Changes in Air-Freezing Index and frost depth between two climate normals periods

Air-Freezing Index (AFI) is a common index for calculating frost depth for mid-latitude regions and is essential for determining the depth of shallow foundation construction. Past research has shown that an estimated frost depth value can be generated using daily mean air temperatures. In this method, the depth of frost penetration is determined by the yearly accumulation of daily AFI values. Daily AFI is defined as the departure of the daily mean temperature above or below 0°C (commonly the index uses Fahrenheit). In other words, the daily AFI is the calculated difference between the daily mean temperature and the freezing point of water. The yearly accumulation of daily AFI values represents the seasonal magnitude and duration of below freezing air temperature. This research updates the existing methodology used in creating AFI values from the 1951-1980 climate normals and applies it to the recently completed climate normals for 1981-2010. AFI values calculated from both climate normals were used to study the temporal and spatial variations of AFI between the two periods for the contiguous United States. The comparison was extended to frost depth penetration as derived using the AFI for the two normals periods. The results show that 58.83% of stations experienced a decrease in the depth of frost penetration, 37.82% experienced an increase, and 3.35% experienced no change. Results were mapped and interpolated across the United States using GIS applications. The goal of this research is to develop accurate digitized frost depth maps across the United States for use by the construction industry, as well as to examine the change in this key variable over time.