As used by the U. S. Army Corps of Engineers, the number of Fahrenheit degree-days (above and below 32°F) between the highest and lowest points on the cumulative degree-days time curve for one freezing season. For a critical review of the topic see Sakari Tuhkanen (1980). “The freezing index is used as a measure of the combined duration and magnitude of below freezing temperatures occurring during any given freezing season. The index determined for air temperatures at 4. 5 feet above the ground is commonly designated as the air freezing index, while that determined for temperatures immediately below a surface is known as the surface freezing-index. ” (from Glossary of Arctic and Subarctic Terms 1955). Compare thawing index.