Glaze (ice)

Glaze (ice), also known as glaze ice, silver thaw, or ice storm, is a coating of ice formed when supercooled liquid precipitation (rain, drizzle) freezes upon contact with surfaces. This occurs when the surface temperature is at or below freezing (0 °C or 32 °F), and the air temperature in the shallow layer above the surface is also near or below freezing, but the precipitation itself falls from a warmer layer aloft.

The supercooled water droplets remain liquid even below freezing because they lack sufficient ice nuclei to trigger freezing. When these droplets encounter a freezing surface, they rapidly freeze, forming a smooth, transparent, and often very strong coating of ice.

Glaze ice can accumulate to significant thicknesses, adding considerable weight to trees, power lines, and other exposed structures. This weight can cause branches and trees to break, leading to power outages and hazardous driving conditions. Glaze ice differs from other forms of ice precipitation like sleet (ice pellets) and freezing rain in its appearance and formation process. Sleet is already frozen before reaching the surface, while freezing rain forms when rain falls through a shallow layer of freezing air but freezes relatively slowly, sometimes forming icicles. Glaze ice, in contrast, freezes almost instantaneously upon contact.

The severity of a glaze ice event is determined by the thickness of the ice accumulation. Even a thin layer can create dangerous conditions for pedestrians and motorists due to the slippery surface. Heavy accumulations can lead to widespread damage and disruption.