You may never hear about the field of psychrometry, but believe me, you are intimately familiar with its effects.  It’s with you in the shower, it’s the reason you sweat, and if you ignore it at home, it will make you very, very sorry.

Very simply, psychrometry deals with understanding air-vapor mixtures, most commonly air and water vapor.  You can use psychrometric tools to understand why the mirror fogs up after a shower, why a “dry heat” really doesn’t feel as bad, and how to avoid mold and mildew in your house.

Now, if you were an engineer, I’d throw this little gem of a chart at you, and you’d be off to the races:

That chart relates temperature, humidity, dew point, enthalpy, and a host of other air/moisture properties to one another so you can set up an HVAC system to cool and dehumidify a house, for instance.  But that graph is cumbersome and too hard to read to be truly useful to “the rest of us.”  Let’s try this again.

Well, at least that’s a little cleaner. And for our purposes, it has just enough information to make some very critical decisions at home.

You should see two X-axes labeled “Dew Point” and “Temperature” and one Y-axis labeled “Relative Humidity”.  The meaning of “Temperature” (in Fahrenheit) should be pretty obvious, but let’s take a closer look at the other two.

Relative Humidity

The name “relative humidity” gives us a clue that the moisture content of air is relative, but you may be wondering “relative to what?”  You guessed it: temperature.  Warm air can hold more water than cold air can, and relative humidity is just a ratio of how much water is currently in the air compared to how much it could possibly hold at that temperature.

The graph is “live” so you can try this out.  Drag the temperature slider up and down, and watch the humidity change.  Assuming you do not add or remove water from the air, raising the temperature should decrease the relative humidity.  This is because as we warm the air, it can hold more, so its “relative” content goes down.  Cooling the air will have the opposite effect, raising the relative humidity.

You now know why the bathroom mirror fogs up and why your glass of iced tea covers in water.  The air around you contains moisture, and when it contacts a cool surface, the relative humidity moves past 100% and the water condenses.

Dew Point

And now you can also figure out the dew point.  Drag the temperature slider down so that it matches the same temperature you see in the “dew point” box.  What is the relative humidity?

You got it, dew point is the point at which dew forms, or the temperature at which air with a given water density will reach 100% relative humidity, leading to condensation.  Isn’t this easy?

And now for a thought experiment:

Imagine a muggy summer day – the kind where you stick to everything and the dog hides under the shrubs.  The thermometer outside is reading a balmy 90 degrees and the weather man said the humidity is pegged at 85%.  But you’re no fool, you’ve got the AC cranked, and your house is a very comfortable 72 degrees.  You think back to the winter when you put off sealing up those leaky windows and around the light fixtures poking into the attic, but you figure, “Hey, I’ve got six more months before I need to worry about that stuff again.”

Question: Can you figure out why there is mold growing in the walls and ceiling of your hypothetical house?

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