The details of the weather algorithm are documented in the support article here:
https://openthings.freshdesk.com/support/solutions/articles/5000823370-using-weather-adjustments
the reason it’s not included in the firmware is that the weather adjustment methods are decoupled from the firmware — it’s hosted on the OpenSprinkler cloud server and new weather algorithms may be added at any time without modifying the firmware.
You can customize the weather algorithm by hosting it on your own server and change the weather url to point to your own server. The weather code and instructions can be downloaded here:
https://github.com/OpenSprinkler/OpenSprinkler-Weather