Dew Point Calculator
Find the dew point temperature from air temperature and relative humidity using the Magnus-Tetens formula.
💧 What is the Dew Point?
The dew point is the temperature air must be cooled to, at constant pressure and moisture content, for water vapor to begin condensing into liquid dew, fog, or frost. It's a direct physical measure of how much water vapor is actually present in the air, expressed as a temperature rather than a percentage.
Meteorologists, pilots, and HVAC engineers rely on dew point constantly, to forecast fog and frost formation overnight, to judge how muggy or oppressive outdoor air will feel regardless of the daily temperature swing, to estimate cloud base height from surface temperature and dew point spread, and to size dehumidification and air conditioning systems that must remove a specific amount of moisture.
A common point of confusion is treating dew point and relative humidity as interchangeable. They are not, relative humidity depends heavily on temperature (the same air mass can show 90 percent RH at dawn and 40 percent RH by afternoon with no moisture added or removed), while dew point tracks the actual water vapor content and stays comparatively stable through the day.
This calculator computes dew point directly from air temperature and relative humidity using the Magnus-Tetens approximation, the formula most operational forecasting tools rely on, and shows the dew point depression (spread) that forecasters use to judge fog risk.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Warm Afternoon
T = 25°C, RH = 60%
Example 2 — Humid Tropical Evening
T = 30°C, RH = 80%
Example 3 — Dry Winter Air
T = 15°C, RH = 40%
❓ Frequently Asked Questions
🔗 Related Calculators
What is dew point?
Dew point is the temperature air must be cooled to (at constant pressure and moisture content) for it to become saturated and start condensing water as dew, fog, or frost. It's a direct measure of the actual amount of water vapor in the air, unlike relative humidity, which also depends on temperature.
How do you calculate dew point from temperature and humidity?
Using the Magnus-Tetens formula: alpha = ln(RH/100) + (17.27*T)/(237.7+T), then Td = (237.7*alpha)/(17.27-alpha), where T is air temperature in Celsius and RH is relative humidity as a percentage. The result Td is the dew point in Celsius.
What is the difference between dew point and relative humidity?
Relative humidity is the ratio of current water vapor to the maximum the air could hold at that temperature, so it changes throughout the day even if the actual moisture stays constant. Dew point measures the actual water vapor content directly, and stays roughly constant as temperature changes unless moisture is added or removed from the air.
Can dew point be higher than air temperature?
No. Dew point can never exceed air temperature. When the two are equal, the air is fully saturated (100 percent relative humidity), and any further cooling causes condensation as dew, fog, or clouds.
What dew point feels comfortable?
Most people find dew points below 15 C (59 F) comfortable, 15 to 20 C (59 to 68 F) somewhat humid, 20 to 24 C (68 to 75 F) muggy, and above 24 C (75 F) oppressive. This scale, from the National Weather Service, is widely used to describe outdoor comfort.
What causes fog or dew to form?
Fog and dew form when air cools to its dew point, usually overnight as the ground radiates heat away, causing water vapor to condense onto surfaces (dew) or into suspended droplets (fog). A small spread between the current temperature and dew point (typically under 2 to 3 C) is the standard forecasting signal that fog is likely by early morning.
What is the Magnus-Tetens formula?
The Magnus-Tetens formula is a widely used empirical approximation relating temperature, relative humidity, and dew point, accurate to about 0.4 degrees Celsius across the 0 to 60 C range typically encountered in weather forecasting. It's simpler to compute than the full Clausius-Clapeyron vapor pressure equation while remaining accurate enough for operational use.
How does dew point relate to comfort compared to the heat index?
The heat index combines air temperature and relative humidity to estimate how hot it feels, and it uses dew point (via humidity) as one of its core inputs. A high dew point drives up the heat index because it means the air is already saturated with moisture, which suppresses evaporative cooling from sweat.
Why does dew point stay more stable through the day than relative humidity?
Relative humidity is temperature-dependent, warm air can hold more moisture, so RH drops in the afternoon heat even if no moisture is added or removed. Dew point tracks the actual water vapor content of the air mass, which typically changes slowly unless a different air mass moves in, so it stays comparatively steady over a single day.
What is dew point depression?
Dew point depression is simply the air temperature minus the dew point (T minus Td). A small depression, a few degrees or less, means the air is close to saturation, useful for forecasting fog, frost, and cloud base height. A large depression means the air is relatively dry for its temperature.
What input range does this calculator accept?
This calculator accepts air temperatures from 0 to 60 degrees Celsius and relative humidity from 1 to 100 percent, the standard valid range for the Magnus-Tetens approximation used in most weather applications.