validation of air freezing index (afi), for determination of frost penetration depth in typical arid and semi-arid zones of iran

depth of frost penetration is one of the main indices in agriculture, civil and transportation engineering. soil temperature is a function of several factors including: topography, solar radiation, air temperature, moisture content and other physical properties of soil such as thermal capacity, coefficient of heat conductivity, and specific heat. the main objective of the present paper is to determine the frost penetration depth in soils based on the air temperature. in this study the daily and hourly temperatures of air and soil at different depths of three climatology stations located at shahr-e-kord, yazd and urmia cities of iran were collected and analyzed for a period of 11 years from 1992 to 2003. in the first stage, air freezing indices (afi) of the three named stations was calculated using three methods known as: american, norwegian and finn and then the results were compared with the observed values accordingly. investigations showed that correlation between the results is significant at one percent level, but the three methods gave different figures. based on other references, it has been shown that the american method is more suitable for regions located at the middle latitudes. thus, the correlations between frost penetration depth (fpd) and afi based on the us method were calculated and found to be 0.88 and 0.82 for shahr-e-kord and urmia stations respectively. however, the correlation for yazd station was much lower (0.65), and significant (p