Urea prills are produced in the prilling towers where a solidification-cooling process takes place. The ambient air is used as the cooling air stream for this process. In hot days, the temperature of the product at the bottom of the tower are hot that cannot be packed directly. In addition, in hot/ humid days, the prills form lamps and cakes with each other and on the scrubber. A mathematical model based on the hydrodynamics, heat, and mass transfer between the urea and the cooling air is developed. A numerical technique with an explicit scheme is used to solve the model. The model results describe the variation of the temperature and moisture along the radius of the particle. Hence, the model results introduce an interpretation of the problem of caking and lamps formation appears because of the incomplete solidification of the prills at the bottom of the tower. In addition, the results predict the quality of the product under different operating conditions of the ambient cooling air.