Why do rainbows always arc up? Why don’t they sometimes arc down, or maybe sideways? Tommeason
The short answer is that the arc of a rainbow has to be centered on a point directly opposite the Sun from the viewer. That is, there’s a line between the Sun, your head, and the apparent center of the rainbow.
Since the Sun is only visible when it’s above the horizon, the center of the rainbow must usually be below the horizon. And so the part you see arcs “up”.
Now, there are cases where you can see almost the whole circle, or even the complete circle. If you’re at a high enough altitude, and the sun is low enough in the sky, the horizon can be far enough below you to let you see the whole rainbow, which is a closed circle (if there is rain at all those points).
It helps if you’re in an airplane.
I once saw a rainbow that arced about 190 degrees, dipping down into Yellowstone Canyon (it was right at sunset). And once, I even saw a 270 degree rainbow from my 9th-floor apartment. In both cases, the portion of the rainbow in front of the backdrop (canyon wall or parking lot) was very faint, but you could make it out if you looked closely.
You can have rainbows that don’t center on the antisolar point (the point – generally underground – that lies ion a direct line from the sum through the viewers eye to the “dome of the sky” (even if it’s underground). Reflected rainbows seen in a body of water, or from a mirrored building will be centered elsewhere. So will the other kind of reflected rainbow that is due to reflection of the sun itself from water or mirrors. None of these will arc “up” though. As others have noted, you can see an upward arc if you are high enough so that you can see below the horizon. This requires you to be in an airplane (or balloon), or to be atop a mountain, or with a gorge below you. The last of these is easily attainable – complete with the requisite water droplets – if you are at a large waterfall at the right time of day.
There are several types of ice crystal haloes which have color separation and arc upwards. Look up the circumzenith arc or the upper tangential arc, for instance.
Larry Niven (A Gift From Earth) describes a world where, perched on the edge of a massive cliff at the edge of the plateau, with the sun at your back, you see a “halo” around the shadow of your head on the clouds.
As Aerodave says, you are seeing the arc of the circle - let’s say the base of a cone between you and the rain droplets, where the center of the cone is the line from the raindrops to your head to the sun. The “spread” of the cone is determined by the size and shape of the rain drops, since really what is happening is this - the light comes from the sun, into the raindrop, bounces off the inside of the raindrop and back to you. If you can position yourself to see more than just a flat hoizon - a mountaintop or an aeroplane - you will see more of the cone.
Since raindrops are… raindrop shaped … the triangular/prism shape means that they break up the reflected light. So the red takes one path to you at one angle, the yellow a slightly more angular path, the blue the biggest angle/path.
Multiple raindrops mean different (layers?) of rainshowers or fogs with different sized raindrops, producing different cones.
Hence the joke about looking for the end of the rainbow, to belabour the obvious. It’s an optical illusion, as you move so does the view. It’s like chasing the water mirage in the desert.
You get that on Earth, too. It’s called a Brokenspekter, or a glory. It’s not the same effect as a rainbow, though.
Spheres aren’t triangular.
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The funny thing is that raindrops are not raindrop shaped. As they are falling, they are actually pretty close to spherical. The rest of the description still works though.
Yep. Sorry, I realized afterwards too late for edit, doh, if it was the triangle effect the result would not be a radially symetrical rainbow. The light spectrum spreads because of the prism effect of the sperical raindrops.