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The Mississippi Triangle
The Planche family saw this strange red triangle while driving eastwards on Interstate 10. Dusk was approaching on May 27, 2015. These iPhone images are by Andrew Planche and span 15 seconds.
All images ©Andrew Planche, shown with permission
The triangle enhanced.

Side A has a strong red edge with weak greens on its right. It looks curved and extends weakly below and above the triangle.

The sky is brighter red on its inside (right) as expected from a sunset rainbow.

It is in the correct position relative to the sun for a rainbow.

Look how side C extends downwards and has a weaker red line above it.
Sunset rainbows can be highly reddened. They light up the sky red inside the bow. This is key to what follows.

Image by Rafael Schmall

Rainbow spokes are always along radii. I.e. at right angles to the curve of the bow and passing through the bow centre.

This image by Larry Phillips in Saskatchewan.


Four separate and quite natural effects:
        A reddened sunset sun
        A rainbow fragment
        An illuminated horizontal cloud bank
        A rain sheet shining with rainbow rays
              came coincidentally together to produce an eerie triangle in the sky.

Obvious in retrospect?     Then so is most magic once explained!
The sighting was both fascinating and enigmatic. Three of us collaborated to get an explanation – Les Cowley (OPOD), Raymond Lee (US Naval Academy & co-author of Rainbow Bridge) and Gavin Pretor-Pinney (Author & founder of The Cloud Appreciation Society).

Thanks to Viktor Golubic and Chris Isbert for alerting one of us and forwarding the images.
The sighting was both fascinating and enigmatic. Three of us collaborated to get an explanation – Les Cowley (OPOD), Raymond Lee (US Naval Academy & co-author of Rainbow Bridge) and Gavin Pretor-Pinney (Author & founder of The Cloud Appreciation Society).

READ NO FURTHER! - If you want to solve the puzzle yourself.

The Sun
The first act in an investigation of a sky sighting is to calculate the sunís position. Always keep an accurate camera clock and know its time zone! Keep original camera images and never erase the EXIF data.

From the image times and the exact vehicle position from GPS (plus comparing an image with Google Earth) we found that the sun was low on the horizon, only 0.2į high in the WNW. The red sunset glow is visible in the car side mirror. The sunset rays illuminate the highway signs with red light.

A Rainbow?
The triangle left-hand side 'looks' like a rainbow fragment and It is correctly placed relative to the sun. One enhancement at right shows its deeper red outer edge. It also extends weakly beneath and above the triangle.
 

Side B
The topmost images show a red line parallel to the horizon extending well past the triangle to the left. This is very likely a cloud edge lit by the low sun. It brightens inside the rainbow as explained above.

Side C - The problem one!

Could it be a rainbows spoke or antisolar ray? No. Its angle to the rainbow's rim is wrong. A spoke or ray would lie along a radius of the rainbow arc.

What about reflections from water? No. Reflection bows do not form at that angle when the sun is almost on the horizon.

Could it be a streak of falling rain (virga or, alternatively, rain reaching the ground) strongly illuminated because it was just inside the rainbow? Strong evidence for this arrived when the image at right became available. Several slanting rain sheets, lit red by the low sun, are along the horizon. The upper part of this image was enhanced by colour subtraction to bring forth the red areas. Slanting rain is numbered 1 to 5.

But.. ..Was there rain? Was the wind in the right direction? Would the rainsheets be tilted at this angle?

We found the ground wind speed and direction from a local weather station and also obtained weather balloon measurements taken at different altitudes close to the time of the photos. There was rain and the wind was in the right direction.

Rain sheets tilt because the wind is slowed close to the ground and so falling rain drops get dragged back against the wind direction as they get lower.

We calculate the angle of the sheets as 40° after taking a raindrop diameter of 0.5mm estimated from the appearance of the rainbow fragment. This agrees reasonably with the tilt of ‘C’ and the other streaks.

The sheet making side C is so bright because the raindrops there are close inside the rainbow edge and reflect back strongly the sun's red light. The weaker line above side C is another rain streak.