The humidity is returning for the summer here in the southeastern US. And as you may know, I don’t use relative humidity to gauge how humid the outdoor air is. Why? For starters, the humidity can get up to 100% even on a cold day so how does knowing only that metric help?
Many people get confused on this matter, saying things like, “It gets up to 98 °F and 95% relative humidity where I live.” Maybe, but almost certainly those two things don’t happen at the same time. If they did, they’d have a new world record for dew point temperature.
Anyway, here are a few fun facts about the dew point temperature.
1. Under normal conditions* here on the surface of Earth, the dew point temperature cannot be higher than the air temperature (dry bulb). That puts a cap on your maximum dew point. So if you wake up and the outdoor temperature is 59 °F (15 °C), you know the maximum dew point.
2. When the dew point and dry bulb temperatures are equal, the relative humidity is 100%.
3. Standard indoor design conditions from the Air Conditioning Contractors of America (ACCA) are 75 °F (24 °C) and 50% relative humidity. Thus, the dew point temperature for indoor air at those conditions is 55 °F (13 °C).
4. Psychrometric calculators are available as apps for your smart phone to let you find the dew point temperature for any dry bulb temperature and relative humidity. I use one called PsychroApp.
5. Here’s my rough guide for the dew point temperature’s impact on comfort when you’re outdoors in summer:
A. Dew point less than 60 °F (~15 °C) ⇨ comfortable
B. Dew point in the 60s Fahrenheit (~15-20 °C) ⇨ moderately comfortable
C. Dew point higher than 70 °F (21 °C) ⇨ uncomfortable
The dew point temperature here in Atlanta this morning is 66 °F (19 °C). We’re definitely feeling the humidity now. I called that moderately comfortable above, but the more active you are, the less comfortable it feels. And I’ve got to get out and mow the yard later today because my wife didn’t like my idea of continuing No Mow May with Just Grow It June.
Post script This is article number 1,000 in the Energy Vanguard Blog! I published my first article on 7 March 2010.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.
* Abnormal conditions would be supersaturation, as can happen high in the atmosphere. The dew point can go higher than the dry bulb temperature—and the humidity higher than 100%—when there are no particles or surfaces for the water vapor to condense on…until a plane goes by.
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