Glaze

Primary reference(s)

WMO, 2017. International Cloud Atlas: Glaze. World Meteorological Organization (WMO). Accessed 20 November 2019.

Additional scientific description

The deposit of ice formed by the freezing of fog or cloud droplets not supercooled at the time of impact with objects at temperatures well below 0°C, is known as glaze. Glaze on the ground must not be confused with ground ice which, on a road surface, is known as ‘black ice’ (WMO, 2017).

Glaze covers all parts of surfaces exposed to precipitation. It is generally fairly homogeneous and morphologically resembles clear ice. At or near the ground, glaze forms when drizzle droplets or raindrops become supercooled as they fall through a layer of air at a sub-frost point temperature. In the free atmosphere, glaze is observed when aircraft are exposed to supercooled precipitation. Glaze forms by the slow freezing of supercooled liquid water and so penetrates the air gaps between the particles of ice before freezing (WMO, 2017).

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

Glaze is denser, harder, and more transparent than either rime or hoarfrost. Its density may be as high as 0.8 or 0.9 g/cm3. Factors that favour glaze formation are large drop size, rapid accretion, slight supercooling, and slow dissipation of heat of fusion. The opposite effects favour rime formation. The accretion of glaze on terrestrial objects constitutes an ice storm; as a type of aircraft icing it is called clear ice. Glaze, as well as rime, may form on ice particles in the atmosphere. Ordinary hail is composed entirely (or nearly so) of glaze; the alternating clear and opaque layers of some hailstones represent glaze and rime, deposited under varying conditions around the growing hailstone (AMS, 2012).