As used by the U. S. Army Corps of Engineers, the period of time between the highest point and the succeeding lowest point on the time curve of cumulative degree-days above and below 32°F; the opposite of thawing season. A less rigorous, but more commonly used, definition is the number of days or months between the first day of fall or winter and the last day of the same winter or the following spring on which the air temperature is below 0°C.