Humidity on PurpleAir Flex

In the data of my new station I notice that the relative humidity is regularly displayed too low. Currently it is 56% for me, 83% for the German DWD. What is the reason for this?
Kind regards

Hi @jheiler

The humidity reported from a PurpleAir sensor is the internal humidity in the housing. Due to the heat generated by the WiFi module and other components of the sensor, the internal air in the housing is typically slighter warmer and drier than ambient air. Thus, the humidity data is not an accurate measure of ambient conditions. Rather, it is meant primarily for certain conversions and other formulas that need to know the operating conditions for the sensor.

That said, in our next iteration of the map (coming soon), we’ll be including a new data layer that will utilize an equation that will alter the humidity reading in an attempt to estimate the ambient humidity. Based on the testing that we’ve done, I’ll point out that this will still be less accurate than a true ambient humidity monitor, but it should more closely match your ambient conditions than what you currently see.


I forgot to mention, that I use the Flex outside. So the station should measure humidity outside.

Sorry, I probably could have been clearer in my wording. By “internal humidity in the housing,” I mean the air inside the housing of the PurpleAir sensor. The air inside the housing (the cap) is warmer and drier than the air outside the sensor housing.

Hope that helps!


If it’s not going to be accurate like a TRUE weather monitor that does humidity, I just won’t look at THOSE numbers on the map. Already, I don’t trust the temperature readings on PurpleAir; they were reading WAY too hot in the summer. I only use the map for real-time air pollution, mainly. I am glad you will be updating the map!!

Perhaps PurpleAir sensors could be modified so they could only sense temperature and humidity OUTSIDE the housing. I can see why for example, the summer temperatures were reading WAY too high, because of the air in the housing when the unit is in direct sunlight.