American Society of Civil Engineers

New Approach for Surface n Factors

by James L. Hanson, Ph.D., P.E., (corresponding author), M.ASCE, (Professor, Dept. of Civil and Environmental Engineering, California Polytechnic State Univ., San Luis Obispo, CA 93407 E-mail:, Nazli Yesiller, Ph.D., A.M.ASCE, (Interim Director, Global Waste Research Institute, California Polytechnic State Univ., San Luis Obispo, CA 93407), Gareth E. Swarbrick, Ph.D., (Associate, Pells Sullivan Meynink Pty Ltd., North Ryde, New South Wales 2113, Australia), and Wei-Lien Liu, Ph.D., A.M.ASCE, (Senior Staff Engineer, CTI Associates, Brighton, MI 48116)

Journal of Cold Regions Engineering, Vol. 24, No. 1, March 2010, pp. 19-34, (doi:

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Document type: Journal Paper
Abstract: Air and surface freezing and thawing indexes and prediction of ground temperatures from air temperatures were investigated. A new method for applying n factors on a daily basis to capture localized temperature extremes is presented and compared to conventional seasonal n-factor analysis. Measured air temperatures from five locations and measured air and surface temperatures from one location were used. Freezing and thawing indexes were determined using daily and monthly average temperatures, different time frames, and seasonal and daily applications of n factors. Air and surface freezing indexes (Iaf and Isf) varied more than air and surface thawing indexes (Iat and Ist). Significant variations were observed in air and surface indexes due to the length of the time period used (1, 10, and 30 years) and frequency of temperature data used (daily and monthly). The surface indexes from seasonal n factors (using daily average temperatures) and daily n factors were similar (within 4%) and higher than the indexes from seasonal n factors (using monthly average temperatures). The average surface temperatures were within 2°C, whereas the maximum and minimum surface temperatures were significantly different (up to 26°C) between the seasonal and daily n factors. Maximum variations between consecutive daily maximum and minimum temperatures were significantly higher using daily n factors (up to 52°C) than seasonal n factors (less than 1°C). Surface indexes from seasonal (using daily average temperatures) and daily n factors can be used interchangeably. Daily n factors are recommended to obtain representative surface and near-surface temperature variations, diurnal extremes, representative timing for temperature change events, and localized freezing or thawing indexes during change-over months.

ASCE Subject Headings:
Temperature effects
Freeze and thaw