Snowfall vs. Snow Accumulation: Why Official Totals Differ From Your Yard

Have you ever looked at a National Weather Service report stating **6 inches of snowfall fell**, only to look out your front window and see bare pavement or two inches of wet slush?

This discrepancy is one of the most misunderstood concepts in winter meteorology: the crucial distinction between **snowfall** and **snow accumulation (snow depth)**.

Defining the Terms - **Snowfall:** The total depth of newly fallen snow measured on an unobstructed, elevated white measuring board before melting, settling, or wind compaction occurs. Measured every six hours by official NOAA weather observers. - **Snow Accumulation on the Ground (Snow Depth):** The actual layer of snow resting on roads, grass, or sidewalks at any given moment.

Why Ground Accumulation is Often Lower

1. Ground and Pavement Thermal Storage Early in the season (October through December), the ground retains solar heat absorbed over summer and autumn. When flakes land on asphalt or concrete, they immediately melt upon contact until the ground temperature cools to 32°F (0°C). A storm may drop 4 inches of actual snowfall, but the first 2 inches may melt into runoff immediately.

2. Compaction and Settling Snow is mostly trapped air. Fresh "fluffy" snow may have a snow-to-liquid ratio of 15:1 or 20:1. As more snow falls on top of it, the weight presses out the trapped air, causing the pack to compress.

3. Air Temperature Above Freezing If atmospheric conditions aloft are cold enough to support snow, but surface temperatures are 34°F to 36°F, heavy flakes will fall rapidly through the air, yet pavement will remain predominantly wet or slushy.

What This Means for School Closures School superintendents and transit officials focus on **road pavement conditions**, not grassy lawn totals. If 4 inches of snow falls onto 35°F salted asphalt, roads remain wet and passable, meaning schools will likely open on time despite impressive snow clinging to tree branches.