While flying in a helicopter today, I noticed that the ground around the helo’s shadow was brighter than the rest of the (non-shadowed) ground. All four people on the flight noticed this. Why would the area around a shadow be brighter than the surrounding terrain?
The weather was clear, blue and 30+ miles visibility (I measured). Our altitude was 700 feet.
The ground was mostly pine forest.
The helicopter is mostly shiny plastic with a high proportion of windows-to-walls.
The effect was constant through more than 90 degrees of turn (Sun aft of helo, sun off left side, sun off right side).
I expect that if you were stood next to the shadow you would see bright reflections from the sillouette-ede of the helicopter. A bit like the diamond ring effect during a solar eclipse. If you are next to the shadow, and not in it you should be able to see the sun (causing the shadow) right next to the edge of the helli. The combined effect of the sun and the relection increases the brightness of that area of land.
Someone with a more sciency answer will pop along soon hopefully.
The situation you describe is very, very common as long as the air is clear. The halo effect is caused by diffraction. In large airplanes (i.e. airliners), you can notice the halo is centered on your part of the plane. For us in the cockpit, the halo is centered on the nose. For the folks back near the tail, the halo appears centered there.
I don’t have a 100% solid explananion for why, but it is a diffraction effect as the sunlight is cut off by the aircraft’s silhouette and the light waves from above & below recombine down-sun from the aircraft. The halo is sharpest a certain distance away from the airplane, and so the best effect is seen when cruising a couple thousand feet above a cloud layer. When we descend into, or climb up out of, a cloud layer the halo will grow & solidify and then fade away as the distance varies towards, and then away from, the ideal distance.
The phenomenon you are describing sounds to me like a “glory”, which is briefly explained here. Another related, and interesting (and perhaps more similar to what you saw) phenomenon is heilegenschein, which can be seen in the linked site causing a bright “halo” type effect around the head of a person’s shadow.
Amazing. Just yesterday afternoon I was flying along looking at our shadow and wondering just what causes the halo. I decided to ask those at the Dope about it but forgot about it. Now I find someone’s asked my question for me.
You also see the halo even if you are too high to cast a distinct shadow.
If you’re in a fighter and want to surprise your opponent you can climb above them and “fly” your halo directly over them, then you will be directly in the sun from their perspective and they won’t see you till you’re going in guns a-blazing. Though airborne radar systems would negate this technique now days.
Under certain conditions you can get the same effect around your own shadow… leading to a Brockenspectre :eek:
I was lucky enough to “create” one of these while walking on the Cuillin Ridge on the Isle of Skye in Scotland earlier this year. Even though I knew what caused it, it was still very freaky.
That page also describes glories around aircraft shadows. If the droplet sizes aren’t quite right to create the rainbow effect you’ll just get a general area of brightness around the shadow.
Another pilot here - the “glory” is pretty common, really, but I think folks on commercial airliners find it hard to see because of the itty-bitty windows on modern jets.
I think it’s cool - never get tired of seeing it, or some of the other neat stuff up there.
