Dew Point — A More Meaningful Measure of Humidity?
Did the grass outside have dew on it this morning where you live? It did here. (That photo to the left is the actual dew on the actual grass on this actual morning just outside my actual house!) The appearance of liquid water occurs because of one simple fact: The temperature of the surfaces that collect this dew is below the dew point of the surrounding air. Everyone knows that, of course, but what does it mean?
Water really is an amazing substance, and if you're wondering why I'm pontificating on it as if I were a college professor again, it's because water and buildings have an interesting relationship. In fact, understanding how water interacts with buildings is one of the most important jobs of building science. I've heard it said that three of the leading causes of building failures are:
I'm not so interested in knowing the relative humidity as I used to be. By itself, it doesn't give you enough information. If you really want to understand the properties of a given volume of air, you have to know at least one more datum in addition to the RH: temperature (dry bulb), absolute humidity, dew point...
Unlike relative humidity, however, if you know the dew point by itself, you've got some good information. If you have air with a dew point of, say, 20° C (68° F), you know that if that air finds any surface with a temperature of 20° C or below, water vapor from the air will condense and form liquid water droplets, as it did on the car roof shown below.
The point made above is important enough to restate. The air we live in contains water vapor. The amount of water vapor in the air—in terms of the number of molecules or the mass of water vapor—determines at what temperature the water vapor starts going into the liquid phase. That generally happens at surfaces. The air temperature doesn't have to be below the dew point. It just has to find surfaces with a temperature at or below the dew point.
You may see this regularly, especially this time of year. In addition to morning dew on grass and cars, you see it also on cold drinks—that glass of ice water in the restaurant, for example—or the food you take out of the fridge and set on the counter.
Yes, even if your air conditioner is working well, the dew point for the air in your home is still probably above the temperature in your fridge. If you keep the temperature at 75° F and the relative humidity is 50%, the dew point is 55° F. Your fridge is probably at 35-40° F.
How do you find what the dew point is? One way I monitor it is to check the Weather Channel on my phone. In addition to the temperature and relative humidity, they also tell me the dew point. You can also find a nice map of dew points across the US on their website. Another really nice website I discovered recently (thanks to David Butler!) is the Dew Point Calculator. It allows you to play around with the sliders for temperature, relative humidity, and dew point to see how changes in one variable effects another.
In a future article, I'll put all these variables together and explain psychrometrics, Willis Carrier's term taken from the Greek words for 'cold' (psychro) and 'measure' (metric). That's where it all comes together, and it's no surprise that the 'father of modern air conditioning' has his fingerprints all over it.
Before I leave the topic for today, let me just give a few examples of where dew point and buildings intersect:
- Condensation on windows
- Calculation of the location of condensing surfaces inside building assemblies
- Removal of water vapor in the air by condensing it on the evaporator coil of an air conditioner
So, if it's true that it really is the humidity, and not the heat, don't tell me the relative humidity. Yeah, it's somewhat useful, but if you're going to give me one number, I'd rather know the dew point.