This atmospheric sounding was taken on the evening of March 25, 2002 at Wilmington, Ohio.  This sounding shows 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 (at the bottom the temperature and the dewpoint are equal, so all we see is the green dewpoint line).  The freezing line in the atmosphere is highlighted in blue.  Notice below 3 km where the temperature rises significantly above freezing (shaded in red).  This allowed for total melting of snowflakes as they fell into this layer.  Below this layer, a very small portion of the atmosphere near the surface is below freezing (shaded in blue). Because this subfreezing layer is so shallow, raindrops falling into this layer did not have time to refreeze, and precipitation fell as supercooled raindrops instead of sleet (this means the raindrops are below freezing, but are not frozen).  Since the surface itself was below freezing, the rain instantly froze when it hit solid objects, resulting in ice accumulation across much of the region.