How to Calculate Dew Point Temperature

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Dew Point Temperature Calculator

°Celsius °Fahrenheit

Calculated Dew Point

function calculateDewPoint() { var temp = parseFloat(document.getElementById('tempInput').value); var unit = document.getElementById('tempUnit').value; var rh = parseFloat(document.getElementById('humidityInput').value); var resultArea = document.getElementById('resultArea'); var display = document.getElementById('dewPointDisplay'); var comfort = document.getElementById('comfortLevel'); if (isNaN(temp) || isNaN(rh)) { alert("Please enter valid numbers for both temperature and humidity."); return; } if (rh 100) { alert("Relative humidity must be between 1 and 100%."); return; } var T = temp; if (unit === "F") { T = (temp – 32) * 5 / 9; } var a = 17.27; var b = 237.7; var gamma = ((a * T) / (b + T)) + Math.log(rh / 100); var dpC = (b * gamma) / (a – gamma); var dpF = (dpC * 9 / 5) + 32; resultArea.style.display = "block"; if (unit === "C") { display.innerHTML = dpC.toFixed(1) + " °C"; } else { display.innerHTML = dpF.toFixed(1) + " °F"; } var comfortMsg = ""; var color = ""; if (dpC < 10) { comfortMsg = "Dry and crisp"; color = "#007baf"; } else if (dpC < 16) { comfortMsg = "Comfortable"; color = "#28a745"; } else if (dpC < 20) { comfortMsg = "Sticky / Humid"; color = "#fd7e14"; } else if (dpC < 24) { comfortMsg = "Uncomfortable / Muggy"; color = "#dc3545"; } else { comfortMsg = "Oppressive / Extreme Humidity"; color = "#721c24"; } comfort.innerHTML = comfortMsg; comfort.style.color = color; }

Understanding Dew Point Temperature

Dew point is the temperature to which air must be cooled to become saturated with water vapor. When the air temperature drops below the dew point, water vapor condenses to form liquid water (dew or fog). Unlike relative humidity, which changes as the temperature fluctuates throughout the day, the dew point provides a more absolute measure of how much moisture is actually in the air.

How to Calculate Dew Point

The calculation is based on the Magnus-Tetens Formula, which is highly accurate for environmental temperatures between 0°C and 60°C. The formula used in this calculator is:

γ(T, RH) = ln(RH/100) + [bT / (c + T)]
Tdp = [c * γ(T, RH)] / [b – γ(T, RH)]

Where:
– T = Temperature in Celsius
– RH = Relative Humidity (%)
– b = 17.27
– c = 237.7°C

Practical Example

Imagine it is a summer afternoon with an air temperature of 30°C (86°F) and a relative humidity of 50%.

  1. We calculate the intermediate value (gamma) using the natural log of humidity and the current temperature.
  2. Applying the Magnus formula, the resulting dew point is approximately 18.4°C (65.2°F).
  3. At this dew point, most people will start to feel that the air is "muggy" or "sticky" because perspiration does not evaporate as quickly from the skin.

Comfort Levels Based on Dew Point

Dew Point (°C) Dew Point (°F) Human Perception
Below 10°C Below 50°F Very dry, refreshing
10°C – 15°C 50°F – 59°F Comfortable
16°C – 20°C 60°F – 68°F Somewhat humid
21°C – 24°C 69°F – 75°F Uncomfortable, oppressive
Above 24°C Above 75°F Extreme danger, very muggy

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