It is hot here and probably where you are. We’re all looking for ways to keep our indoors comfortable. Although the AC is tempting, it is also expensive and tough on the environment and our infrastructure. So can’t we just open our windows when the outside temperature is lower than indoors? Not so fast!
We’ve all seen the “feels like” temperature in our forecasts. The other day it was in the high 80°Fs here, but I started to feel loopy, because the humidity made it feel like it was over 100°F. That’s because as the humidity rises, our bodies are less able to cool themselves. Sweat turns our skin into an evaporative cooler. When the liquid sweat turns into water vapor, it absorbs heat, cooling your body.
The wet-bulb temperature calculation was created in 1956 by the US Marine Corps to limit corps members from getting ill in hot weather training. They wrapped a thermometer in a damp cloth and the more efficient the evaporation, the cooler the thermometer reading. Today, we can use the temperature and relative humidity to calculate the wet-bulb temperature.
The formula for this is the following monstrosity:
Tw = T × arctan[0.151977 × (RH% + 8.313659)^(1/2)] + arctan(T + RH%) – arctan(RH% – 1.676331) + 0.00391838 ×(RH%)^(3/2) × arctan(0.023101 × RH%) – 4.686035
Where:
Tw = Wet Bulb Temperature
RH% = Relative Humidity Percentage
T = Temperature
Obviously we’re not typing this into our TI-83 graphing calculators (ask someone who went to school in the 1990s) every evening to figure out if it is time to open the windows. One way to reckon this is by using an online calculator, such as this one from the National Weather Service or another from OmniCalculator.
But typing the indoor and outdoor temperature and humidity into calculators repeatedly also gets old fast, so I created this chart, which hangs by our weather station monitor. The right-hand side is the temperature in degrees Fahrenheit while the top row is relative humidity. To find the wet-bulb temperature, simply cross reference the current temperature and humidity values and compare between indoor and outdoor wet-bulb temperatures.
For example, as I write this, it is 77°F and 62% humidity indoors, which correlates to 66.9°Fw. Outside it is only 70°F but with higher humidity, at 78%. This correlates to 65.0°Fw, so I know I should open the windows. An hour earlier, though, even when the temperature outside was below our indoor temperature, the high humidity made it feel warmer, so we left the windows shut.
You can also download it as a PDF right here:
And finally, a thank you to Mark Thomas, who turned me on to this comparison.
Thanks Scott!