OCR Text |
Show 2 - In reaching this general approximation, some considerations have necessarily been slighted. It will be noted that a term for the porosity f , vanished on formation of equation ( 14), Field experience suggests that high porosity ( low density) snow types are more amenable to depth hoar formation. Time profile records indicate that depth hoar formation commonly is initiated in snow layers of density 0.10 to 0.25 g/ cnH. ( This crystal form appears to originate with difficulty in snow of density greater than 0.30 g/ cm^.) Porosity decreases only 18% from 0.10 g/ crn^ to 0.25 g/ cm^, hence this factor must actually play a rather limited role. Variations in initial density and crystal type apparently do have a more pronounced effect on the ultimate size of the depth hoar crystals. Presently available evidence does not give any factual data on whether the formation rate of depth hoar is constant with time, other factors being constant. Qualitative field observations suggest that the initial formation of depth hoar crystals may be more rapid than the subsequent growth of these crystals, but this has not been confirmed by direct experiment. A variable formation rate would require further modification of Equation ( 20). |