How to automate messages to open windows when outdoor humidity < inside

I have a calculation that converts the outside humidity to inside humidity which takes into account the temperature inside when doing the calculation (values taken from sensors)

That gives me a % relative humidity, how can I use this information with an automation to prompt me to open the windows in a room with relatively high humidity?

The other factor is if it is cold outside, I would like a prompt to tell me to close the window if the temperature is dropping too quickly.

Is this possible with automation?

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Yes this is doable.
You might want to consider calculating the Dewpoint for inside and out. Just converting the humidty from outside to inside (if I understand correctly) will not work, since that won’t account for your shower, breathing or anything else.
The way I understand is the you have sensors for:
Humidity Outside
Temperature Outside
Temperature Inside

Most humidity sensor give you relative humidity, I have never seen one that gives absolute humidity (which you can calculate but that is a lot of if statements since it is a different equation depending on the temperature range).

One thing you should also utilize for determining to open your windows is air quality. I have a purpleair myself that gives me outdoor air quality (I don’t have anything inside yet). You can pull from any purpleair sensor that may be near you.
https://www2.purpleair.com/
Just click view that map and find one near you.
There is a good thread here on how to import that info.

Ok, once I have the data, how would I form that into an automation? Would I use trend in some way so that the windows arent left open too long and allow indoor temperature to drop too far?

First you need to get a general idea of what you want to do and when you want to do it.
From my understanding you want a notification to open the windows when dewpoint outside is less than dewpoint inside (dewpoint will take humidity and temperature into consideration; the higher the dewpoint the higher the volume of water in the air). You probably don’t want to get prompted to open the windows when you have the heat or AC on; and (even if you don’t have HVAC) then you want a temperature range to open/close the windows. You don’t want to be prompted to open/close the windows if they are already in that state.

I would set up a trend for outside temperature and play with it.
Do you have door/window sensors?
What are you using to determine inside & outside temperature and humidity?
Do you have an HVAC system that you can get status of? (could be as simple as energy monitor)
Do you plan on opening windows and turning on an exhaust fan?
Do you have the Home Assistant app installed on all the devices you want the notification to go?
Do you have any smart speakers or media servers that can get Text-to-Speech? (I have a couple of Pis with USB speakers running KODI headless and use them for TTS notifications.
Some of my Outside sensors:


Some others:

Give me an idea of what sensors you have so we can work on your notifications.