As used by the U. S. Army Corps of Engineers, the number of Fahrenheit degree- days (above and below 32°F) between the lowest and highest points on the cumulative degree- days time curve for one thawing season. The thawing index determined from air temperatures at 4. 5 ft above the ground is commonly designated the air thawing index, while that determined from temperatures immediately below a surface is called the surface thawing index. Compare freezing index; See Also growing season.