differences exceeding 6°." Cox evidently was not 

 entirely satisfied with the possible explanations he advanced for this difference 

 though they doubtless explain it in part. He states, "In a marsh grasses and 

 uprights from the vines interfere slightly with radiation from the thermometers 

 placed on the surface and it is probable that a thermometer or leaf exposed at an 

 elevation above the surface loses its heat more rapidly by radiation than if it 

 rested upon the surface because the upper one is not shielded in any way and 

 while the radiation is going on from the lower one, at the same time heat is being 

 conducted to it from the ground beneath. A thermometer resting upon the 

 surface of the bog becomes a part of the soil or vegetation upon which it rests, 

 as it were, and is benefited by the free conduction of heat to it from the ground, 

 while the conduction to and through the air is very slight in comparison ; because 

 of these differences in radiation and conduction, the surface thermometer usually 

 registers a higher temperature than the instrument a few inches above. For 

 the same reason, the temperature of the vegetation at the surface and 5 inches 

 above would vary as these temperatures have varied, especially when the surface 

 vegetation is shielded above. It is a matter of common knowledge that in the 

 bogs the cranberries growing at the tops of the uprights a few inches above the 

 ground are often damaged by frost while those lying on or near the ground escape 

 injury." 



Cox reports also two series of observations on temperatures at various 

 heights up to 36 inches above the surface. On the bog the 5-inch height had 



