Estimate the lowest temperature achievable by evaporative cooling.
°C
°F
Estimated Wet Bulb Temperature:
Understanding Wet Bulb Temperature
Wet Bulb Temperature (WBT) is the temperature read by a thermometer covered in a water-soaked cloth (wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature); at lower humidity, the wet-bulb temperature is lower than the dry-bulb temperature because of evaporative cooling.
Why is Wet Bulb Temperature Critical?
Human bodies cool themselves through the evaporation of sweat. If the wet-bulb temperature reaches 35°C (95°F), the air is so saturated with moisture and so hot that the human body can no longer cool itself by sweating, leading to potential heatstroke even in healthy individuals after prolonged exposure.
Heat Stress Thresholds (Wet Bulb):
Below 26°C (79°F): Generally safe for outdoor activity.
26°C – 29°C (79°F – 84°F): Caution; use caution during intense physical activity.
Above 32°C (90°F): Extreme Danger; high risk of heat exhaustion and heatstroke.
Calculation Example
If the ambient air temperature is 35°C (95°F) and the relative humidity is 45%, the estimated wet bulb temperature is approximately 25.2°C (77.4°F). While the dry temperature feels very hot, the low humidity allows for effective evaporative cooling.
function calculateWetBulb() {
var T = parseFloat(document.getElementById('airTemp').value);
var RH = parseFloat(document.getElementById('humidity').value);
var unit = document.getElementById('tempUnit').value;
var resultWrapper = document.getElementById('resultWrapper');
var wetBulbDisplay = document.getElementById('wetBulbValue');
var riskDisplay = document.getElementById('heatRisk');
if (isNaN(T) || isNaN(RH)) {
alert("Please enter valid numbers for both temperature and humidity.");
return;
}
if (RH 100) {
alert("Relative humidity must be between 0 and 100%.");
return;
}
// Convert to Celsius for calculation if in Fahrenheit
var tempC = (unit === 'F') ? (T – 32) * (5 / 9) : T;
// Stull's Formula (Valid for RH 5% to 99% and Temp -20 to 50C)
var tw = tempC * Math.atan(0.151977 * Math.pow(RH + 8.313659, 0.5))
+ Math.atan(tempC + RH)
– Math.atan(RH – 1.676331)
+ 0.00391838 * Math.pow(RH, 1.5) * Math.atan(0.023101 * RH)
– 4.686035;
var finalResult = tw;
var displayUnit = "°C";
if (unit === 'F') {
finalResult = (tw * 9 / 5) + 32;
displayUnit = "°F";
}
wetBulbDisplay.innerHTML = finalResult.toFixed(1) + displayUnit;
resultWrapper.style.display = 'block';
// Risk Assessment Logic based on Celsius wet bulb
if (tw < 26) {
riskDisplay.innerHTML = "Low Risk";
riskDisplay.style.backgroundColor = "#d4edda";
riskDisplay.style.color = "#155724";
resultWrapper.style.backgroundColor = "#f3fff5";
} else if (tw < 29) {
riskDisplay.innerHTML = "Moderate Risk (Caution)";
riskDisplay.style.backgroundColor = "#fff3cd";
riskDisplay.style.color = "#856404";
resultWrapper.style.backgroundColor = "#fffcf0";
} else if (tw < 32) {
riskDisplay.innerHTML = "High Risk (Danger)";
riskDisplay.style.backgroundColor = "#f8d7da";
riskDisplay.style.color = "#721c24";
resultWrapper.style.backgroundColor = "#fff5f6";
} else {
riskDisplay.innerHTML = "Extreme Risk (Critical)";
riskDisplay.style.backgroundColor = "#343a40";
riskDisplay.style.color = "#ffffff";
resultWrapper.style.backgroundColor = "#eeeeee";
}
}