Dewpoint, relative humidity, and frost point
Relative humidity is computed from the same Magnus / Bolton (1980) approximation used on this site's cloud-base calculator: es(t) = 6.112 exp(17.67 t / (t + 243.5)) with t in °C, and RH = 100 × es(Td) / es(T). You can enter temperature and dewpoint, or temperature and RH, and the other is filled in. The temperature–dewpoint spread is reported in both Celsius and Fahrenheit.
Frost point is the temperature at which the current vapor pressure would saturate over a plane ice surface. It is computed from a Magnus formula over ice (constants 21.87 and 265.5). When the dewpoint is below 0 °C, frost point is slightly warmer than dewpoint, because ice saturates at a lower vapor pressure than supercooled liquid water. At 0 °C the two temperatures coincide.
PHAK Figure 7-12 carburetor-icing zones
The Pilot's Handbook of Aeronautical Knowledge chart of carburetor-icing potential, Figure 7-12, is plotted against temperature and relative humidity. This page uses the zones as stated in that figure and its supporting text:
High potential: OAT 20–70 °F and RH 80–100%
Icing possible: OAT 20–100 °F and RH 50% or more
The high-potential region is a subset of the possible region. The caption of the figure states that icing is possible under conditions not depicted on the chart — so “outside the shaded zones” does not mean icing is impossible. Advisory Circular 20-113 notes that a temperature–dewpoint spread of about 20 °F or less corresponds roughly to a relative humidity of 50% or more, which is the threshold of the “possible” zone. That screen is reported alongside the PHAK classification.
This is the PHAK training chart, not a digitized version of the more detailed “serious icing — descent / cruise / any power” envelope published in some older FAA and Transport Canada material. Those finer polygons are not recreated here because they are not given as exact numerical boundaries in PHAK Figure 7-12.
Sources
FAA Pilot's Handbook of Aeronautical Knowledge, Figure 7-12 (carburetor icing); FAA Advisory Circular 20-113; Bolton (1980) Magnus formula for saturation over water; Alduchov & Eskridge-style Magnus constants for saturation over ice. For cloud base from the same spread, see the cloud-base calculator.
Frequently asked questions
When is carburetor icing in the PHAK high-potential zone?
When the outside air temperature is between 20 and 70 °F and the relative humidity is 80 to 100 percent, according to FAA PHAK Figure 7-12. Icing can still occur outside that box.
What is frost point, and how does it differ from dewpoint?
Frost point is the temperature at which the current vapor pressure would saturate over ice rather than liquid water. Below 0 °C it is slightly warmer than the dewpoint. At 0 °C the two are the same.
If I am outside the shaded zones, am I safe from carb ice?
No. The PHAK figure’s caption says icing is possible under conditions not shown on the chart. Treat an “outside” result as “not in the shaded training zones,” not as a clearance.