These two images show atmospheric soundings taken on February 13, 2007 at Wilmington, Ohio, showing measurements of temperature, dewpoint and wind from the surface to several kilometers up in the atmosphere.  We will focus on the red line, which shows the atmospheric temperature profile.  Both graphs show the freezing line highlighted in yellow.  Notice that at 7 am (on the left), almost the entire temperature profile is below freezing, including at the surface.  The exception is about 1.5 kilometers off the surface, where there is a very small portion of the atmosphere is above freezing.  At this time, snowflakes falling into this small layer did not have time to melt, and continued falling to the surface as snow.  12 hours later (on the right), a much thicker portion of the atmosphere has warmed above freezing.  This allowed for total melting of snowflakes as they fell into this layer.  The subfreezing portion of the atmosphere below this warm layer was not deep enough to cause the liquid drops to refreeze, and precipitation fell as rain instead of sleet.  Since the surface itself was below freezing, the rain instantly froze when it hit solid objects, resulting in ice accumulation across much of southwest and central Ohio. You may have noticed that the sounding on the right stops about 4 km off the surface..  This is because ice accumulation on the weather balloon forced it down.