Calculate Dew Point

Celsius (°C) Fahrenheit (°F)

Calculation Result

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 output = document.getElementById("dewPointOutput"); var comfort = document.getElementById("comfortLevel"); if (isNaN(temp) || isNaN(rh)) { alert("Please enter valid numbers for temperature and humidity."); return; } if (rh 100) { alert("Relative humidity must be between 1% and 100%."); return; } // Convert to Celsius for calculation if in Fahrenheit var T = (unit === "F") ? (temp – 32) * 5 / 9 : temp; // Magnus-Tetens Approximation Formula var a = 17.27; var b = 237.7; var alpha = ((a * T) / (b + T)) + Math.log(rh / 100.0); var dpC = (b * alpha) / (a – alpha); var dpF = (dpC * 9 / 5) + 32; resultArea.style.display = "block"; if (unit === "C") { output.innerHTML = dpC.toFixed(2) + " °C"; } else { output.innerHTML = dpF.toFixed(2) + " °F"; } // Dew Point Comfort Categories (Based on DP in Fahrenheit) var comfortText = ""; var dF = dpF; if (dF = 50 && dF = 55 && dF = 60 && dF = 65 && dF = 70 && dF < 75) { comfortText = "Comfort Level: Very uncomfortable."; } else { comfortText = "Comfort Level: Oppressive and miserable."; } comfort.innerHTML = comfortText; }

Understanding the Dew Point

The dew point is the temperature at which air becomes saturated with water vapor. When the air temperature drops to its dew point, water vapor condenses to form liquid water, which we see as dew, fog, or clouds. Unlike relative humidity, which changes based on the current temperature, the dew point provides a more absolute measure of how much moisture is in the air.

The Science Behind the Calculation

To calculate the dew point, we use the Magnus-Tetens formula. This equation relates the saturation vapor pressure to the actual air temperature and the relative humidity. The formula used by this calculator is:

γ(T, RH) = ln(RH/100) + [17.27T / (237.7 + T)]
T_dp = [237.7 × γ(T, RH)] / [17.27 – γ(T, RH)]

Where T is temperature in Celsius and RH is relative humidity expressed as a percentage.

Dew Point vs. Relative Humidity

While many people look at relative humidity to determine "mugginess," it can be misleading. For example, 100% humidity at 40°F (4°C) feels very dry because cold air cannot hold much water. However, 50% humidity at 90°F (32°C) results in a dew point of approximately 69°F (20.5°C), which feels quite oppressive. Meteorologists generally prefer the dew point as a metric for human comfort.

Practical Examples

• Example 1: Air temperature is 75°F and RH is 50%. The dew point is approximately 55°F. This is considered very comfortable.
• Example 2: Air temperature is 85°F and RH is 70%. The dew point jumps to 74°F. At this level, the air feels extremely humid, and most people will find it difficult to cool down through sweating.
• Example 3: Air temperature is 30°F and RH is 90%. The dew point is 28°F. Even though the humidity is high, the actual amount of water in the air is very low because the air is cold.

Why Dew Point Matters

1. HVAC Performance: Engineers use dew point to calculate the latent cooling load required to remove moisture from indoor air.
2. Agricultural Planning: Farmers monitor dew point to predict frost formation on crops. If the dew point is below freezing, frost is likely to form instead of liquid dew.
3. Industrial Applications: In compressed air systems or chemical processing, maintaining a specific dew point is critical to prevent pipe corrosion and equipment failure.
4. Aviation: Pilots use the spread between temperature and dew point to predict the likelihood of carburetor icing or fog formation on runways.