Joe Jordan led us on a glorious tangent filled romp through the sky — chasing rainbows and other phenomena with photos, chalkboard drawings, and props.
Joe, former physicist NASA, current director of the Sky Power Institute (http://www.sky-power.org) talked to us at the Randall on November 20th, 2014 about all manner of sky phenomena and the physics behind them: the complexities of rainbows, sun dogs, sun pillars, moonbows, haloes, glories, contrails, green flashes, castles in the air, the directionality of meteor showers, and more!
A lot of what we see looking up, or what we see downwards looking out of a plane is the result of ice crystals in the air, their orientation to the sun, and the observers particular perspective.
Sun pillars for instance — where a column appears to rise from the setting sun — is sunlight reflecting off the undersides of ice crystals. It is the same effect as the sun reflecting off the waves toward you, just in the sky. Other effects depend on the angle that light is hitting the crystal, and where that crystal is in relation to the light and you.
A more complicated phenomena, but one of my favorites, are glories. Sitting on the shadow side of the aircraft — as you are coming into land and passing over clouds, you might see a glory in the clouds, a circular rainbow. At the center of that you might be able to make out the shadow of the aircraft.
(If you are a SCIAM subscriber you can read more at http://www.scientificamerican.com/article/the-science-of-the-glory/)
Joe talked a fair amount about the appearance of sun rays fanning out. Sun beams are essentially parallel given the distance the travel from the sun. The fanning out is an optical illusion akin to how train tracks look when we look down them.
We spent the longest time with the intricacies of rainbows, the bending of light through raindrops and how that gets to our eyes. Multiple bounces leads to double and triple and more rainbows (in the lab, this has been done up to the 15th order, but in nature you’d be lucky to see a 5th order rainbow).
My favorite factoid of the night is that although what we see a rainbow as a static thing up in the sky, what that rainbow actually is is an animated mosaic projected onto our eyes — millions of rain drops bending light in our direction, changing colors until they fall “out of the picture.” followed and replaced by the raindrops above.
SF Natural History Series 