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

Sunburn and the warmth of sunlight are caused by completely different mechanisms and operate at vastly different energy scales, making warmth an unreliable indicator of sunburn risk. Sunburn results from UV-B radiation damaging DNA, while the sun's warmth comes from visible and infrared light exciting molecules in the skin—with UV-B being hundreds of times less intense than near-IR at Earth's surface. Additionally, UV intensity drops off more rapidly than infrared as the sun moves away from noon, so the sun can feel hot in the morning while posing minimal sunburn risk.