Cryogen spray cooling (CSC) is a heat extraction process that protects the epidermal layer during skin laser surgery of port wine stain (PWS) birthmarks and other specific dermatoses. The objective of the present work is to investigate temporal and radial variations on the heat transferred at the surface of a skin model during CSC. A fast-response thermal sensor is used to measure temperature across the radius of the sprayed surface of a skin model. These interior measurements along with an inverse heat conduction algorithm are used to determine the heat transferred at the surface. Results show that radial and temporal variations of the boundary conditions have a strong influence on the homogeneity of heat extraction from skin. However, there are subregions of uniform cooling. It is also observed that the surface heat flux undergoes a marked dynamic variation, with a maximum heat flux occurring at the center of the sprayed surface early in the spurt followed by a quick decrease. The study shows that external conditions must be taken into account and ideally controlled to guarantee uniform protection during CSC.