The ratio of the surface freezing or Thawing Index to the air freezing or Thawing Index. The difference between air and surface temperatures at any specific time and location is greatly influenced by
climatic, surface and subsurface conditions (e.g. latitude, cloud cover, time of day or year, relative humidity, wind speed, type of surface and thermal properties of the ground). Freezing Index: n_f = I_sf/I_af where I_sf = ground surface freezing index; I_af = air freezing index Thawing Index: n_f = I_sf/I_at where I_st = ground surface thawing index; I_at = air thawing index