Have you earned your pilot’s halo?

Well, perhaps the shadow of one, with a halo? Don’t worry, it’s not the ghost of some lost aircraft, but your “pilot’s halo.” Also called the “pilot’s glory” or “glory of the pilot,” this phenomenon appears as a colorful halo encircling an airplane’s shadow on the clouds. I’d wager we’ve all seen one, but for VFR pilots, much more often from airliner windows.

A pilot’s halo will always appear directly opposite the sun, with the aircraft’s shadow in the center, at what is called the anti-solar point. Scientists have determined that much of the light that forms a glory doesn’t pass into and reflect back from the droplets, the process that creates rainbows. Instead, the pilot’s halo is formed when water droplets in the cloud, or fog, causes backscattering of the sunlight. In physics, backscattering is the reflection of waves or particles back toward the source, a 180-degree reflection. That’s like throwing a baseball directly at a wall so it comes back into your mitt. Precisely how this happens is still being debated by scientists, although one theory is that when a light ray grazes the surface of a droplet, it travels along the surface as a surface wave before being refracted and reflected back toward the source.Also called the “pilot’s glory” or “glory of the pilot,” this phenomenon appears as a colorful halo encircling the airplane’s shadow on the clouds.

The shadow in the center of the pilot’s halo is not created by it, or vice versa. They appear concurrently because the same light rays create both, and that’s why the halo always encircles the shadow.

Before the days of aviation, this phenomenon was first seen by mountain climbers. Under the right conditions, they’d sometimes see a shadow of themselves cast on clouds below them with a halo encircling the climber’s shadow. German climbers observed this phenomenon on the Brocken, the highest peak in the Harz Mountain range. Being only 3,743 feet high, the mountain is easily climbed, and it’s also often shrouded in clouds and fog. These are perfect conditions to observe what became known as the “Brocken Specter”—a ghost?—or a Mountain Specter, Brocken Bow, or Specter of the Brocken. A Brocken Specter may appear whenever the sun is low on the horizon and the climber is above the clouds or fog. The specter became a local legend and was described in 1780 by Johann Silberschlag, a German naturalist. The same phenomenon was earlier observed in the Andes Mountains by the 1735 French Geodesic Mission to the Equator. The phenomenon has been observed in China for centuries and labeled as “Buddha’s Light.”

If you want to look for the Specter of the Brocken today, the mountain is located in the Harz National Park about halfway between Frankfurt and Berlin. If climbing is not your thing, a historic steam railway carries visitors to the top and there’s even a hotel on the peak. The pilot’s glory is also frequently observed from very tall buildings. But whether they are called specters, halos, or glory, it’s more fun to look for them from the cockpit.