It’s early summer in SE Michigan, so the cottonwood trees are launching seemingly endless quantities of fluffy seeds into the air, sometimes to the point of making it look like it’s snowing. I’ve only ever perceived this to be a ground-level phenomenon, with the fluff cloud extending up only as high as the tops of the trees. I recently realized that this is only because I couldn’t see the fluff against a bright, clear blue sky; the stuff stands out against a backdrop of dark green foliage, but it blends in when you’re looking upward toward the sky.
Last week I drove to the grocery store. When I parked, my tinted sunroof was closed, but the shade was open, so I could see the sky. The head of a nearby streetlamp was perfectly eclipsing the sun. I stared at it for a couple of seconds, amused by the coincidence, and then something interesting happened. Gazing just next to the head of the streetlamp, I realized that I could now see cottonwood fuzz hundreds of feet up, as if we were under a bona fide blizzard falling from great height. It was a really amazing phenomenon, and I found myself staring with deep fascination for a few minutes.
So what was happening?
Physicists will tell you that small particles scatter more light in a forward direction (nearly parallel to the incident light ray) than sideways. So if the particle is between you and the light source - such that you’re staring pretty much toward the light source - the particle will be brighter than if the particle is not between you and the light source. So if the sun is overhead and you’re looking horizontally toward some fluff, it won’t be as bright as if the fluff is nearly overhead, close to the sun. However, the problem is that the bright sun makes your iris contract down to a pinhole, making it impossible to see that overhead fluff, despite its actually being brighter. But once you eclipse the sun, your iris opens up, and you can see those overhead fluff particles - quite clearly, and for a very long ways up.
I was able to duplicate this situation yesterday with a piece of cardboard held at arm’s length to block the sun. However, the view, the sense of atmospheric depth, wasn’t as dramatic as last week. It may be that the mild tint of my sunroof helped my iris open up just a little more, letting me see some of the more faint bits of fuzz even higher up.
You might still give this a try, though. The next time you see cottonwood fuzz blowing around your neighborhood on a sunny afternoon, hold up a piece of cardboard at arm’s length, and block out the sun with it. Now look toward it, and give your eyes a few seconds to adjust to the reduced brightness. Look at the area just off of the edge of the cardboard, and see if you can spot the fuzz blowing around at high altitudes. Experiment with the position of the cardboard, moving it around so that it blocks more (or less) of the sky right next to the sun, and see what works best.