Dewpoint Calculator

Dewpoint Calculator

Calculated Dew Point

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What is 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 begins to condense into liquid water, forming dew, fog, or frost. 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 This Calculator Works

This tool uses the Magnus-Tetens Formula, which is a highly accurate approximation for Earth's atmospheric conditions. The formula calculates the dew point temperature (Td) based on the current air temperature (T) and the relative humidity (RH).

γ(T, RH) = ln(RH/100) + [bT / (c + T)]
Td = [c * γ(T, RH)] / [b – γ(T, RH)]
(Where b = 17.67 and c = 243.5 °C)

Dew Point vs. Comfort Levels

The dew point is often used by meteorologists to describe how "sticky" or humid the air feels to humans. Here is a general guide for dew point values in Celsius and how they affect your comfort:

• Below 10°C: Very dry and comfortable.
• 10°C to 15°C: Comfortable for most people.
• 16°C to 20°C: Noticeably humid; starting to feel "sticky."
• 21°C to 24°C: Uncomfortable; oppressive humidity.
• Above 24°C: Extreme discomfort; potentially dangerous levels of moisture.

Practical Examples

Example 1: If the temperature is 30°C and the relative humidity is 50%, the dew point is approximately 18.4°C. This would feel somewhat humid to most people.

Example 2: If the temperature is 20°C and the relative humidity is 90%, the dew point is 18.3°C. Even though the air is cooler than the first example, the high humidity means condensation (dew) will form very easily if the temperature drops even slightly.

Why Dew Point Matters

Understanding the dew point is critical in several fields:

• HVAC Systems: Engineers use dew point to prevent condensation inside wall cavities and on windows.
• Aviation: Pilots monitor the spread between air temperature and dew point to predict the formation of fog or carburetor icing.
• Agriculture: Farmers use it to predict frost formation, which can damage sensitive crops.
• Paint and Coatings: Industrial painters must ensure the surface temperature is at least 3°C above the dew point to ensure proper adhesion.

function calculateDewPoint() { var t = parseFloat(document.getElementById('airTemp').value); var rh = parseFloat(document.getElementById('relHumidity').value); var resultDiv = document.getElementById('resultArea'); var dpDisplay = document.getElementById('dpResult'); var dpFDisplay = document.getElementById('dpFahrenheit'); var comfortDisplay = document.getElementById('comfortLevel'); if (isNaN(t) || 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; } // Magnus-Tetens Formula Constants var b = 17.67; var c = 243.5; // Calculation Logic var gamma = Math.log(rh / 100) + (b * t) / (c + t); var dewPointC = (c * gamma) / (b – gamma); var dewPointF = (dewPointC * 9 / 5) + 32; // Update UI dpDisplay.innerHTML = dewPointC.toFixed(1) + " °C"; dpFDisplay.innerHTML = "Equivalent to " + dewPointF.toFixed(1) + " °F"; // Comfort logic var comfort = ""; var color = ""; if (dewPointC = 10 && dewPointC = 16 && dewPointC = 20 && dewPointC < 24) { comfort = "Uncomfortable / Muggy"; color = "#ef6c00"; } else { comfort = "Extremely Oppressive"; color = "#c62828"; } comfortDisplay.innerHTML = comfort; comfortDisplay.style.color = color; resultDiv.style.display = "block"; }