Sun Day Tracker

When I am outside in August for 2 hours beginning at sunrise, this early-morning window produces little to no meaningful vitamin D in the US zone 8 (34°N). Yet, this tracker has me at 2000 IU. Vitamin D synthesis requires UVB (roughly 290–315 nm). These short wavelengths are strongly absorbed and scattered when the sun is low because the path through the atmosphere (and ozone) is much longer. Practical thresholds for useful vitamin D production are generally solar elevations above ~35–45° (when your shadow is shorter than your height is a common rough guide, corresponding to ~45°). Below that, UVB is minimal or effectively absent for cutaneous vitamin D production even under clear skies.

Need Apple Watch version tho