Ever tried to build a snowman with powdery snow, only to find that it just won't stick? Or have you noticed how much harder it is to shovel sticky snow compared to when it's in its light, fluffy form?
The cause for these differences is the surface temperature. When surface temperatures are just above freezing, snow can melt slightly, adding more moisture and creating heavy, wet snow. When surface temperatures are below freezing, you get powder.
Powdery snow contains less water, on average 5 inches of dry snow will melt to only 0.5 of an inch of water. Wet snow, however, can equal up to an inch of water for every 5 inches of snow. Not only is wet snow heavier and therefore harder to shovel, it's also more likely to accumulate an ice layer than powdery snow.