Abstract
Observations of frost and snow depths were conducted at the Obihiro University site
located in eastern Hokkaido, Japan, from November 1981 to April 2007, and the characteristics of
ground freezing and thawing under snow cover were analyzed. The results indicated that the
annual maximum frost depth ranged from 5 to 55cm during the 26 winters. Frost depth is influenced
by seasonal changes in snow depth, and it was confirmed that it is proportional to the square root of
the freezing index when snow depth reaches 20cm. The thawing of frozen ground starts from the
surface at the seasonal end of snow cover and finishes from late March to early May (last frost day).
A linear relationship was found between the vanishing frost depth and the maximum frost depth.
Further, the vanishing frost depth was found to be proportional to the square root of the thawing
index. Based on these findings, the seasonal maximum frost depth, vanishing frost depth, and last
frost day were calculated using meteorological data from 1961 to 1980 obtained from Obihiro
Meteorological Observatory. Taken together, we can conclude that the annual maximum frost
depth has decreased in the last half century and that the last frost day has shifted from mid to early
April.
