Window seat

When coming or and going from this region, I usually drive; this weekend was different: I flew. (I like airplanes; it’s airports I don’t like.)

Airplanes provide a wonderful platform for viewing the natural world, so I always request a window seat. If one looks at a physical object, say, a mountain or cloud, the perspective is different, but the object is clearly the same. The rules change, however, when one looks at optical phenomena: some occur only above the horizon, some only below, and some either above or below. For the ones that can only be seen below the horizon, an aircraft’s window seat provides a superb view.

First, I show two strictly sub-horizon phenomena seen on this weekend’s jaunt; then a feature of the aircraft window, itself. Finally, I include a lagniappe.

The glory appears at the antisolar point: the spot directly opposite the Sun. So, the antisolar point is as far below the horizon as the Sun is above but it is on the opposite side of the sky. This is, of couse, where you see your shadow, or your airplane’s shadow. The glory is formed in clouds of water drops; clouds made of ice crystals will not produce it. It is explained by wave interference: light waves that exit a water drop after entering and reflecting off the back inside of the drop interfere with those that exit the other side of the drop.

The subsun requires different particles (ice crystals rather than water drops) and is seen in a different part of the sky (the subsolar point rather than the antisolar point). The subsolar point is directly below the Sun, as far below the horizon as the Sun is above. It is as if the Sun were reflected in a horizontal mirror. Indeed, that is what is happening, but not in one mirror, but many: myriad plate-like ice crystals all falling with their surfaces horizontal. As it is merely a reflection of the Sun, the subsun shows no colour.

The aircraft window, itself, can show some interesting features. The stressed plastic of the window is birefringent and so produces colours when seen with polarized light. The light from most scenes is not strongly polarized, but a reflection from a body of water is, so the colours seen here are a consequence of both the reflection (from, in this case, Georgia Strait) and the aircraft window.

And the lagniappe? When back on the ground and nearly home, I was greeted by an elk.