Understanding the four different intensities of icing allows for better decision making and better PIREPS.
Trace— Icing is barely perceptible and doesn’t require the use of anti/de-icing equipment for over an hour. On graphical PIREPs, the symbol for trace icing is a green ‘”U.”
Light— The rate of accumulation only creates a problem for flights over an hour. Occasional use of anti/de-icing prevents accumulation. Indicated by a green “U” with a vertical line through it.
Moderate— Even short encounters are potentially dangerous and require the use of anti/de-icing or diversion. Moderate icing is indicated by an orange “U” with two vertical lines through it.
Severe— This rate of accumulation overcomes even anti/de-icing equipment and requires an immediate diversion. Commercial operators are prohibited from operating in areas of known severe icing. Severe icing is indicated by a red “U” with two (one bold) vertical lines through it.
In addition to intensity, icing PIREPS can also describe duration. Duration is divided into thirds; occasional is less than 1/3 of the time, intermittent is 1/3 to 2/3, and continuous is greater than 2/3. Not coincidentally, this is the same breakdown as for turbulence reports.
Given these intensities, encountering trace or light icing isn’t usually a big deal. The pilot has plenty of time to find a better altitude or land at an alternate. The key is taking proactive action once ice is detected.