The growth potential converts air temperature to a 0 to 1 (or 0 to 100%) scale. Of course, there are a lot more things that affect actual growth, including light, nitrogen, and water. I discuss these and why I'm not so convinced that adding a light factor to GP makes it more useful. The blog post discussed here is https://www.asianturfgrass.com/post/light-and-gp-again/ This calculator finds full sun PPFD and DLI for any location on any date: https://asianturfgrass.shinyapps.io/ppfd_by_time/ Park Nobel's environmental productivity index is described in this article: https://esajournals.onlinelibrary.wiley.com/doi/abs/10.2307/1938497 It's also mentioned in his text book Physiochemical and Environmental Plant Physiology: https://www.sciencedirect.com/book/9780128191460/physicochemical-and-environmental-plant-physiology Travis Shaddox's Turfgrass Epistemology channel: https://www.youtube.com/@TurfgrassEpistemology Read more about all kinds of turfgrass topics at https://www.asianturfgrass.com/Get ATC newsletters at https://www.asianturfgrass.com/newsletter/Turfgrass information and decision-making tools at https://www.paceturf.org/PACE Turf YouTube channel at https://www.youtube.com/user/paceturfATC's YouTube channel at https://www.youtube.com/asianturfgrasscenter