Icing (Including Ice)

Primary reference(s)

WMO, 1992. International Meteorological Vocabulary, WMO-No. 182. World Meteorological Organization (WMO). Accessed 25 November 2019.

Additional scientific description

Icing, in general, is any deposit or coating of ice on an object, caused by the impingement and freezing of liquid (usually supercooled) hydrometeors; to be distinguished from hoar frost in that the latter results from the deposition of water vapour (NOAA, 2019).

An ice deposit may form on different parts of an aircraft when flying in supercooled clouds or precipitation. The intensity and characteristics of the icing vary, but depend primarily on the degree of supercooling, the droplet diameters and concentration, and the characteristics of the airflow around the aircraft. The main types of icing are soft rime, hard rime, clear ice, and glaze (WMO, 2017, 2020a,b,c,d).

Necessary conditions for icing include air temperatures at or below 0°C and supercooled liquid water droplets or wet snowflakes.

However, If an aircraft has been in below freezing temperatures and then in above freezing temperatures, the aircraft’s surface temperature can remain below freezing for some time. Thus, icing still may be possible in ambient temperatures above freezing (NOAA, 2019).

NOTE: Supercooled liquid water droplets are predominantly found at temperatures ranging from 0°C to -20°C. Although rare, small amounts of supercooled water droplets can be found at temperatures as low as -40°C. The smaller and purer the droplets, the lower their freezing points.

NOTE: When a supercooled droplet strikes an object such as the surface of an aircraft, the impact destroys the internal stability of the droplet and raises its freezing temperature. This is known as aerodynamic heating – the temperature rise resulting from adiabatic compression and friction as the aircraft penetrates the air (NOAA, 2019).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Factors which affect the icing threat for aeroplanes include: particle size; particle concentration; shape of aircraft surfaces; aircraft speed; environmental temperature; and aircraft surface temperature (must be 0°C or less) (NOAA, 2019).This table illustrates icing effects on primary forces and the resulting effect on aircraft (NOAA, 2019).

Heavy accumulations of ice can bring down trees and topple utility poles and communication towers. Ice can disrupt communications and power for days while utility companies repair extensive damage. Even small accumulations of ice can be extremely dangerous to motorists and pedestrians. Bridges and overpasses are particularly dangerous because they freeze before other surfaces (AMS, 2012).

An example of a National Alerting Parameter for icing is that by China for road icing (China Meteorological Administration, 2012).