The dew point is a critical meteorological metric that represents the temperature to which air must be cooled at constant pressure and water content to reach saturation. At this temperature, water vapor will condense into liquid water (dew).
Why is the Dew Point Important?
Comfort Level: A higher dew point indicates more moisture in the air, making it feel more humid and muggy. Dew points above 20°C are often considered uncomfortable.
Fog and Cloud Formation: When the air temperature cools down to the dew point, fog or clouds can form.
Agriculture: Dew point affects crop transpiration and can indicate potential for fungal diseases.
Industrial Processes: Many manufacturing and storage processes require precise control over humidity, making dew point a key factor.
Aviation: Low dew points can indicate dry air, while high dew points can increase the risk of icing conditions.
The Mathematics Behind Dew Point Calculation
Calculating the dew point directly from temperature and relative humidity involves a few steps using approximations based on Magnus's formula or similar empirical relationships. A commonly used and reasonably accurate approximation is the one implemented in this calculator. The formula involves calculating the saturation vapor pressure (e_s) at a given temperature, then the actual vapor pressure (e) from the relative humidity, and finally solving for the dew point temperature (T_d).
The steps are typically:
Calculate Saturation Vapor Pressure (e_s): This is the maximum amount of water vapor the air can hold at a given temperature (T). A common formula (in hPa or millibars) is based on the August-Roche-Magnus approximation:
e_s(T) = 6.112 * exp((17.62 * T) / (243.12 + T))
where T is the air temperature in degrees Celsius.
Calculate Actual Vapor Pressure (e): This is the amount of water vapor currently in the air, derived from the relative humidity (RH) and the saturation vapor pressure at the air temperature.
e = (RH / 100) * e_s(T)
where RH is the relative humidity in percent.
Calculate Dew Point Temperature (T_d): Now, we use the actual vapor pressure (e) to find the temperature at which this vapor pressure would be saturation. This is the dew point (T_d). The inverse of the Magnus formula is used:
T_d = (243.12 * ln(e / 6.112)) / (17.62 - ln(e / 6.112))
where T_d is the dew point temperature in degrees Celsius.
How to Use This Calculator
Simply enter the current air temperature in degrees Celsius and the relative humidity percentage into the fields above. Click the "Calculate Dew Point" button, and the calculator will provide the dew point temperature in degrees Celsius.
So, the dew point for these conditions is approximately 18.1 °C.
function calculateDewPoint() {
var tempC = parseFloat(document.getElementById("temperature").value);
var rh = parseFloat(document.getElementById("relativeHumidity").value);
var resultContainer = document.getElementById("result-container");
var dewPointResultElement = document.getElementById("dewPointResult");
// Input validation
if (isNaN(tempC) || isNaN(rh) || rh 100) {
alert("Please enter valid numbers for Temperature (in °C) and Relative Humidity (0-100%).");
resultContainer.style.display = 'none';
return;
}
// Using the formula based on the August-Roche-Magnus approximation
// Step 1: Calculate saturation vapor pressure (e_s) in hPa
var saturationVaporPressure = 6.112 * Math.exp((17.62 * tempC) / (243.12 + tempC));
// Step 2: Calculate actual vapor pressure (e) in hPa
var actualVaporPressure = (rh / 100) * saturationVaporPressure;
// Step 3: Calculate Dew Point Temperature (T_d) in °C
var dewPointC = (243.12 * Math.log(actualVaporPressure / 6.112)) / (17.62 – Math.log(actualVaporPressure / 6.112));
// Display the result
dewPointResultElement.textContent = dewPointC.toFixed(1); // Display to one decimal place
resultContainer.style.display = 'block';
}