If you've ever been on a chairlift when warmer temperatures call for rain, but instead noticed snowflakes soaking into your coat, this question applies to you: Can it snow when temperatures are above freezing? In most cases snow temperature are cold at cloud level, causing the precipitation to start out as snow. If the air temperature is colder than freezing all the way from the cloud to the ground then the snow flakes retain their flakiness and fall as snow.
Cold vs. Warm Air
However, in certain situations the air between the cloud and the ground is not always below freezing. This actually happens more frequently along the east coast of the U.S.; When big storms move south to north along the coastline (Nor' Easters), the flow around the storm pulls in air from the Atlantic Ocean. This is good because oceanic air is very moist and can produce lots of snow. This is also bad at times, though, because the same oceanic air is also warm. If the air is warmer than freezing, the snowflakes that fall from the cloud will hit the warm air on their way to the ground and melt into a rain drop.
That's not the end of the story, though. As the raindrop falls, it might encounter air that is once again colder than freezing. Since colder air is heavier than warmer air, sometimes it can ‘hang out' very close to the ground and refuse to move despite the best efforts of the warm air trying to push it out of the way. So back to our example, the snowflake that melted into a raindrop would hit this colder air near the ground. If the cold air is thick enough and extends from the ground up a few thousand feet, the raindrop will have enough time to melt back into a solid ball of ice, which is called sleet. If the cold air is just near the ground and only extends a few hundred feet above the ground, the rain drop won't spent enough time in the cold air to freeze back into sleet but it will freeze when it hits a cold surface on the ground (power line, grass, road, etc).