You observe that the snow pile on your yard has decreased in size although the temperature has not been above freezing since the last snowfall what most likely occurred

Answer: - On a sunny day, the heat energy is being transferred from sun to snow pile and it melts. Due to melting, it's size decreases as a phase change from ice to liquid takes place where there is no change to temperature.

The size decrease is because of your hand heat.

Explanation:

You can't forget the body heat.

The weather temperature could be the same, but another variable is implicitly involved: your hand natural heat. All living creatures sprout energy and human beings are not the exception.

So, in this case your hand heat is causing this phenomenon.