What is Wet Bulb Temperature?
Wet Bulb Temperature (WBT) is the lowest temperature that can be reached by evaporating water into the air at a constant pressure. Unlike the standard "dry bulb" temperature you see on a thermometer, the wet bulb temperature accounts for the cooling effect of evaporation. This makes it a critical metric for understanding how human bodies cool themselves through sweating.
Why Wet Bulb Temperature Matters for Safety
Humans regulate body temperature primarily through the evaporation of sweat. As the relative humidity increases, the air's ability to absorb more moisture decreases. If the wet bulb temperature reaches 35°C (95°F), the human body can no longer cool itself by sweating, even in the shade with unlimited water. This is considered the theoretical limit of human survivability for more than a few hours.
Heat Stress Categories
Using the wet bulb temperature, we can categorize the risk of heat-related illnesses:
- Low Risk (Below 27°C): Generally safe for outdoor activities, though hydration is still necessary.
- Moderate Risk (27°C – 31°C): Risk of heat exhaustion. Frequent breaks and heavy hydration are required.
- High Risk (31°C – 34°C): Severe risk of heatstroke. Outdoor work and exercise should be extremely limited.
- Extreme Risk (35°C+): Critical danger. The body cannot cool down. Immediate shelter in a climate-controlled environment is vital.
Calculation Example
If the air temperature is 35°C (95°F) and the relative humidity is 60%:
- The dry bulb temperature is 35°C.
- High humidity prevents rapid evaporation.
- The calculated Wet Bulb Temperature is approximately 28.5°C (83.3°F).
- This falls into the Moderate Risk category, suggesting caution for physical labor.
function calculateWetBulb() {
var temp = parseFloat(document.getElementById('airTemp').value);
var rh = parseFloat(document.getElementById('humidity').value);
var unit = document.getElementById('tempUnit').value;
var resultArea = document.getElementById('resultArea');
var wbtValue = document.getElementById('wbtValue');
var riskLevel = document.getElementById('riskLevel');
var explanation = document.getElementById('explanation');
if (isNaN(temp) || isNaN(rh)) {
alert("Please enter valid numbers for temperature and humidity.");
return;
}
if (rh 100) {
alert("Humidity must be between 0% and 100%.");
return;
}
// Convert to Celsius for formula if input is Fahrenheit
var tCelsius = unit === 'F' ? (temp – 32) * 5 / 9 : temp;
// Stull's Formula for Wet Bulb Temperature
// Tw = T * atan(0.151977 * (rh + 8.31359)^0.5) + atan(T + rh) – atan(rh – 1.676331) + 0.00391838 * (rh)^1.5 * atan(0.023101 * rh) – 4.686035
var tw = tCelsius * Math.atan(0.151977 * Math.pow(rh + 8.31359, 0.5))
+ Math.atan(tCelsius + rh)
– Math.atan(rh – 1.676331)
+ 0.00391838 * Math.pow(rh, 1.5) * Math.atan(0.023101 * rh)
– 4.686035;
var finalTw = tw;
var unitLabel = "°C";
if (unit === 'F') {
finalTw = (tw * 9 / 5) + 32;
unitLabel = "°F";
}
// Display Results
wbtValue.innerHTML = "Wet Bulb: " + finalTw.toFixed(1) + unitLabel;
resultArea.style.display = "block";
// Risk Level Analysis (using Celsius for logic)
if (tw >= 35) {
riskLevel.innerHTML = "EXTREME RISK";
riskLevel.style.backgroundColor = "#000000";
riskLevel.style.color = "#ffffff";
explanation.innerHTML = "Critical survival threshold reached. The human body can no longer cool itself via sweating. Seek air conditioning immediately.";
} else if (tw >= 31) {
riskLevel.innerHTML = "HIGH RISK";
riskLevel.style.backgroundColor = "#f44336";
riskLevel.style.color = "#ffffff";
explanation.innerHTML = "Severe danger of heatstroke. Avoid all outdoor physical activity and stay in the shade with plenty of fluids.";
} else if (tw >= 27) {
riskLevel.innerHTML = "MODERATE RISK";
riskLevel.style.backgroundColor = "#ff9800";
riskLevel.style.color = "#ffffff";
explanation.innerHTML = "Increased risk of heat exhaustion. Take frequent breaks and drink water regularly if working outdoors.";
} else {
riskLevel.innerHTML = "LOW RISK";
riskLevel.style.backgroundColor = "#4caf50";
riskLevel.style.color = "#ffffff";
explanation.innerHTML = "Conditions are generally safe, but maintain hydration during sustained activity.";
}
}