Freezing Rain vs. Sleet vs. Snow: The Physics of Winter Road Hazards

Winter storms in the United States rarely produce uniform snow from start to finish. Often, a storm features a complex mix of **snow, sleet, and freezing rain**.

Understanding the vertical temperature profile of the atmosphere explains how each form develops—and why freezing rain poses the greatest risk to morning school operations.

The Atmospheric Temperature Sandwich

1. **Snow:** The entire column of air from cloud top to ground level remains below freezing (<32°F). Ice crystals grow into delicate snowflakes and reach the surface intact. 2. **Sleet (Ice Pellets):** Snow falls from aloft, passes through a shallow warm layer above freezing and melts into rain drops, then enters a deep layer of sub-freezing air near the surface, refreezing into tiny, hard ice beads that bounce off windshields. 3. **Freezing Rain:** Snow falls into a thick warm layer of air and melts completely into liquid raindrops. As the rain falls into a very shallow layer of sub-freezing air right at ground level, the liquid does not have time to freeze in the air. Instead, it **supercools and freezes instantly upon contact with frozen surfaces**, creating a clear glaze of ice.

Why Freezing Rain Trumps Snow for School Cancellations While modern municipal plows can push 6 inches of powdery snow aside, **plows are powerless against sheet glaze ice**: - **Braking Distance:** On glaze ice, a 30,000 lb school bus has virtually zero traction, even with tire chains. - **Power Grid Collapse:** Glaze ice just 0.25 to 0.5 inches thick adds thousands of pounds of weight to tree branches and utility wires, causing widespread blackouts. - **Refreezing:** Melting sleet or freezing rain can rapidly turn into invisible "black ice" if temperatures plummet after sunset.