Make Dew Point Your Friend
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I’ve been on a bit of a mission for the past several years. My goal is to get more people thinking about humidity in terms of dew point rather than relative humidity. Why? Because relative humidity is misleading. That whole “relative” thing is a bit slippery, you know.
This shouldn’t be a big deal for HVAC pros, who are used to thinking in terms of wet bulb temperature, but it’s best not to assume to much. For example, I’ve had a building industry professional who deals with this stuff tell me that in his area, they regularly see both temperature and relative humidity in the 90s (Fahrenheit and percent, respectively). It’s a common misconception. Recently I was speaking with a client in the mid-Atlantic who told me it gets up to 98° F and 95% relative humidity where he builds.
Putting humidity in perspective
I don’t doubt either of those people. I just think their timing is a bit off. Yes, hitting temperatures in the 90s, even up to 98° F, is certainly possible in the majority of the United States. And having relative humidity at 90% or higher is also possible, especially in the Eastern half. They just don’t happen at the same time. When they do, new records are set.
Yes, meteorologists and climatologists keep records of such things as highest dew point. Dew point is the quantity you have to look at for records because it actually tells you how much water vapor there is. Relative humidity tells you how close you are to saturation (100% RH), which happens when the water vapor starts condensing out as you try to add more.
So, let’s look at some records. Here are the highest dew points recorded in the US:
Let’s now take a look at what the air temperature (dry bulb) and relative humidity was for each of those:
You’re probably wondering now about the rest of the world. The highest dew point temperature ever recorded was 95° F on 8 July 2003 at Dhahran, Saudi Arabia. The air temperature at the time was 108° F, making the relative humidity a relatively dry-sounding 68%.
Yes, the mythical 90/90 day for air temperature and relative humidity is possible. It’s happened at least once before…and it set a record. But those are far from normal conditions.
An 80/80 day, however, is quite likely on the humid side of North America. The dew point in those conditions is 73° F. If you live in Houston or Mobile or Florida, that’s about the low end of humidity for you this time of year. Here in Atlanta, that’s about where we top out.
Here’s one more perspective on dew point: When you keep your house at recommended design conditions of 75° F and 50% relative humidity, the dew point is 55° F. That’s comfortable. In my condo, we’re usually at 74° F and about 58% relative humidity. The corresponding dew point is 58° F, and that’s still comfortable. (But you might want to read about how I made one little change to jack up our relative humidity to over 70% back in 2014.)
Make dew point your friend
High relative humidity certainly does happen. It usually coincides with lower air temperatures, though. As the temperature rises, the relative humidity goes down. This morning when I awoke, for example, the relative humidity here in the Atlanta area was 87%. Although that sounds bad, the air temperature was 65° F and the dew point was a nice, dry 61° F. If we keep that same dew point as the temperature rises to the predicted high of 89° F, our relative humidity will drop to a desiccating 39%.
The weather app on your phone probably shows the dew point for your location so if you get in the habit of looking at it, you’ll have a better idea of the humidity. Here in Atlanta, our normal summer range is from the high 60s to the low 70s. On the Gulf coast, it’s mid to upper 70s.
Here’s a screenshot of the dew point map on usairnet.com, showing dew points all across the United States at 7:30 on the morning I wrote this article.
Also, our bodies respond to dew point because one of the ways we cool off is through evaporation of sweat. Higher dew point makes that process more difficult because there’s more water vapor in the air, which makes the vapor pressure higher, too.
Checking the dew point gives me an idea of how humid it really is. When I get up in the morning and see a dew point of 61° F, as it is this morning, I know it’s a great day to be outdoors because it’s going to feel like I’m in Santa Monica, California…well, almost anyway.
Relative humidity isn’t what a lot of people think it is. Dew point will always be your friend, though.
Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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Posted In: Building Performance, Residential Buildings
