What is the Dew Point?
The dew point is the temperature to which air must be cooled, at constant pressure, for water vapor to condense into liquid water. It is one of the most reliable indicators of how humid the air actually feels: the higher the dew point, the more moisture in the air. A dew point above about 16 °C feels noticeably muggy, while a value above 21 °C is oppressive. This calculator converts an air temperature and relative humidity reading into a dew point using the widely used Magnus-Tetens approximation.
How to Use This Calculator
Enter the current air temperature in degrees Celsius and the relative humidity as a percentage (0–100). Press calculate to see the dew point in °C plus the dew point spread — the difference between the air temperature and the dew point. A small spread means the air is close to saturation (fog or rain is more likely); a large spread means dry air.
The Formula Explained
The Magnus-Tetens approximation uses two empirical constants, \(a = 17.625\) and \(b = 243.04\) °C. First an intermediate term gamma is computed:
$$\gamma = \ln\!\left(\frac{\text{RH}}{100}\right) + \frac{a\cdot T}{b + T}$$
Then the dew point follows directly:
$$T_d = \frac{b\cdot\gamma}{a - \gamma}$$
The natural logarithm of the relative humidity fraction captures how far the air is below saturation. The method is accurate to within a few tenths of a degree for the 0–60 °C range typical of weather conditions.
Worked Example
Suppose the air temperature is 25 °C and relative humidity is 60%. Then $$\gamma = \ln(0.60) + \frac{17.625 \times 25}{243.04 + 25} = -0.5108 + 1.6437 = 1.1329.$$ The dew point is $$T_d = \frac{243.04 \times 1.1329}{17.625 - 1.1329} = \frac{275.34}{16.4921} \approx 16.69 \text{ °C}.$$ The spread is \(25 - 16.69 \approx 8.31\) °C.
FAQ
Is dew point the same as relative humidity? No. Relative humidity depends on temperature, while dew point is an absolute measure of moisture content. Two readings with the same humidity but different temperatures have different dew points.
What units does this use? Temperatures are in degrees Celsius. To convert °F to °C, use \((\text{°F} - 32) \times \frac{5}{9}\).
Why does the air feel sticky at high dew points? When the dew point is high, sweat evaporates slowly because the air is already near saturation, so your body cools less efficiently.
