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Horizontally Moving Rainbow

Description:

A single and sometimes double rainbow can be seen moving from left to right as time progresses. This timelapse is from the Iowa State University Agronomy Farm located just west of Ames, Iowa.

Video:

YouTube Movie

Questions:

  1. Are rainbows a result of reflection or refraction or both reflection and refraction of sunlight?
  2. Why does the rainbow appear to move from left to right?

Discussion:

Rainbows are commonly seen in Iowa after the passage of a rain shower or thunderstorm. We often see afternoon showers that pass from west to east allowing them to interact with the setting sun in the western sky. The result is a colorful display in the eastern sky. While it is possible to see a rainbow during the morning hours, the combination of the sun's position and clouds rarely produce a scenario for a morning rainbow in Iowa.

The video shows a little known phenomena of how a rainbow is actually moving from the viewer's perspective. The movie speeds up time enough for us to visually notice it is moving. The reason it is moving is due to the change in the sun's azimuth as the sun sets in the western sky. If you were to trace a path of the sun in the sky on a given day, it would not make a vertically straight line into the horizon. Think about how a sun dial works with its shadow moving to indicate the passage of time. The same concept applies to seeing the rainbow move.

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