Hi,
Until now I’d been using the new app but I’m currently migrating to the classic app due to problems with the new one.
In the new app I can set an automation to run when a temperature is above or under a value in a sensor, in the classic app I cannot find a way to set a routine to perform automatically using the temperature as a trigger.
Is there a way to do this in the classic app?
Temp is not a valid trigger for a routine, which is why it is not an option. You’ll want to look at core or webcore for anything more than very basic automations in the classic app.
The classic app is much more powerful, but also less intuitive. So you can definitely do it, it can just take a little research to figure out how.
The following might be of help (the topic title is a clickable link). This includes discussion of Webcore. 
There is also a “virtual thermostat” smartapp in the marketplace of the classic app that might do what you need. ( don’t be scared off by the name, everything in the “marketplace” is free as of this writing. ) it just depends on the details, which is true of pretty much everything in the classic app.
Thank you for your answer, I’ve been reading the forum and I’m aware of WebCore and SharpTools, they do seem very powerful and flexible but until now I hadn’t had the need to use them, as I have been able to accomplish everything I needed with the built-in options. So I’m a bit surprised I’ll need to start using them for a very simple scenario (I only need to receive a push notification when a certain temperature is reached on a room).
But since it’s like it is, what would you recommend for my use case? SharpTools, WebCore or any other?
In your specific case, for a simple approach check the custom notification options under smart home monitor. Note that this is not the same smart home monitor that you use in the new app: they just happen to have the same name. 
Anyway, I think there’s a temperature and humidity trigger there. But check to see for sure.
I would also add that the SharpTools.io Rule Engine is easy to get started with and a nice visual rule builder. Just sign-in, authorize your devices, and you are ready to start building rules. We put a big focus on making sure it was easy to get started with a minimal learning curve!
Cool, I checked the smart home monitor and under custom I was able to create the notification for temperature reached, super useful hint, thank you!
I just solved my need with @JDRoberts hint, but I’ll definitely be having a look at SharpTools and WebCore now that I’ve moved to the classic app and things seem more “interesting” 
