Relative Humidity Calculator

Relative Humidity Calculator

Determine the moisture content in the air based on air temperature and dew point.

Calculated Relative Humidity:

0%

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What is Relative Humidity?

Relative humidity (RH) is a measure of the current amount of water vapor in the air relative to the maximum amount of water vapor the air can hold at that specific temperature. It is expressed as a percentage. When the relative humidity reaches 100%, the air is saturated, and water vapor begins to condense into liquid water (dew or fog).

How This Calculator Works

This tool uses the Magnus-Tetens Formula to determine the saturation vapor pressure and actual vapor pressure based on your input of ambient air temperature and dew point temperature. The calculation follows these scientific steps:

1. Determine the Actual Vapor Pressure using the Dew Point.
2. Determine the Saturation Vapor Pressure using the Air Temperature.
3. Divide the Actual by the Saturation and multiply by 100.

Formula Used

RH = 100 × (exp((17.625 × Td) / (243.04 + Td)) / exp((17.625 × T) / (243.04 + T)))

Where:
T = Air Temperature in Celsius
Td = Dew Point Temperature in Celsius

Practical Example

Suppose the current air temperature is 25°C and the dew point is 15°C. Using the Magnus formula approximation:

• The saturation vapor pressure at 25°C is approximately 31.67 hPa.
• The actual vapor pressure at a dew point of 15°C is approximately 17.04 hPa.
• Calculation: (17.04 / 31.67) × 100 = 53.8% Relative Humidity.

Importance of Monitoring Humidity

Relative humidity plays a critical role in human comfort, health, and building maintenance. High humidity (>60%) can encourage mold growth and dust mites, while very low humidity (<30%) can cause dry skin, respiratory irritation, and damage to wooden furniture or musical instruments.

function calculateRH() { var t = parseFloat(document.getElementById('airTemp').value); var td = parseFloat(document.getElementById('dewPoint').value); var resultDiv = document.getElementById('rhResultBox'); var output = document.getElementById('rhOutput'); var comfort = document.getElementById('comfortLevel'); if (isNaN(t) || isNaN(td)) { alert("Please enter valid numbers for both Temperature and Dew Point."); return; } if (td > t) { alert("Dew point cannot be higher than air temperature. In such cases, humidity is 100% and condensation occurs."); td = t; } // Magnus-Tetens approximation constants var a = 17.625; var b = 243.04; // Numerator: exp((a * Td) / (b + Td)) var numerator = Math.exp((a * td) / (b + td)); // Denominator: exp((a * T) / (b + T)) var denominator = Math.exp((a * t) / (b + t)); var rh = 100 * (numerator / denominator); // Fix bounds if (rh > 100) rh = 100; if (rh < 0) rh = 0; output.innerHTML = rh.toFixed(1) + "%"; resultDiv.style.display = "block"; // Comfort Level Logic var comfortText = ""; if (rh = 30 && rh 50 && rh <= 70) { comfortText = "Humid: Might feel slightly muggy or damp."; } else { comfortText = "Very Humid: High risk of mold growth and significant discomfort."; } comfort.innerHTML = "Status: " + comfortText; }