The UV index is not the warm sensation of sunlight on bare skin

This explanation separates the sensations of sunlight warmth from the biological process that causes sunburn. Sunburn arises from DNA damage produced mainly by UV‑B (and to a lesser extent UV‑A) photons altering molecular structure, while the skin’s warm feeling comes from visible and infrared light exciting vibrational modes in water and melanin. UV‑B at ground level is hundreds of times less intense than visible/near‑IR, so the energy that heats skin is orders of magnitude larger than the energy that damages DNA. That difference explains why darker skin can feel hotter yet resist sunburn longer: heating and DNA damage are distinct mechanisms on different energy scales.

Time of day, clouds, and surface reflections change UV and IR differently. UV‑B intensity peaks sharply at solar noon and falls off faster than IR because of atmospheric scattering, so mornings or late afternoons can feel warm while posing lower burn risk. Uniform cloud cover reduces both warmth and UV, but cloud gaps, snow, or ice can locally amplify UV. The spectral plots and examples come from pvlib’s SPECTRL2 solar‑spectrum simulations for a clear sky over Toronto on a specific date; caveats include that pvlib targets solar panels, assumes a horizontal surface and clear conditions, and SPECTRL2 omits wavelengths below 300 nm.