Why does snow melt when the temperature is 28 degrees?

Your readings could be wrong.

It could but then, under certain conditions. Here is why: First of all, snow (or clouds, or the fur of the polar bear) is not white but partly transparent. We observe it white because it reflects nearly all the visible light frequencies. But all frequencies have energy that turns into heat, like the infra-red ones. If white has a high albedo (light reflection) it still absorbs some energy. Now, the strange thing is that, in order to melt, ice or snow needs to take heat energy and creates ... cold! This is exactly the same that happens when your sweat evaporates from your skin, it cools it down. So, how can snow melt in temperature below freezing point? Well, where did you measured that temperature? You see, on a windless day, the difference of temperature from the ground to a few feet above it can be very different! Early in the morning, it can be much colder and at midday, when the sun shines, it can be much warmer. Next time, try to actually put your thermometer flat on the snow. Here is another fact about ice and snow; it evaporates naturally all the time. It is then called, sublimation, i.e. going from solid to gas without the liquid state. The opposite can also happen; undercooled water vapour can go directly to form ice crystals in the atmosphere. It is then called, deposition.