Staying on Top of the Freezing Level

Winter and airframe icing are like peanut butter and jelly; it’s hard to imagine one without the other.

Monday, April 10, 2023 at 09:46 AM ET

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Winter and airframe icing are like peanut butter and jelly; it’s hard to imagine one without the other. [Credit: iStock]

Key Takeaways:

Airframe icing occurs in visible moisture at or below the lowest freezing level, making its accurate determination crucial for flight safety.
The standard lapse rate (2°C/1,000 ft) is an unreliable method for estimating the freezing level due to variable atmospheric conditions like temperature inversions or non-standard lapse rates near the surface.
Misjudging the freezing level can lead to pilots unexpectedly encountering airframe ice at lower altitudes or incorrectly avoiding flights when conditions are safe.
Pilots should use official meteorological resources, such as the Aviation Weather Center's lowest freezing level forecasts, G-AIRMETs, Skew-T diagrams, or vertical route profiles, for accurate preflight icing risk assessment.

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Short of an erroneous forecast or calibration issue with your immersion thermometer, if you can remain below the lowest freezing level during your entire flight, there’s typically no chance for an encounter with airframe ice. Induction ice is certainly possible, but not airframe ice.

If you plan an altitude where the temperature aloft is zero degrees Celsius or less, airframe icing becomes exceedingly more likely while flying in visible moisture. Therefore, the freezing level is one key variable that you need to determine during your preflight analysis to better quantify your risk of airframe ice.