Relative humidity is what everyone likes to talk about. It gets the attention, but it can be a bit confusing, especially when the temperatures drop. For example, at one point yesterday, we had a relative humidity (RH) of 97%. Seems humid, eh?
It's not really. Not in terms of how much water vapor is actually in the air, that is. The psychrometric chart below shows how this works.
The two points I've highlighted on the chart are:
- Point A: 32° F, 100% RH
- Point B: 70° F, 20% RH
They're connected by the arrow, indicating that when that cold, seemingly humid, outdoor air leaks into a home by infiltration, it warms up. Let's assume that the mass of air leaking in doesn't gain or lose any water vapor molecules along the way.
The psychrometric chart rocks!
On the psychrometric chart, an unchanging number of water molecules means the movement is purely horizontal. The temperature changes but the absolute humidity does not. But as that mass of air warms up, the relative humidity does change. In simple terms, warmer air can hold more moisture, so rather than being saturated, as it was outdoors at 100% RH, that same air can hold a lot more moisture when it moves inside and warms up.
The three main variables on the psychrometric chart are:
- Temperature - Dry bulb, what we normally mean when we say the word; it's shown along the horizontal axis.
- Absolute Humidity - How much actual water vapor is in the air; measured in grains per pound of dry air, where 1 grain = 1/7000 of a pound (or 7000 grains = 1 pound).
- Relative Humidity - What we usually mean we say the word 'humidity.' It's actually the partial pressure of water vapor in the air divided by the vapor pressure when the air is saturated at that temperature.
As you can see from the psych chart above, there are only about 22 grains/lb of water vapor when the temperature is 32° F and the RH is 100%. That temperature has a special name: dew point. It turns out that 22 grains/lb (often referred to as simply 22 grains) isn't a lot of water vapor.
On a nice spring day when the temperature has risen to 70° F and the RH is 50%, the absolute humidity is about 50 grains/lb. Can you find that on the chart above? Can you find what the dew point is for that condition? (Answer below)
One of the worst kind of days we have here in the Atlanta area is when it's about 80° F an 80% RH. Ugghhhhh. The absolutely humidity is about 122 grains/lb, and the dew point is 73° F. And that's not even the worst I've experienced in the Southeast. Atlanta has nice, fairly mild summers compared to where I was born: Houston, Texas.
Back to the main point, though, if we think of air by its absolute humidity, it's easy to see that cold air is dry air. 100% RH at the freezing point has only 22 grains per pound. 80% RH at 80° F has 122 grains per pound. If you had 80° F air with only 22 grains/lb, the relative humidity would be less than 15%. You'd be in a desert.
Psychrometrics on your smart phone
If you have a smart phone, you can find apps that do psychrometric calculations for you. I have two on my iPhone. One is the Ultra-Aire Psychrometric Calculator made by Thermastor. They make great dehumidifiers, and their app can help you find how much water removal you'll get based on the input and output conditions of the air. You can also do basic psychrometric calculations with relative humidity, absolute humidity, dry bulb temperature, and dew point. I also have one called Psychrometrics Lite, which does the basic calculations and allows you to adjust for altitude. Both of these are free. You can get more advanced apps that cost a few bucks.
Answer to Question: At 70° F and 50% RH, the dew point is ~50° F. You can see that by finding the point for 70° F and 50% RH and then going horizontally to the left until you hit the 100% RH curve.