WEATHER NEWS: Double rainbow brightens D.C., Maryland and Northern Virginia on Sunday
After a storm passed through the D.C. area on Sunday, a double rainbow illuminated the sky for anyone who looked up. Capital Weather Gang received hundreds of reports of the flying colors shortly before sunset from residents in Poolesville, Md., to Fairfax, Va. While rainbows are not a rare phenomenon, this one had a few special elements that caught people’s attention.
One of the most prominently-visible features in last night’s resplendent display was “Alexander’s Dark Band,” or a region of darkness that occupies the sky in between the primary and secondary rainbows.
It has to do with a concept known as minimum deviation, which explains how much a ray of light can be bent/refracted. The properties of water and the source of the light mean that there are certain ray paths that can’t exist due to minimum deviation.
The primary bow, for example, is a 137.5 degree deviation — meaning the light that comes out toward the observer is doing so at a 137.5 degree difference compared to what went into the raindrop. (That’s why the primary arc makes a circle 42 degrees outward from the “antisolar” point, or the point in the sky opposite the sun. 180 degrees, a straight line, minus 137.5 degrees roughly equals 42 degrees). The secondary arc is a 230 degree deviation. Since that’s greater than 180 degrees, all the colors get flipped.
In between those two ray paths, there’s a sort of dead zone. Light that enters raindrops in that part of the sky can’t be bounced back toward the observer, leaving an unlit section in the sky. The result? Alexander’s Dark Band.
Sunday night’s rainbow was also very high in the sky. The lower the sun, the higher the rainbow. That’s because the center of the rainbow (which really traces a full circle, but half of it is generally below the horizon) is anchored on the “antisolar point.” If the sun is high in the sky, the antisolar point around which the rainbow opens is below the horizon, so we only see a fraction of the arc. Around sunset, we can see up to about half of the full circle.
You want a full 360 degree rainbow? Airplanes or mountaintops are your best bet!
Enjoy these beautiful captures on social media from around D.C., Maryland and Virginia.