The loss of heat from the snow surface via thermal radiation.
The snow surface constantly emits thermal radiation (heat) to the surrounding atmosphere. When the sky is clear, that heat escapes into space, allowing the snow to efficiently cool. If the sky is cloudy, there is significantly less radiative cooling. Cloud cover re-radiates heat back toward the snowpack.
Radiative cooling is the primary driver that causes a wet snowpack to refreeze. During the springtime, clear nights are the most important factor in determining if the snowpack refreezes. Ambient air temperature also plays a role, but even if the air temperature drops below freezing, the snowpack may not freeze well if skies are cloudy. Conversely, the snowpack may refreeze during a clear night even if the ambient temperature is above freezing.
Radiative cooling is also a culprit for promoting weak layer growth. Under clear skies, the snow surface cools efficiently in contrast to underlying layers, causing an imbalance in temperatures. This creates large temperature gradients near the snow surface, which in turn, promotes faceting. Furthermore, the cooling snow surface facilitates lowering the surrounding air temperature to its frost point, which can promote surface hoar growth.