In practical applications frost formation often takes place with varying environmental parameters (air velocity, temperature, and humidity). Most correlations that have been developed are for constant parameters. Since the frost formation process is dependent on both time and on the history of the frost layer, the correlations for constant environmental parameters will not work when the parameters vary. For this reason, a study was made to develop a method to predict frost formations with varying parameters. A theoretical model of frost growth based on molecular diffusion of water vapor at the frost surface and using energy and mass balances was developed. The model can incorporate varying environmental parameters. The model uses functions relating heat and mass transfer coefficients at the frost surface to the environmental parameters. The model was first tested and calibrated using data taken by previous investigators. A facility was then built to test the validity of the model for varying environmental parameters. Tests were made for changes in humidity and air velocity. The model showed good agreement with the basic trends observed for varying parameters. The qualitative results were generally within 25 percent of the predicted values for time periods up to 3 hr.