In situ data from around the globe are used to evaluate the parameterization of the fraction of soil ice over total moisture (fi) as used in four weather and climate models. Based on these evaluations and data analysis, an overall realistic (empirical) formulation is developed to compute fi as a function of soil temperature and the ratio of total moisture over its saturated value. This new formulation is found to reduce the winter ground temperature by up to 3 K over high latitudes in the offline Community Land Model (CLM3) simulations.