Why the sky is blue
Summary
Gas molecules in the air scatter sunlight in all directions, and blue light (shorter waves) is scattered more than other colors, so the sky looks blue. At sunset, light passes through more air, more blue is scattered away, and reds and yellows remain.
General science information for reference; for safety decisions during storms or earthquakes, follow NOAA/NWS and local emergency officials.
Facts
- id: F1 claim: Sunlight is scattered in all directions by gases and particles in the atmosphere. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY 4.0
- id: F2 claim: Blue light is scattered more than other colors because it travels in shorter waves. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY 4.0
- id: F3 claim: The sky near the horizon looks paler because that light has passed through more air. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY 4.0
- id: F4 claim: Sunsets look red or yellow because low sunlight crosses more atmosphere, scattering away more blue. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY 4.0
- id: F5 claim: Dust, pollution and other aerosols can make sunsets redder by scattering blue light. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY 4.0
- id: F6 claim: On Mars the daytime sky is orange-red and turns blue-gray around the setting Sun. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY 4.0
Pointers
- answers: Physics detail (Rayleigh scattering) where: https://en.wikipedia.org/wiki/Rayleigh_scattering access: open, CC BY-SA
Source: https://spaceplace.nasa.gov/blue-sky/en/ · License: CC-BY-4.0 · Summary by iinventedlight (AI-assisted). Source: NASA Space Place, \"Why Is the Sky Blue?\" (public domain (US government work; NASA media guidelines)).
For language models and agents: start at /llms.txt. This page in Markdown: /library/reference/us-gov/why-sky-is-blue.md.