Dew Point
Calculator
Calculate dew point from temperature and relative humidity, convert between dew point and RH, and find absolute humidity — using the precise Magnus formula. Supports °F and °C.
Dew Point Comfort Scale
How different dew point temperatures feel to the human body.
Dew Point, Temperature & Relative Humidity
Approximate relative humidity at different temperature / dew point combinations.
| Air Temp | Dew Point | Rel. Humidity | Comfort Level | Conditions |
|---|---|---|---|---|
| 35°C / 95°F | 24°C / 75°F | ~55% | Dangerous | Tropical heat wave; heat stroke risk |
| 30°C / 86°F | 21°C / 70°F | ~65% | Oppressive | Typical summer in southeastern US |
| 28°C / 82°F | 18°C / 64°F | ~60% | Sticky | Warm humid summer day |
| 25°C / 77°F | 15°C / 59°F | ~55% | Pleasant | Comfortable summer conditions |
| 20°C / 68°F | 10°C / 50°F | ~52% | Comfortable | Ideal spring/fall conditions |
| 15°C / 59°F | 5°C / 41°F | ~54% | Comfortable | Cool and dry; comfortable jacket weather |
| 10°C / 50°F | 2°C / 36°F | ~63% | Dry | Cool and mildly humid |
| 10°C / 50°F | 10°C / 50°F | 100% | Foggy | Air fully saturated; fog likely |
Dew Point Is More Reliable Than Relative Humidity
RH changes all day as temperature rises and falls — even if the air's actual moisture content stays exactly the same. On a warm afternoon, RH may drop to 40% while dew point stays at 15°C. Meteorologists use dew point to measure actual moisture because it stays constant unless moisture is added or removed.
Dew forms when a surface cools to the dew point temperature — water vapor in contact with that surface condenses into liquid droplets. This is why grass is wet in the morning even without rain.
When the dew point is below 32°F (0°C), it becomes the frost point — water vapor condenses directly into ice crystals rather than liquid water. Frost can form on surfaces even when air temperature is slightly above freezing, if the surface itself cools below the frost point.
Meteorologists use dew point as a key forecasting variable. A high surface dew point provides the moisture needed for thunderstorm development. The difference between air temperature and dew point — the "temperature-dew point spread" — is used to forecast fog: when the spread is 5°F (3°C) or less, fog is likely to form.
In severe weather forecasting, surface dew points above 65°F (18°C) are associated with increased severe thunderstorm potential, while dew points above 70°F (21°C) are associated with significant tornado risk in the US.
How Dew Point Is Calculated — The Magnus Formula
The dew point is calculated using the August-Roche-Magnus approximation, one of the most widely used formulas in meteorology and HVAC engineering. It provides accuracy within ±0.4°C for temperatures between −40°C and 60°C, making it suitable for virtually all real-world applications.
Dew Point from Temperature and Relative Humidity
Relative Humidity from Temperature and Dew Point
Absolute Humidity
Absolute humidity is the mass of water vapor per unit volume of air (g/m³). It is calculated from temperature and relative humidity using the Tetens saturation vapor pressure formula:
Dew Point vs Relative Humidity — What's the Difference?
Relative humidity and dew point both measure atmospheric moisture, but they behave very differently — and understanding the difference helps explain why weather apps and meteorologists prefer dew point for comfort assessments.
Why Relative Humidity Changes Through the Day
Relative humidity is the ratio of actual water vapor to the maximum water vapor air can hold at that temperature. Warmer air can hold more water vapor. So as temperature rises through the morning, the air's capacity to hold moisture increases — and relative humidity drops, even though no moisture has left the air. At night when temperature falls, relative humidity rises. A typical summer day might start at 90% RH at dawn and fall to 50% RH by afternoon, with the dew point remaining constant throughout.
Why Dew Point Is a Better Comfort Indicator
Since dew point only changes when actual moisture content changes, it gives a stable, consistent measure of how humid the air truly is — regardless of time of day or temperature. A 70°F dew point feels equally oppressive at 75°F air temperature (95% RH) or at 95°F air temperature (50% RH). In both cases, your body's ability to cool itself by sweating is severely impaired because the air is already heavily loaded with moisture.
Dew Point and Indoor Comfort (HVAC)
In buildings and HVAC engineering, indoor dew point is used to control condensation on surfaces and prevent mold growth. The recommended indoor dew point range is 40–55°F (4–13°C), corresponding to roughly 40–60% relative humidity at typical indoor temperatures of 68–72°F (20–22°C). A dew point above 55°F indoors creates conditions where mold can thrive on cooler surfaces.