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