The Air Freezing Index (AFI) for a location is the number of degree-days below freezing over a winter. Calculated using the average air temperature of a day, in degrees below freezing. For example, if a daily average temperature is 25 degrees Farenheit, there are 7 degree-days towards the Freezing Index: (32 - 25) x 1 day. A location has an "average" AFI, and a "design" AFI. The design AFI is based on a design cold winter, such as the coldest over a 30-year period. Explained better/in more detail in various publications, such as
