@franzstein I agree with you there is a discrepancy in the quality of data we have now versus with WUnderground. We just upgraded our OWM account to provide better forecast data and we are planning on implementing a caching system so we can lower the number of redundant calls to the API (both the app and the device are querying the same location for the same data now). Once we have this caching system in place, we will add historical API to our Weather API and allow it to properly get the data needed when it’s missing or use the cached data when appropriate.
This isn’t a small undertaking but the problem before was the large array of weather APIs we interfaced with caused me not to truly focus on any single one. I am hoping my choosing just OWM, we can continue to refine and improve this service for everyone. I agree with you the current state is not very usable for Zimmerman calculation and we are hoping to improve that.
For the record, @Peter added a PR to the Weather API we provide which allows you to query a local weather station directly and allow OpenSprinkler to directly use this data. There is more information on this post here: https://opensprinkler.com/forums/topic/cutting-the-weather-underground-cord-homebrew-solution/. I am reviewing this and hoping to include it in our master branch moving forward to assist people with weather stations that would allow local polling.