The Maximum Icing Severity chart provides an aid to flight planning and situational awareness through graphical depiction of forecast icing conditions across an area or along a route of flight. The chart should always be used in combination with icing information from all available sources including AIRMETs, SIGMETs, and PIREPs. Pilots of aircraft that are not certified for flight into known or forecast icing conditions should be especially cautious of areas displaying any type of icing severity, regardless of the probability indicated on the graphical display.

Expected icing severity is depicted by five categories of color shading: none, trace, light, moderate, and heavy. The red areas of "SLD threat" are warnings for the presence of supercooled large drops, an indicator of severe icing potential. 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.