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

The warmth of sunlight has no direct correlation with the risk of sunburn. It is possible to experience sunburn on a cool, cloudy day, while the sun may feel intense in the morning without causing immediate damage. Sunburn occurs when DNA absorbs UV-B light, altering its molecular structure and impairing protein synthesis. This damage is painless and takes a significant amount of UV-B energy to cause.

Sunlight feels warm due to visible and infrared light exciting molecules in the skin, particularly water and melanin. This process generates heat, but it does not directly damage molecular structure. Darker skin tones feel warmer in sunlight but burn more slowly. Visible light, near-IR, and mid-IR all contribute to the sensation of warmth, with UV-B being hundreds of times less intense than near-IR at Earth's surface.

Despite the intensity of the sun at solar noon, the risk of sunburn is lower due to the rapid decrease of UV intensity with distance from noon. Clouds, which contain water, scatter UV and absorb IR, reducing both UV index and perceived warmth. However, gaps in clouds can increase UV intensity at ground level. Snow and ice also amplify this effect. Simulated measurements using the SPECTRL2 model show that sunlight's spectrum varies throughout the day, impacting the risk of sunburn.