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ADDS - Icing Help Page (2 of 4)
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Notes on Icing Severity

CIP/FIP provide information on expected icing severity as five categories: none, trace, light, moderate, and heavy. On the display this field can be shown for all icing probabilities or masked (gray areas) to show only icing severity at icing probabilities greater than 25 or 50%.

The severity estimations are roughly based on the accretion rate of ice on an airplane, and the levels are determined by the time it would take for an airfoil to accrete 1/4 in on ice: trace, 1 h; light, 15 min-1 h; moderate, 5-15 min; and severe < 5 min. The rates are, in turn, estimated from the amount of supercooled liquid water expected with a nominal drop diameter of 15 microns, and are further tuned by nearby pilot reports of encountered severity. These are relative values and the use of which should take into account the airframe and the level of icing protection provided by the aircraft. The ultimate safety factor is the vigilance demonstrated by the pilot in potential icing situations.

Different aircraft and different flight configurations (airspeed, angle of attack, etc.) will experience variations in accretion rate. These rates have been simulated for a range of aircraft and are a "broad brush" approach to severity prediction. They are presented here as guidance and supplement the primary forecasts produced as AIRMETs by the Aviation Weather Center.

For additional information, consult this: How to Properly Use an Icing Forecast.