The characteristic blue-black color is seen when all other wavelengths are filtered out. UVA rays, however, are not visible when they occur naturally as part of the solar radiation spectrum. Like visible light, UVA is poorly absorbed by the ozone layer and therefore reaches the earth's surface in abundance. It is important in cutaneous vitamin D metabolism, especially if dietary vitamin D intake is poor. A source of UVC that does reach our skin is from halogen lightbulbs.
Vasodilation causes mild, transient erythema as the skin attempts to cool down.
Infrared radiation is absorbed by the water droplets in clouds, so on a cloudy day, its heating effect is attenuated. UVA and UVB, however, are not well absorbed by clouds and though the skin will not feel as hot on a cloudy day, it is still susceptible to the effects of ultraviolet light. Cutaneous Effects of Ultraviolet Light Sunburn. In contrast, the longer-wavelength UVA penetrates the skin more deeply and is absorbed by melanocytes, elastin, and collagen. UVA contributes relatively little to sunburn erythema.