As used by the U.S. Army Corps of Engineers, the number of Fahrenheit degree- days (above and below 32F) 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.
The cumulative number of degree-days above 0C for a given time period. Four main types of air thawing indices have been used: _ Approximate Thawing Index - calculated from the mean monthly air tempera
tures for a specific station without making corrections for negative degree-days (T < 0C) in spring and fall (Boyd, 1973, 1979); I_af = SUM (NT) where N = number of days per month for months with a mean monthly temperature above 0C during one complete year. T = mean monthly temperature. _ Total annual Thawing Index - calculated by adding all the positive mean daily temperatures (C) for a specific station during a calendar year (Harris, 1981); I_af = SUM (T) where T = mean daily air temperatures (C) above 0C for one complete calendar year. _ Seasonal Thawing Index - calculated as the arithmetic sum of all the positive and negative mean daily air temperatures (C) for a specific station during the time period between the lowest point in the spring and the highest point the next fall on the cumulative degree-day time curve (Huschke, 1959); I_af = SUM (T) where T = mean daily air temperatures (C) during the time period between the lowest temperature in the Spring and the highest temperature in the Fall. _ Design Thawing Index - calculated by taking the average of the seasonal thawing indices for the three warmest summers in the most recent 30 years of record. If data for 30 years are not available, then the index is based on the warmest summer in the latest 10-year period of record (U.S. Army/Air Force, 1966).
As used by the U.S. Army Corps of Engineers, the period of time between the lowest point and the succeeding highest point on the time curve of cumulative Fahrenheit degree- days above and below 32F; t
he opposite of freezing season.
Perennially frozen ground which, upon thawing, will experience significant thaw settlement and suffer loss of strength to a value significantly lower than that for similar material in an unfrozen cond
ition
Perennially frozen ground which, upon thawing, will experience significant thaw settlement and suffer loss of strength to a value significantly lower than that for similar material in an unfrozen cond
ition.