The cumulative number of degree-days above 0C for a given time period. Four main types of air thawing indices have been used: _ Approximate Thawing Index - calculated from the mean monthly air temperatures for a specific station without making corrections for negative degree-days (T < 0C) in spring and fall (Boyd, 1973, 1979); I_af = SUM (NT) where N = number of days per month for months with a mean monthly temperature above 0C during one complete year. T = mean monthly temperature. _ Total annual Thawing Index - calculated by adding all the positive mean daily temperatures (C) for a specific station during a calendar year (Harris, 1981); I_af = SUM (T) where T = mean daily air temperatures (C) above 0C for one complete calendar year. _ Seasonal Thawing Index - calculated as the arithmetic sum of all the positive and negative mean daily air temperatures (C) for a specific station during the time period between the lowest point in the spring and the highest point the next fall on the cumulative degree-day time curve (Huschke, 1959); I_af = SUM (T) where T = mean daily air temperatures (C) during the time period between the lowest temperature in the Spring and the highest temperature in the Fall. _ Design Thawing Index - calculated by taking the average of the seasonal thawing indices for the three warmest summers in the most recent 30 years of record. If data for 30 years are not available, then the index is based on the warmest summer in the latest 10-year period of record (U.S. Army/Air Force, 1966).