Peter Forister has chased and photographed more than his fair share of East Coast thunderstorms and tornadoes, but one of his weirdest photos was captured when he set out to enjoy the sunrise Friday morning. Shortly after the sun rose above the horizon, a bright stripe of green appeared directly atop the sun.

Then the sun rose further, and the green splotch appeared flat and disk-like.

Forister, a graduate student at Virginia Tech, knew instantly he had captured the elusive “green flash.” It’s a rare phenomenon that can be spotted only at sunrise or sunset, and under ideal circumstances at those times.

The ephemeral scene took real effort to see. To set up for the sunrise meant braving temperatures in the teens, plus a wind chill of minus-5 degrees, but that’s nothing new to Forister, an intrepid photographer.

“In an average spring week, I would maybe go out two or three times,” Forister said. “I’m intentional about choosing mornings that have potential to see something. I knew it would be cold and windy. … It woke me up real good.”

Forister awoke around 6:30 a.m. and ventured to Afton Overlook, about 15 miles west of Charlottseville, where Interstate 64 intersects the Blue Ridge Parkway. The elevation there is 1,895 feet.

“I’ve known for several years [the green flash] existed,” Forister said. “I feel like I have seen it just strolling down the beach, but I couldn’t be sure. It’s not something I intentionally looked for before.”

This time, however, he was looking, and had his camera trained on the sun when it happened.

“I was able to see it with my naked eye,” said Forister, who added that he was stealing occasional glances at the rising sun. “I saw the flash and had about two seconds to get my camera in the right spot and get the right photo. It lasted three to five seconds.”

Most green flashes are seen at sunrise or sunset directly above the horizon and usually over an open ocean.

Two effects combine to yield a green flash. The first is refraction, or the bending of light. Green light is refracted more strongly than red light, and thus the sun will appear tinged green on top and red at bottom. This is the case at all times to some degree, but the effect is very minor. It has to be bolstered by a mirage to be noticed.

Mirages form when the temperature profile of the atmosphere, or decay of temperature with height, results in a change in refractive index. In other words, changing temperatures affect how the light is bent. When there is a strong inversion, or increase in air temperature with height just above the ground, it makes it possible to see the green flash for longer and more dramatically.

Since refraction is greatest for light of short wavelengths, such as green, blue and violet, those are the colors we should technically see last before sunset and first before sunrise. But atmospheric “extinction” — the scattering of light of certain wavelengths by air molecules — often eliminates the violet and some of the blue.

In other words, purple shades are often scattered or bounced away by air and pollution. If the air is clean enough, some aquamarine and violet can remain intact.

Forister didn’t witness his green flash over the horizon though. This one crested atop a cloud.

He thinks it was probably an M-mir, or mock-mirage type flash, which is especially rare. Little is known about this phenomenon, though it’s believed that it stems from an inversion at the top of the cloud layer.

In the case of Friday’s setup, there was a stout inversion at about 4,700 feet. There was also evidence of a much shallower inversion just above the surface. It seems possible that the rays of sunlight Forister witnessed were affected by both as they traveled over the mountains to the east and to his viewfinder.

The presence of an inversion is also supported by later images Forister took of a distorted sun that appeared stretched horizontally and vertically stout.

Forister, who’s known for photography including rocket launches, tornadoes and night-sky scenes, says the biggest key to catching striking phenomena like the green flash is simply waking up and looking up.

“There’s so much beauty around us,” he said.

Oh — and one more thing.

“Coffee,” he joked. “I drink so much coffee. I’m drinking coffee right now.”