OpenWeatherMap: Dew Point wrong?

[schneich]

Hi,

I just installed the openweathermap integration. I get a lot of values, but I am only interested in 3 values: temperature, humidity and Dew Point.
(I want to compare Dew Point outside against inside to start a fan for ventilating our basement.)
Where as temperature and humidity seem to be correct, the Dew Point seems to be a constant at 2.9°C.

When I call the webserivce, I do get correct values for Dew Point. The error must be somewhere within homeassistant…

How can I debug this? Is this a known issue? The support team of openweathermap was not very helpfull.

Any help is very much appreciated.

Chris

[schneich]

This morning, I did an update from core version 2021.6.1 to core-version.7.1 - et voilà, the dew point is shown correctly.

My issues is solved, unfortunately I don’t know, what the cause was…

[HouseOfCards]

Mine is wrong on 2021.9.2. It says my current dew point is 137 degrees Fahrenheit. Not likely, and also not what the web site reports.

[schneich]

I had the discribed issue at the beginning, but after the update of HA, it is working almost perfectly fine. Do you still have invalid values?

[HouseOfCards]

Yup. Unless the current dew point is 162 degrees. LOL

This has surpassed numerous updates, and is still wrong. Running on a Pi4.

[evilscarp (MIke Scarpellino)]

This has been annoying me too, so I did some poking at the UI and the code. Seems the dewpoint math for °F is broken in the integration. The API call returns all temperatures in °K. If you select Metric units in the General Settings area, the number for dewpoint presented in °C is correct. If you select Imperial units in General Settngs, then the number for dewpoint is double-converted to °F. (20°C → 71°F → 154°F, approximately).

The module that fetches the data from the OWM API uses a different function to convert temperature from Kelvin to Celsius than is uses to convert other temperatures, which is odd but the math checks out. I am still looking through the codebase to see where the double conversion happens, since somethine either drops the units or fails to verify the units before the second conversion.

[ratatine]

So any news on how this went? I’m seeing the same issue as you with obscenely high dew points and before I dig into the component code myself I figured I’d ask!

[vontrapp (Von Fugal)]

I’m still seeing this now.

I created my own dewpoint sensor for indoors, using the formula (degC) - (100-humidity)/5.

To see how “good” that formula was, I also applied it to the outside temperature and humidity. When I went to chart everything I noted that the formula followed the outdoor dewpoint pretty well, with some offset. But then yeah, dewpoint was above temp, which is … odd, for both values. I looked at the values and decided if I was getting the outside temp as F instead of C and doing the above formula, I get the incorrect values I was getting. I don’t know if something similar is happening in the openweathermap service, but it seems possible if the dewpoint is being calculated from the temp, and the temp was taken as C when the value was the F value, it could do the same thing.