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

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.

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.