Abstract: Based on air temperature and ground temperature measured at 0-320 cm soil depths of nine meteorological observation stations in Hu'nan province from 1981 to 2013,the comparison of variation characteristics of ground temperature and air temperature was made using the relevant statistical methods.The results show that ground temperature of each depth exists two typical spatial structures,i.e.high in the southeast versus low in the northwest and high in the southeast versus low in the northeast and northwest.Under a certain depth,variation of monthly ground temperature is not synchronized with air temperature,and the transition layer is at 40 cm depth.The minimum monthly ground temperature for the layers between the surface and 40 cm depth occurs in January,and the maximum monthly ground temperature occurs in July,which are in accordance with the occurrence times of the minimum and maximum monthly air temperature.Along with increasing of soil depth (below 40 cm depth),the occurrence times of the minimum and maximum monthly ground temperature delay.The minimum ground temperature of 320 cm occurs in April,and the maximum occurs in October.The correlation coefficients of shallow ground temperature and air temperature are more than 0.97,which reduce along with the increasing of soil depth below 40 cm layer.Inter-annual variation of shallow ground temperature is large in winter,followed in spring.Inter-annual variation of deep ground temperature in winter and spring are obviously larger than that in summer and autumn.With increasing of soil depth,the ground temperature is gradually decreasing in spring and summer and increasing in autumn and winter.As a result,the deep ground temperature is significantly higher than air temperature in winter,and lower than air temperature in summer.The annual average ground temperature of different layers shows an increasing trend significantly.The increasing rates of ground temperature of different layers are slower than air temperature except for 0 cm ground temperature.The warming trends of ground temperature at different layers are consistent with air temperature in all seasons,and temperature increasing rate is the most rapid in spring,followed in autumn,not obvious in winter.Annual average ground temperature at different layers presented increasing abrupt during 1993-1995.Abrupt in winter are the earliest,followed in spring and autumn,late in summer.Quasi-oscillations of 2-3 years,3-4 years and 6-9 years are found for annual average ground temperature of different layers.

Key words: Ground temperature, Transition layer, Correlation analiysis, Period