The Ice Accretion Indicator is an L-shaped piece of aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 38 cm (15.0 in) long by 5 cm (2.0 in) wide. It is used to indicate the formation of ice

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, frost

Frost

Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...

 or the presence of freezing rain

Freezing rain

Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...

 or freezing drizzle

Freezing drizzle

Freezing drizzle is drizzle that freezes on contact with the ground or an object at or near the surface. Its METAR code is FZDZ. When freezing drizzle accumulates on land it creates an icy layer of glaze...

.

It is normally attached to a Stevenson screen

Stevenson screen

A Stevenson screen or instrument shelter is an enclosure to shield meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard weather station...

 but may be mounted in other areas away from any artificial heat sources. The weather station

Weather station

A weather station is a facility, either on land or sea, with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind...

 would have two on site and they would be exchanged after every weather

Weather

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...

 observation. The spare indicator should always be at the outside air temperature to ensure that it is ready for use.

If the observer notes the presence of ice or frost on the indicator then a remark to that effect should be sent in the next weather observation. Examples of these are 'Rime Icing on Indicator' and 'FROIN' (Frost on Indicator). As the indicator is at air temperature and is kept horizontal it provides an excellent surface on which to observe freezing precipitation.