Answer: 45 and cloudy will melt snow faster in most cases. Snow is pretty reflective, so the heat transfer is more efficient when it comes from the air than straight from the sun, especially when there is wind to move and replace cooled air.

Since snow-melting is a thermodynamic process the investigation pertained largely to a consideration of the various factors influencing the transmission of heat to the snow-mantle. Of these factors, it was found that for high melting rates the heat contributed by convection and condensation of moisture through turbulent diffusion of warm moist air are the important heat-sources. The problem is, then, largely a consideration of the upper limit values of air-temperature, humidity, and wind-velocity compatible with an adequate snow-cover, and the relationship of these values to the rate of snow-melt. Light, 1941