Research brief
First and last light: a warmer sunlight spectrum
Why the low sun can appear warmer: a longer journey through the atmosphere reduces short-wavelength light more strongly before it reaches us.
A longer atmospheric path
At first and last light, sunlight reaches us at a shallow angle and travels through more atmosphere than it does when the sun is high. Shorter visible wavelengths are scattered more strongly along that path, shifting the light we see toward warmer reds and oranges. Zagury and Fujii examine this red scattered sunlight at sunrise through atmospheric optics.
Less short-wave light, relatively more long-wave light
The near-horizon effect changes the balance of daylight rather than turning sunlight into a single wavelength. Relative to direct midday sunlight, less short-wavelength visible light and less UV reaches the ground, while longer visible wavelengths remain a more prominent part of the scene. The exact spectrum varies with location, season, ozone, weather, aerosols and viewing direction.
Lower UV does not mean no UV
Early morning and late afternoon usually have lower UV because sunlight takes a longer path through the atmosphere. However, ultraviolet exposure is not zero, and UVA is less filtered by the atmosphere than UVB. Use the local UV Index and sun-protection guidance when needed, particularly at high altitude, near reflective surfaces or when UV levels are 3 or above.
What this does and does not show
This is a story about the changing spectrum of natural sunlight, not proof that a particular time of day is risk-free or that red and near-infrared light delivers every health benefit. A red-light device also does not reproduce sunlight. Product-specific results depend on wavelength, irradiance, dose, treatment area and the study outcome.
Sources
- Zagury F, Fujii M. Spectral analysis of red scattered sunlight at sunrise. New Astronomy. 2003;8(6):549-556. DOI: 10.1016/S1384-1076(03)00048-4.
- World Health Organization. Radiation: Ultraviolet (UV) radiation. Notes that early morning and late afternoon sunlight travels at a greater angle through the atmosphere, so more UV is absorbed and less reaches Earth; UVA is less strongly filtered than UVB.