Dew Point Calculator
Print| Air Temperature | - |
| Relative Humidity | - |
| Dew Point | - |
| Absolute Humidity | - |
Managing indoor comfort, predicting agricultural frost, and ensuring aviation safety requires understanding atmospheric moisture. The amount of water vapor in the air determines when condensation will occur, dictating the feel of the weather and the likelihood of fog or dew formation.
Our free online Dew Point Calculator allows you to input any two of the following three variables to automatically compute the third:
- Air Temperature: The physical temperature of the ambient air.
- Relative Humidity: The current moisture saturation percentage of the air.
- Dew Point Temperature: The temperature at which moisture will condense.
What is Humidity? Relative vs. Absolute
Humidity represents the amount of water vapor (the gaseous state of water) present in the air. Saturated air acts as a buffer for the formation of dew, frost, fog, and precipitation. The maximum amount of water vapor air can hold is dictated by air temperature; warmer air can hold significantly more water vapor before reaching saturation than cooler air.
Absolute Humidity
Absolute humidity measures the actual mass of water vapor present in a given volume of air, typically expressed in grams per cubic meter (g/m^3). If the volume of air is fixed, absolute humidity does not change with temperature. However, in the open atmosphere, pressure and temperature shifts cause air volume to expand or contract, altering the absolute humidity value even when the total water content remains constant.
Relative Humidity
Relative humidity compares the current absolute humidity to the maximum possible moisture the air could hold at its current temperature, expressed as a percentage. Because cooler air holds less water vapor, cooling down a volume of air will increase its relative humidity even if no new water is added. A relative humidity of 100% indicates that the air is fully saturated, meaning dew or condensation will begin to form.
What is the Dew Point?
The dew point is the temperature to which a volume of air must be cooled (at constant atmospheric pressure) for it to reach 100% relative humidity, causing water vapor to condense into liquid water (dew).
Key properties of the dew point include:
- Moisture Level: More humid air contains more water vapor, resulting in a higher dew point than dry air.
- Temperature Proximity: The higher the relative humidity, the closer the dew point is to the actual air temperature. At 100% relative humidity, the dew point is exactly equal to the air temperature.
- Frost Point: If the dew point is below freezing (
32°For0°C), the water vapor undergoes deposition, turning directly into frost rather than liquid dew.
Human Comfort and OSHA Standards
The dew point is a highly reliable indicator of human comfort. High dew points inhibit the evaporation of sweat from our skin, making it difficult for the body to cool down. This increases the apparent temperature (see our Heat Index Calculator).
Conversely, very low dew points indicate dry air, which can irritate skin, crack lips, and dry out respiratory airways.
| Dew Point Range | Human Perception & Comfort |
|---|---|
| Below 50°F (10°C) | Very dry and crisp. Can feel uncomfortably dry in heated indoor spaces. |
| 50°F to 55°F (10°C to 13°C) | Highly comfortable for most individuals. |
| 56°F to 60°F (13°C to 16°C) | Comfortable, though humidity is becoming slightly noticeable. |
| 61°F to 65°F (16°C to 18°C) | Sticky and humid. Outdoor activities feel warmer. |
| 66°F to 70°F (18°C to 21°C) | Very humid and oppressive. Highly uncomfortable. |
| Above 70°F (21°C) | Extremely oppressive and hazardous, severely restricting sweat evaporation. |
To promote health and safety in offices and factories, the US Occupational Safety and Health Administration (OSHA) recommends keeping indoor temperatures between 68°F and 76°F, with a relative humidity managed between 20% and 60%.
General Aviation and Dew Point Meters
In aviation, pilots monitor the dew point to predict visibility hazards and mechanical failures:
- Carburetor Icing: In piston-engine aircraft, fuel vaporization combined with low pressure in the carburetor throat can drop air temperatures by up to 70°F. If the air is humid and the dew point is near this reduced temperature, ice forms in the throat, potentially causing engine failure.
- Fog Prediction: When evening temperatures drop close to the dew point, water vapor condenses to form surface fog, dramatically reducing landing visibility.
Meteorologists use dew point meters to measure these thresholds. These instruments cool a polished metal mirror while air passes over it; the temperature at which condensation first forms on the mirror is recorded as the dew point.
To calculate feel-like cooling in winter, visit our Wind Chill Calculator. To compute perceived summer heat ranges, use the Heat Index Calculator.
Frequently Asked Questions (FAQ)
Can the dew point be higher than the air temperature?
No. The air temperature cannot cool below the dew point without moisture condensing out, which releases latent heat and matches the air temperature to the dew point. In normal atmospheric conditions, the dew point will always be equal to or lower than the current air temperature.
How does relative humidity affect sweat evaporation?
Sweating cools the body because the phase change of liquid sweat turning into water vapor absorbs heat. When relative humidity is high, the air is already close to saturation, which slows down the evaporation of sweat. This leaves you feeling hot and sticky because your body is unable to shed excess heat effectively.
What does a dew point below 32°F indicate?
When the dew point is below 32°F (0°C), it is referred to as the frost point. If the air temperature drops to this level, water vapor will skip the liquid phase and deposit directly as solid ice crystals (frost) on surfaces.
Why is dew point considered a better comfort indicator than relative humidity?
Relative humidity changes constantly as the temperature rises and falls throughout the day, even if the absolute amount of moisture in the air remains the same. The dew point remains stable regardless of temperature fluctuations, providing a consistent measure of the actual moisture content in the air.