Quantifying Cumulative Ultraviolet Radiation Dose in Desert Microclimates: A Gaussian Model and Peak-Weighted Shade Factor Classification System for the Las Vegas Valley

Current horticultural guidance for the Las Vegas Valley relies on single-point, peak-intensity ultraviolet (UV) radiation measurements to characterize plant exposure environments. This approach fails to account for the cumulative nature of UV-induced tissue damage, which is a function of both radiation intensity and duration of exposure—the total dose. This paper presents three contributions to desert microclimate science: (1) a mathematical model demonstrating that clear-sky diurnal UV irradiance at Las Vegas latitude (36.17°N) follows a Gaussian distribution characterized by amplitude A, center μ (solar noon), and width σ (seasonally variable from 1.81 to 2.75 hours); (2) a three-point field measurement protocol that characterizes the complete daily UV dose curve from just three strategically timed readings; and (3) the Peak-Weighted Shade Factor (PWSF), a novel quantitative metric that classifies desert microclimates (MC-0 through MC-5) based on the fraction of cumulative daily UV dose blocked by shading features, weighted by the timing of shade relative to peak irradiance. The PWSF enables, for the first time, a rigorous mapping between microclimate shading characteristics and plant species viability that accounts for the dose-response relationship between UV exposure and plant tissue damage. Results demonstrate that a two-hour midday shade window centered on solar noon reduces cumulative UV dose by 24%, while the same two hours of shade in early morning provides only a 10–12% reduction—a difference invisible to single-point measurement approaches.