RainbowChase

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About Rainbows

RainbowChase uses knowledge about the physics of light and data about local weather conditions to help in your chase to bring the beauty of rainbows into your life.

Download the PDF of Dr. Businger’s paper, “The Best Rainbows on Earth.”

Animation 1. The arch of a rainbow is higher when the sun is lower in the sky (early in the morning or late in the afternoon) and lower when the sun is higher. Animation showing how a semi-circular rainbow descends as the sun elevation increases in the morning. Note how the angle between the left and right sides of the rainbow also decreases as the rainbow shifts downward with the rising sun. Click on the image to start the animation.
Animation 2. Animation showing the height of the top of the rainbow above the horizon as the sun makes its daily journey across the sky.  The angle of the sun above the horizon is given and it relates directly to the height of the top of the rainbow.  When the sun rises higher than about 40 degrees above the horizon, the top of the rainbow moves below the horizon and is no longer visible. Click on the image to start the animation.
Figure 1.
Figure 1. a) Path of light rays through a rain droplet. Note the concentration of rays in the close vicinity of and comprising the rainbow-colored Descartes ray. b) A cone of colored light projected onto a wall by a polished quartz crystal sphere held by the author, as an example of what a sunlit raindrop also produces. The refractive index of crystalline quartz for red light is 1.520, for a raindrop it is 1.333. Thus, a spherical raindrop will result in a larger circle.
Figure 2
Figure 2. a) Each raindrop projects a cone of colored light at an angle of 42° from incoming sunlight (e.g., see Fig. 1b). If a viewer’s line of sight aligns with the raindrops cone of colored light, the viewer will see one bright ray of color in that direction, which will be at 42° from the antisolar point. b) The rays colored light that make up the primary bow also form a cone, where the tip is at your eye. Its center axis is parallel to the sun’s rays and directed to the antisolar point or the head of your shadow. Myriad raindrops along the cone’s surface, be they close or far, send rays (one color per drop) of refracted, reflected, and dispersed sunlight to your eyes to create the bow.
Figure 3
Figure 3. The tallest rainbow occurs just after sunrise (rightmost panel), followed by diminishing rainbow height as the sun rises higher in the sky during the morning, until the rainbow sets below the horizon when the sun’s angle exceeds 42° (leftmost panel). The red portion of the curve on the right side of each figure indicates the sun angles that can produce a visible rainbow above the horizon when the observer is standing on flat ground. The numbers inside the yellow suns give the sun’s angle above the horizon. The shading illustrates that the rainbow is not a two-dimensional object in the sky, but rather follows a conical surface.
Figure 4
Figure 4. a) Example of Alexander’s dark band. Note the enhanced light scattering by rain inside the primary bow and increased cloud brightness outside the secondary bow. b) Schematic of scattering from spherical raindrops illustrating the reason for Alexander’s dark band. The black arrows denote the Descartes’ rays for the primary and secondary rainbows.

RainbowChase: bring more rainbows into your life!


To download the app:
For iOS devices, please visit the App Store
For Android devices, please visit Google Play

RainbowChase was conceived and is being developed by Steven Businger* and Paul Cynn
*Professor, Department of Atmospheric Sciences (ATMO) in the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawai‘i at Mānoa (UHM)
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