An improved liquid water absorption model is developed for frequencies between 0.5 and 500 GHz. The empirical coefficients for this model were retrieved from a dataset that consists of both laboratory observations of the permittivity of liquid water (primarily at temperatures above 0 degree C) and field observations collected by microwave radiometers in three separate locations with observations at temperatures as low as -32 degree C. An optimal estimation framework is used to retrieve the model's coefficients. This framework shows that there is high information content in the observations for seven of the nine model coefficients, but that the uncertainties in all of the coefficients result in less than 15% uncertainty in the liquid water absorption coefficient for all temperatures between -32 degree and 0 degree C and frequencies between 23 and 225 GHz. Furthermore, this model is more consistent with both the laboratory and field observations over all frequencies and temperatures than other popular absorption models.