Calculated and Measured Air and Soil Freeze-Thaw Frequencies

Freeze-thaw frequencies calculated by eight different counting methods were compared using daily maximum and minimum temperatures from eight north-central United States National Weather Service (NWS) stations. These frequencies were also compared to those obtained using hourly air temperature data from six of the same NWS stations. In addition, the calculated frequencies were compared to measured freeze-thaw frequencies at several depths in a bare soil and a sod-covered soil at the University of Minnesota St. Paul campus climatological observatory. The necessary acceptance of the idealized daily heating cycle when using daily maximum and minimum air temperature data resulted in a higher occurrence of calculated freeze-thaw events than those obtained with hourly data; one method gave 23% more freeze-thaw events with the daily maximum and minimum temperatures. With the freeze-thaw phenomenon centered upon those months in which the mean temperature hovers near O°C, a bimodal frequency occurs at the northern stations (October and April, as at International Falls, Minnesota, and November and March at Fargo, North Dakota), while in warmer climates the bimodal characteristic is replaced by a single-peak frequency in January as at Sedalia and West Plains, Missouri. In the comparison between the calculated freeze-thaw frequencies based on daily maximum and minimum values and the hourly temperature measurements at several heights between the surface and the temperature shelter at the climatological observatory, it was found that the annual total frequencies increased as the height above the surface decreased. For the shallowest height above the surface there was an approximate 13% increase over those measured in the shelter with hourly temperature data. The annual total frequencies of the calculated freeze-thaw events obtained with the daily maximum and minimum temperature measurements in the shelter approximated those actually occurring at the 1-cm depth in a bare soil at the climatological observatory. Results also indicated that on the same day as a thaw at 1 cm the average shelter maximum temperature was 6.3°C, while for a freeze at the same depth the average shelter minimum was −3.5°C. These values are explained, in part at least, by the frequent lack of direct relationship between the timing of a soil freeze-thaw event in a day and the occurrence of the daily maximum or minimum air temperature. The freeze-thaw phenomenon extended below 10 cm in a soil bare of vegetation at St. Paul but averaged only about eight events per year at both 10 and 20 cm, compared to about 62 occurrences at 1 cm; even at 5 cm the average was less than 17 per year. Once frozen to a depth of about 20 cm, the soil remained frozen throughout the winter.