The cumulative number of degree-days below 0C for the air temperature during a given time period. The Air Freezing Index differs from the corresponding surface Freezing Index (see n-factor). SUM (T_1)
< 0
A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), a
nd 2) undergoing specific modifications while in transit away from the source region.