The size and temperatures of the keyhole have been calculated through the thickness of the plate being welded. The temperature calculations are based on a critical intensity and the power loss resulting from vaporization. The evaporative power loss is set equal to the Gaussian power density, which is used to represent the laser beam. The result is a temperature distribution across the surface of the plate. The temperatures through the thickness of the plate are calculated on the basis of a minimum intensity at the bottom of a drilled hole. The intensity at the bottom of the plate is assumed to exceed this minimum intensity. This permits an absorption coefficient to be calculated. Thus, the intensity through the thickness can be calculated and the temperatures corresponding to this intensity can also be evaluated. The results are used to explain the “nail head” appearance of the fusion zone, which is quite common in laser beam welds. The results also explain some visual observations of laser beam welds.