It's rare to see a green flash, even in the ideal viewing conditions of Hawaii -- where the Sun rises and sets over the ocean and where it is often clear all the way to the horizon. Nevertheless, clouds may linger on the horizon, making a green flash an infrequent event, even in the best of circumstances.
A green flash occurs shortly before sunrise or after sunset, when the area at the top of the Sun's disk suddenly and briefly changes color from orange or red to green -- in a flash that usually lasts for, at most, a few seconds. This optical phenomenon happens because the atmosphere acts like a prism, refracting the sunlight into various colors; when the light rays bend, the green flash is the last burst of light visible from the setting Sun.
Astronomer Dr. Mai Shirahata had the good fortune to view and photograph this remarkable sight from Mauna Kea's summit area on April 5, 2013, just before conducting open-use observations with the Subaru Telescope. Her experience is testimony to the clear air at Subaru Telescope's site, where the light from the setting Sun can reach an observer without scattering. Such clear, unpolluted air benefits not only astronomers in their observations of the night sky but also those who appreciate the gifts and beauty of the natural setting.