2021-05-23 rant

So, here are two facts about me: I love collecting metrics about my life and my environment, and I have been working as part of distributed teams for many, many years now. As such, I have learned that the air quality in my office room not only affects some numbers, but it actually has a significant impact on how I work and how I feel. So naturally, I have distributed tiny climate sensors all over my apartment (and outside) to get a better insight into how the air … looks like.

While I have used and am still using some self-built sensors, my main source of data these days are the small Apple HomeKit-compatible Eve Room, which not only measure and collect Temperature readings, but they also keep track of VOC¹ and relative humidity. The last one, relative humidity, always struck me as very odd. I have been told it’s a good idea to always keep the relative humidity between 40% to 60% for a good living climate and to lower the chance of mold buildup on or inside walls. But why is it called “relative humidity”, and what’s the other side of the relation?

Air is weird. We all know that air can store a bit of water, also known as humidity, inside it without it raining. However, how much water the air can hold depends on its temperature: warm air can hold much more water than cold air can². That’s part of the reason why you frequently feel hot and sticky during summer and why the air is super dry in cold winters. The temperature component is pretty significant: if we calculate the absolute humidity in g/m³ water in the air for two realistic temperatures…

• 10 °C at 50% RH ≈ 4.7 g/m³ water
• 20 °C at 50% RH ≈ 8.6 g/m³ water

… we’ll see that the increase from 10 to 20 degrees Celsius can almost double the amount of water stored at 50% relative humidity. This fact makes the relative humidity numbers problematic in a few instances, so let’s talk about…

When Relative Humidity breaks down

Let’s imagine you are me and have been working 8 hours in your office room. You notice that the relative humidity in your office is a bit too high, but unfortunately, it’s a horrible day outside, and it has been raining all day long. Here are the numbers you can measure:

• Inside: 20.1 °C, 58% relative humidity
• Outside: 10.3 °C, 81% relative humidity

So while the humidity in the office is almost too high, you are out of luck because it’s just super humid outside. I mean, it’s raining, so there’s no point in even trying to open the window to make your inside air dryer. Right?

This is where you have fallen into the evil traps the Relative Humidity people set up for you to fall in. That assumption is wrong. If we calculate the water content in the air in those cases, we’ll see that the inside air contains roughly 10 g/m³ of water, while the outside air only contains 7.8 g/m³! If we took the outside air and magically heat it up instantly to our inside temperature of 20.1 °C, the relative humidity would show only a low value of 45%. So the real answer to the question is: heck yeah, you should absolutely open the window and get some nice, fresh, and dry air in. Yes, even though it’s raining³.

So far, I have been talking a lot about the absolute water content in the air in grams of water per cubic meter of air. That’s a good enough metric, but what if I told you there’s an even cooler number we could look at together?

Let me introduce you: The Dew Point

The dew point is a temperature reading, and it describes the temperature at which the current air will be 100% saturated with water. This means that if the air temperature is higher than its dew point, it can take a bit more water, but if the air temperature falls below the dew point, some water will fall out in the form of condensation. If the dew point is below zero and the temperature drops below that, the water won’t really condense, but it’ll just start freezing to surfaces - we call that frost, and it’s very gorgeous.

If the humidity rises, so does the dew point. And luckily, the dew point is completely independent of the actual air temperature, so it’s a fairly absolute measurement, which means it’s a great number to compare. It takes all the “guessing” out of comparing temperatures and relative humidities. To make a good point, here is the example from earlier, but this time with dew points added:

• Inside: 20.1 °C, 58% RH, dew point: 11.6 °C
• Outside: 10.3 °C, 81% relative humidity, dew point: 7.8 °C

That’s much clearer: the dew point outside is lower, so letting the air in makes sense because it contains less water. And sure enough, since I started writing this post, the relative humidity is now down to 48%, and the inside dew point is now at 8.4 °C. Good enough for me!

In addition to the ease of telling “which volume of air is dryer”, the dew point has a few additional perks to it. You know that ugly feeling you get on humid summer days when everything is hot and sticky? Air generally feels that way once it’s above a dew point of somewhere around 16 degrees. So if the outside dew point is higher than that, you know it’s going to be ugly.

Also, if you live in a part of the world where winters are cold, I am sure you will have experienced the condensation on windows you frequently get overnight. This happens if the glass gets colder than the inside air’s dew point, so the humidity in the air starts condensing on your windows. While you can’t really predict what temperatures your windows will have, you can take note of the cases where you do see condensation and can act against that: either by agressively ventillating before you go to bed, or by drying the air with other tools like air dehumidifiers. What’s even worse is when your walls fall below the dew point. In those cases, humidity can settle into the walls or the wallpapers, which causes mold buildup. Again, while you can’t really predict how cold your walls will be, you can maybe use a thermal camera to look for cold spots in winter, so you know when and where to be careful.

If you are still not convinced about how great of a number the dew point is, I don’t know what to say!

Please, just show us the dew points

If you’re manufacturing thermometers or climate-related IoT thingies, then please, just show us the dew points. I understand that you might not want to do that on an LCD with limited space, but if your product has a dot-matrix display, then why not add the option? And if you’re a company like Eve, building IoT sensors, why not add a dew point approximation to your applications? If you’re working at Apple as part of the HomeKit team, then please consider adding the dew point to the list of supported characteristic types⁴.

If you’re building your own sensors and your own measurement dashboards, calculating a good approximation for the dew point without knowing the air pressure is relatively easy. I hope to have convinced you that the dew point can be useful!

Anyway, that’s enough for now. Thanks for your attention.