Pedagogy in Action > Library > Conceptual Models > Conceptual Model Examples > Blue Skies & Sunsets

Blue Skies & Sunsets

Starting Point page organized by R.M. MacKay.
This material was originally created for Starting Point:Introductory Geology
and is replicated here as part of the SERC Pedagogic Service.

Summary

This conceptual model explains why skies are blue and sunsets are red. Related hyperlinks are provided with text and images to make this easy to present and discuss in an introductory Earth science course with atmospheric content.

Learning Goals

Students will learn about
  • Scattering of light in the atmosphere
  • The visible spectrum
  • Why sunsets are red and the sky is blue.

Context for Use

Appropriate for use in Climate Change, Earth System, or Meteorology courses to introduce the electromagnetic spectrum and the scattering of electromagnetic waves by the atmosphere in a context familiar to the everyday experience of students.

Description and Teaching Materials

Included are:
  • Figure for blue sky and red sunset geometrical orientation. (click to enlarge)

    The figure on the left shows that the perceived color of light reaching your eyes when in this position relative to the sun's rays is blue. The reason for this is that the atmospheric scattering of blue light away from the forward direction is much stronger than for red light. The figure on the right shows that the perceived color of light reaching your eyes when looking at the sun near sunset is red for essentially the same reason; much less of the red light is scattered out of the direct beam compared to the larger scattering for blue. The sunlight has the deepest color of red right at sunset when the rays travel through the greatest amount of air before reaching your eyes. Red sunsets are also enhanced by particulate matter in the atmosphere.

  • Good hyperlink reference on sky color (more info)

Teaching Notes and Tips

After discussing this model for the scattering of visible light by the atmosphere, other parts of the electromagnetic spectrum can be introduced (UV and IR). This conceptual model can also help begin a more general discussion of light scattering by aerosols and cloud dropplets.

Assessment

Follow-up questions on exams, homework, or in class discussion can be used to assess student understanding.

References and Resources

The link above, Good hyperlink reference on sky color (more info) , also contains information on Mie scattering, Rayleigh scattering, and other related topics.