On the assumption of single spherical frog droplets, the motion and evaporation characteristics of frog droplets under the coupled effect of thermal radiation and forced convection are studied by establishing a model for the motion and evaporation of frog droplets. It is found that the main factors which have influences on the frog droplet lifetime in outdoor environment are ambient humidity and the initial diameter of frog droplets. The influences of thermal radiation and forced convection are negligible. The thermal radiation can cause the temperature of frog droplets to increase, even above the ambient temperature. The temperature of frog droplets may firstly increase and then decrease. The influence of forced convection on the temperature of frog droplets is negligible. The larger initial frog droplet diameter is helpful to the formation of dense frog. However, the oversize diameter may cause the lifetime of frog droplets to decrease due to their sedimentation. So there exists an optimum initial diameter for frog droplets.