Refraction at Work

To refract light is to bend light. More specifically, refraction describes the bending of light rays upon passing through the juncture of two dissimilar masses (such as from air into water), or through the juncture of two regions of the same composition but of dissimilar densities, such as when light travels through warm air into colder, denser air. Called atmospheric refraction, light traveling through the air constantly changes course, usually imperceptibly, but sometimes dramatically, depending on the magnitude of the change in density of air layers through which the light travels (light only travels in a straight line in a perfect vacuum or in a perfectly homogenous gas, which are conditions that do not exist outside of extreme deep space). Interestingly, due to terrestrial refraction, virtually every landmark before our eyes rests, in reality, just slightly offset from where we “see” that landmark.

The terminology and science of terrestrial light travel and atmospheric refraction can confuse and confound even the most ardent sky and weather enthusiasts, but most can grasp the abridged overview: light rays bend toward denser air during atmospheric refraction, and the greater the difference in density between two air masses, the greater the bending a light ray passing between those two air masses will experience.

Can layers of air turn simple headlights into space ships? Of course not, but they’ll look something like extraterrestrial craft if the temperatures of the respective adjacent layers vary enough—creating strong temperature gradients—then those layers transform the headlights into the ethereal, the animated, and yes, even the mysterious.

Geometry in Motion

As I plunged deeper into my journey to unravel the science behind these mysterious dancing lights, I realized that a proper explanation must incorporate geometry as well—both the static geometry of the Earth’s curve as well as the dynamic geometry of the abundant and ever-changing layers of air through which light rays pass. The simplest of the Marfa geometry equations I could formulate involved a single car speeding northbound on Route 67, its headlights reaching a curious traveler more than 20 miles away, who is waiting at the viewing turnoff. The traveler sees a bright dot of white creeping slowly along the horizon. But then the car’s light rays pass through a boundary of cold air lying underneath warm air, with a huge temperature gradient between the two.

I wondered: how does that change what the person sees? Because light will bend into the denser air, and because the temperature gradient is large, the light disappears for the tourist as the rays angle toward the road surface. But then the car—moving at a clip of around 70 or 80 mph—sends its headlight beams out of the boundary layer and the light reappears to the now wide-eyed traveler. He has just witnessed the most basic incarnation of the Mystery Lights.

In general, however, far more complex geometry governs the machinations of the Mystery Lights. I didn’t take into consideration the curvature of the Earth, and my example asserts that juncture through which the light rays passed stood between two layers of uniform air, with no subtle gradations in density. Though my equations were sufficient for solving the basic lightshow, I needed to delve more into the science of the atmosphere to explain the dazzling and ever-changing orbs that keep people believing in the phenomenon. But before I could definitively identify which specific atmospheric mechanisms transform ordinary car headlights into supposedly extraterrestrial beings, I needed to step back and take a look at the region itself, the lay of the land of the Marfa Mystery Lights, and the sky above in order to see what regional-specific factors render the situation unique.

The area lies in the heart of the Chihuahuan Desert, on a high plain not quite 5,000 feet in elevation. Summers here cook up warm, sometimes hot days, with average daily July highs reaching the upper 80s; rarely do temperatures break the 100ºF mark, but they can. Average winter daily lows hover in the upper 20s, but they can drop into the teens and single digits.

This is important because one of the most important weather factors influencing the Marfa Lights relates to temperature. Not extremes or average highs or lows, but the average diurnal temperature variation—how far the temperature swings back and forth over the course of a day. In the Marfa region, that swing surpasses 30ºF on average, meaning vertically adjacent layers of air can form with strong, sometimes extreme temperature gradients, creating wide angles of refraction.

Studying the geography of the area yielded more clues to the headlights-atmosphere interaction puzzle. Most everyone who reports seeing the lights spies them hovering above Mitchell Flat, a virtually featureless swath of the Marfa region. Standing at the observation point, visitors can gaze for nearly 40 miles to the southwest toward Chinati Peak and see nothing but a high plain. Mitchell Flat isn’t absolutely devoid of vertical features, but it is close, with gentle depressions and rises of no more than 200 feet over the course of miles in most places, creating shallow bowls into which cold air sinks and creates inversion layers. Sometimes layers of air of varying temperatures and densities stack atop each other throughout Mitchell Flat, ready to alter the course of light passing through them.

With more pieces of the puzzle snapping into place, I discovered yet another small bit to the mystery: the high point of Route 67, which vehicles crest when driving toward Marfa, lies about 25 miles to the southwest of the viewing point. The topography to the north of the high point, up to the Marfa area and observation point, is unrelentingly flat, but I found it to be flat to the south of it too. One should, I figured, be able to first catch a glimpse of headlights of vehicles once they crested the high point, and then watch the lights as they traversed Mitchell Flat. Without any strong refraction, 25 miles is a lot of atmosphere to traverse, and even mild temperature gradients should cause some shimmering and visual aberration. But as I learned more about refraction, I wondered if somehow light could be refracted from car headlights to the south of the high point and bend toward visitors at the viewing area. The answer was a surprising yes, but only in areas with layers of air of sufficiently high temperature gradients—like Mitchell Flat.