But warm air is less dense than cold air, so eventually, a boundary sets up where the warm air pushes up against the cold air. That's known as a warm front.

And in this instance, we were sitting far enough to the north that we were still sitting underneath a cold air mass.

But the warm air doesn't just hit a wall and stop. It runs up and over that cold air mass.

Not only does this process itself create precipitation, it often leads to many different types of precipitation in the winter months.

This is a cross section of what you can see.

Farthest away from the warm front, to the north, you have all snow.

There isn't enough -- or any -- above-freezing air left to melt any of the precipitation on the way down to the ground. So, it falls as all snow.

A little closer to the warm front, there is still a wedge of above-freezing air sitting just above the surface, which is still below freezing.

Though the precipitation starts as snow, it hits that warm layer and melts.

When there is just a little bit of warm air, it has time to re-freeze and fall to the ground as sleet -- little pellets of ice.

And where that layer is very thick, reaching almost down to the surface, you can get freezing rain.

It doesn't have enough time to re-freeze before it hits the ground, but because the ground is below freezing, that water freezes on impact and coats everything in ice.

And, finally, when the air all the way down to the surface is above freezing, you get plain old rain.

It's a tricky balance that can change significantly from mile to mile and with differences of just a couple degrees.