Meteorologist captures rare ‘green flash’ in Half Moon Bay

Bay Area meteorologist Jan Null had been trying to see the so-called green flash for nearly 50 years, looking for a hint of emerald any time he was at a beach during sunset.

But it wasn’t until Null moved to Half Moon Bay a year and a half ago and and made frequent trips at sunset to the beach that he finally caught a glimpse of the phenomenon. Once he knew what to look for, he saw it again and again. 

“It actually occurs fairly regularly,” said Null, who worked for the National Weather Service’s Bay Area office for more than 24 years before launching his consulting company, Golden Gate Weather Services. “I’d say half the time I go out and photograph the sunset, I see some of that green above the setting sun.”

Last week, Null spotted the biggest green flash he has seen yet and posted the image to Twitter, catching the eyes of many in social media. 

“Mondo Green Flash,” Null wrote. “From last evening, this is the largest example of the Green Flash that I have ever photographed plus some cool mock-mirage effects. (The colors are right out of the camera with no post-processing).” 

An inferior-mirage green flash taken at Mavericks Beach at Pillar Point near Half Moon Bay.Jan Null

The existence of the green flash has long been debated, especially in Hawaii among groups of tourists standing on their condo balconies overlooking the ocean. “Look, the green flash,” one person will say. “I didn’t see it!” another will say.

Scientists have established that the green flash is indeed real, and not an optical illusion, though it is only occasionally observed at sunset (most people don’t know what to look for) and even less often at sunrise.

Andy T. Young, an astronomer who is widely recognized as the world’s leading expert in the green flash, explains on his website dedicated to the phenomenon that two different effects come together to make a green flash that lasts for one to two seconds.

One can be explained through a physics lesson in the refraction of light. Just as light passing through a prism disperses, the light emitted by a sinking sun can be bent by the atmosphere, separating different colors.

Each color bends a different amount based on its wavelengths; shorter wavelengths (blue, violet and green) refract more strongly than longer ones (yellow, orange and red). Blue and violet light are scattered by the atmosphere while red, orange and yellow are absorbed, with a vivid green light being most visible during the few seconds when the sun sinks below or rises above the horizon.

The second is where the atmosphere acts like a lens, magnifying the green hue and creating a mirage.

“When the density profile is curved, the curvature is what produces this lensing (focusing) effect,” Young explained in an email. “A curved density gradient (or, approximately, a lapse rate that changes with height) produces this effect.”

The flashes can be other colors, too. In rare instances, on an especially clear day, blue light may make it through the atmosphere, Young explains.

There are four different types of green flashes, as outlined on Young’s site, but the two most common are inferior-mirage flash and mock-mirage flash.

The inferior-mirage is the classic green flash that’s seen as a thin disc of color just as the sun dips below the horizon. This occurs when the water is warmer than the air above and is best viewed from sea level.

“This is the type succinctly described by James P. Joule [an English physicist] as the ‘last glimpse’ of the setting sun,” Young wrote in an email.  “When the waves are small, you will see this kind of flash is a little above the sea horizon.”

The mock-mirage is caused by a thermal, most commonly occurring when the water is colder than the air. The inversion can also be produced by other processes such as subsidence of air in a high-pressure weather system.

“You need to be above the bottom of the inversion to see those well,” Young explained. “When you’re beneath the inversion, the flashes tend to be skimpy little slivers that are only visible when there’s enough wind shear to makes waves on the inversion.”

The green flash Null photographed last week from the bluffs of Redondo Beach, about 50 feet above the ocean in Half Moon Bay, and shared on social media was a mock mirage, as he noted on Twitter. 

“The fine structures in the upper part of the picture are little mirages of the sun’s upper edge, probably produced by waves on the inversion,” Young said. “These little bits appear to pinch off from the top of the disk as the Sun sets, and each one turns green for a second or so before it shrinks to nothing and vanishes.”

This would make sense, as Null took the photo July 8 when Half Moon Bay was unusually warm and reached the high 70s in the afternoon.  

“It was a hot night,” Null said. “There was this dry warm layer above the cold water and cold air right above the cool surface. You get a temperature inversion. You get this wavy shape around the outside of the sun. That’s the light getting ducted through these different densities of air. You have warmer air and cooler water. The light is coming through different through each layer.”

A photographer captures the green flash at sunset at Poplar Beach in Half Moon Bay.Jan Null

A green flash isn’t always visible, but if you want to see one, it helps to keep these tips in mind. 

– Pick a clear day with minimal haze and cloud cover. 

– Don’t look at the sun until it’s just sinking below the horizon — your retina can temporarily get bleached because of the bright red light from the sun.

– Pick the right location. Being right at the ocean, standing on the beach or on a boat, is best, but viewing from a tall building or ocean bluffs works, too. 

-Most green flashes are barely visible and it helps to use binoculars.

-You says that most people don’t know what to look for when they’re seeking the green flash. His website provides additional advice and tips on how to see it. 

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