There are a few things that can cause snow to melt even if temperatures are well below freezing. These include evaporation, sunlight, and wind.
First, evaporation occurs when water vapor from the cloud is drawn into the cold air below. This causes the temperature to drop below the freezing point of the water vapor. This results in a layer of cool, moist air below the snow.
Second, sunlight can also jump-start this process. This is called sublimation, and it happens when the sun's warm direct rays create enough energy to allow the solid snow to turn directly into water vapor without melting.
Light plays a huge role in how snow looks, as it can give the crystals a white appearance. This happens because the snow grains have a lot of surfaces to reflect the sun's light.
However, if the snow is too close to the ground and the light waves get stuck in the grains, they can't be reflected back. That's why the light that bounces off the snow is often blue instead of white.
If the temperature is below freezing, it can be difficult to get the snow to melt. That's because the frozen surface of the snow acts like an insulator, which prevents heat and moisture from escaping.