Greatest risk of icing conditions is experienced in cirrus clouds.
A cloud consisting of both supercooled water droplets and ice crystals produces aircraft icing.
Greatest risk of icing when cloud temperature decreases well below minus 12 degrees C.
Probability of icing increases when dry snow starts to fall from a cloud.
Refer to figure.
SEVERITY OF ICE ACCRETION is defined as the rate of accumulation of ice by weight per unit area per unit time.
Among the meteorological factors determining this rate are the amount of liquid water present and the size of the droplets. These characteristics are not the same throughout a particular cloud, even at one level:
- A cloud containing both liquid water and ice crystals may have large patches where one or the other predominate and icing will tend to be severe when the temperature is not far below 0 °C. An analysis of reports on ice accretion shows a preponderance of occasions at temperatures above about - 10 °C and indicates that the frequency diminishes rapidly when the temperature falls below - 20 °C, although occasional icing has been reported at temperatures below - 40 °C.
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