Relative Humidity Calculator
Find relative humidity from temperature and dewpoint, see the temperature-dewpoint spread, and check carburetor icing risk. A small spread means the air is near saturation — more likely fog, low cloud, and induction icing.
Carburetor Icing Risk (FAA AC 61-23C)
| Risk Level | Condition | Meaning |
|---|---|---|
| Serious (Red) | Temp 20–70°F, spread ≤ 5°F | Serious carburetor icing risk at any power setting |
| Moderate (Amber) | Temp 20–70°F, spread 5–15°F | Moderate carburetor icing risk at descent/low power |
| Moderate (Yellow) | Temp 20–70°F, spread 15–30°F | Moderate carburetor icing risk at cruise power |
| Low (Green) | All other conditions | Low carburetor icing risk |
This indicator is based on the FAA carburetor icing probability chart (AC 61-23C). It applies to carbureted engines only. Fuel-injected engines are not susceptible to carburetor ice but may experience induction icing under certain conditions. Always follow your aircraft’s POH for carburetor heat procedures.
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Frequently Asked Questions
This calculator is for educational and planning purposes only. Always refer to your aircraft's POH/AFM, current NOTAMs, and official weather briefings. No calculator is a substitute for pilot-in-command judgment or CFI guidance. Read our full Safety Disclaimer.
Relative Humidity in Depth
Relative humidity expresses how much water vapor the air holds compared with the maximum it could hold at that temperature, stated as a percentage. For pilots it is more than a comfort figure: it hints at fog and low-cloud risk, contributes to density altitude, and signals conditions favorable for carburetor and induction icing. Computing it from temperature and dewpoint turns two routine observations into a useful picture of the air's moisture state.
The link between relative humidity and the temperature-dewpoint spread is direct. When temperature and dewpoint are far apart, the air is dry and relative humidity is low; as they converge, the air approaches saturation and humidity climbs toward 100 percent. Understanding this relationship lets a pilot read moisture risk straight from the numbers on a METAR.
Step-by-Step: How the Calculation Works
Relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure at the current temperature, times 100. Saturation vapor pressure rises sharply with temperature, so the same amount of moisture produces a higher relative humidity in cold air than in warm air. The dewpoint fixes the actual vapor pressure, and the temperature fixes the saturation value.
The calculator estimates both vapor pressures from the temperature and dewpoint you enter, using a standard approximation, and returns the ratio as a percentage. Because the calculation depends only on temperature and dewpoint, it works directly from the values reported in any METAR, giving a humidity figure that most raw observations do not state explicitly.
More Worked Examples
Example 1 — Dry air
Temperature 30°C, dewpoint 5°C — a wide 25-degree spread. The actual vapor pressure is far below saturation at 30°C, giving a relative humidity around 20 to 25 percent. Fog is not a concern, and the air is comfortably clear of saturation.
Example 2 — Near saturation
Temperature 12°C, dewpoint 11°C — a 1-degree spread. Relative humidity is near 93 percent, and only slight cooling, such as after sunset, could drive the air to saturation and form fog or low stratus. This is a classic setup to watch on an early-morning or evening flight.
Pilot Decision-Making Context
High relative humidity with a small temperature-dewpoint spread is a leading indicator of fog and low ceilings, particularly overnight and near dawn when temperatures fall toward the dewpoint. A pilot who notes near-saturated air in the forecast plans for the possibility that a VFR morning departure could be delayed by fog that burns off only after the sun warms the surface.
Humidity also feeds two performance concerns. Moist air is slightly less dense, nudging density altitude upward on hot, humid days, and certain temperature-humidity combinations favor carburetor icing even in visibly clear conditions. Reading relative humidity alongside temperature keeps both effects on the pilot's radar rather than letting them surprise the flight.
More Questions Answered
How does relative humidity relate to the dewpoint?
The closer the dewpoint is to the temperature, the higher the relative humidity. When they are equal, the air is saturated and relative humidity is 100 percent.
Why does the same moisture give different humidity at different temperatures?
Warm air can hold much more water vapor than cold air, so a fixed amount of moisture is a smaller fraction of the maximum in warm air, producing a lower relative humidity.
Does high humidity affect aircraft performance?
Yes, modestly. Moist air is less dense, which slightly raises density altitude, and some humidity and temperature combinations increase the risk of carburetor icing.
What relative humidity signals a real fog risk?
Fog becomes a genuine concern as relative humidity climbs toward 100 percent with a temperature-dewpoint spread of only a degree or two, especially overnight and near dawn when the surface cools toward the dewpoint. Watching the spread narrow through the evening is a practical early warning.
Is relative humidity or dewpoint the better icing indicator?
For carburetor icing, the combination of temperature and dewpoint (and therefore relative humidity) matters, because serious carburetor icing can occur at surprisingly warm temperatures when the air is moist. Consult a carburetor-icing probability chart, which uses temperature and dewpoint together, rather than relying on relative humidity alone.
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