Wet Bulb Temp Calculation

Wet Bulb Temperature Calculator

Enter values and click 'Calculate'

function calculateWetBulbTemp() { var dryBulbTempInput = document.getElementById("dryBulbTemp").value; var relativeHumidityInput = document.getElementById("relativeHumidity").value; var resultDiv = document.getElementById("wetBulbResult"); var Tdb = parseFloat(dryBulbTempInput); var RH = parseFloat(relativeHumidityInput); if (isNaN(Tdb) || isNaN(RH)) { resultDiv.innerHTML = "Please enter valid numbers for both fields."; return; } if (Tdb 50) { resultDiv.innerHTML = "Dry Bulb Temperature must be between -20°C and 50°C."; return; } if (RH 100) { resultDiv.innerHTML = "Relative Humidity must be between 5% and 100%."; return; } // Stull (2011) empirical formula for Wet Bulb Temperature (Tw) in Celsius // Tw = T * atan[0.151977 * (RH + 8.313659)^(1/2)] + atan(T + RH) – atan(RH – 1.676331) + 0.00391838 * (RH)^(3/2) * atan(0.023101 * RH) – 4.686035 var Tw = Tdb * Math.atan(0.151977 * Math.pow(RH + 8.313659, 0.5)) + Math.atan(Tdb + RH) – Math.atan(RH – 1.676331) + 0.00391838 * Math.pow(RH, 1.5) * Math.atan(0.023101 * RH) – 4.686035; resultDiv.innerHTML = "Calculated Wet Bulb Temperature: " + Tw.toFixed(2) + " °C"; }

Understanding Wet Bulb Temperature

The Wet Bulb Temperature (WBT) is a critical meteorological parameter that measures the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure. Unlike the standard "dry bulb" temperature you see on a thermometer, WBT takes into account both heat and humidity, providing a more accurate indicator of how hot it "feels" to the human body and the potential for heat stress.

Why is Wet Bulb Temperature Important?

WBT is particularly important for assessing heat stress risk, especially in hot and humid environments. When the WBT rises, the body's ability to cool itself through sweating (evaporation) is reduced. This is because the air is already saturated with moisture, making it harder for sweat to evaporate from the skin. If the WBT reaches a certain threshold (often cited around 35°C or 95°F), the human body can no longer cool itself effectively, even in the shade, leading to potentially fatal heatstroke.

• Human Health: High WBT values pose significant health risks, particularly for outdoor workers, athletes, the elderly, and those with pre-existing conditions.
• Climate Change: Rising global temperatures and humidity levels due to climate change are increasing the frequency and intensity of dangerous WBT events in many regions.
• Agriculture and Livestock: High WBT can also impact crop yields and livestock health.

How is it Calculated?

While a true wet bulb temperature is measured using a thermometer with its bulb wrapped in a wet cloth and exposed to airflow, it can also be estimated using mathematical formulas based on the dry bulb temperature and relative humidity. The calculator above uses an empirical formula developed by J. D. Stull (2011) which provides a good approximation for a wide range of conditions.

The key inputs for this calculation are:

• Dry Bulb Temperature (°C): This is the standard air temperature measured by a regular thermometer.
• Relative Humidity (%): This indicates the amount of moisture in the air relative to the maximum amount it can hold at that temperature.

Interpreting Wet Bulb Temperature Results

The higher the wet bulb temperature, the greater the risk of heat stress. Here's a general guide:

• Below 25°C (77°F): Generally safe for most activities.
• 25°C – 30°C (77°F – 86°F): Caution advised, especially for strenuous activity or prolonged exposure. Risk of heat exhaustion increases.
• 30°C – 35°C (86°F – 95°F): Dangerous conditions. Strenuous outdoor activity should be limited or avoided. High risk of heatstroke.
• Above 35°C (95°F): Extremely dangerous. Sustained exposure can be lethal even for healthy individuals at rest.

Examples of Wet Bulb Temperature Calculation:

Let's look at a few scenarios:

1. Hot and Humid Day:
   • Dry Bulb Temperature: 35°C
   • Relative Humidity: 60%
   • Calculated Wet Bulb Temperature: Approximately 29.3°C
   • Interpretation: This indicates significant heat stress, making outdoor activities challenging and potentially dangerous.
2. Warm and Very Humid Day:
   • Dry Bulb Temperature: 25°C
   • Relative Humidity: 80%
   • Calculated Wet Bulb Temperature: Approximately 23.1°C
   • Interpretation: While the dry bulb temperature isn't extremely high, the high humidity makes it feel much warmer and reduces the body's cooling efficiency.
3. Very Hot and Dry Day:
   • Dry Bulb Temperature: 40°C
   • Relative Humidity: 30%
   • Calculated Wet Bulb Temperature: Approximately 24.2°C
   • Interpretation: Despite the very high dry bulb temperature, the low humidity allows for more effective evaporative cooling, resulting in a lower WBT compared to the first example. However, dehydration is still a major concern.

Using this calculator can help you understand the true impact of heat and humidity on your environment and take necessary precautions to stay safe.