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What produces the "blue flash" during sunset?

Tom Hamilton, San Clemente, California
Earth's atmosphere acts as a prism to refract light
Earth's atmosphere acts as a prism to bend — or refract — light, with red bending the least and blue/purple the most. When the Sun is close to the horizon, an observer can see blue light for a second or two if the atmosphere is extremely still and the sky clear. // Hinrich Bäsemann
The blue flash during sunset is a relatively uncommon phenomenon. It appears just before the last rays of the Sun slip beneath an unobstructed horizon — where both the bending, or refraction, of light and mirage effects (multiple or inverted images) are greatest. As the Sun approaches the horizon, the atmosphere acts as a prism and begins to separate the Sun’s image into its component colors. Because blue light is refracted more than red light (due to its shorter wavelength), the top rim of the Sun turns blue. It is this blue bit of sunlight that you might rarely see just above the horizon (a mirage) at the last instant of sunset — a phenomenon we call the blue flash.

Seeing the blue color, however, requires extremely clear and transparent sky conditions. The reason is that the light path of the last ray of sunlight to your eye passes through the densest part of the atmosphere where air molecules, water vapor, and atmospheric contaminants effectively scatter blue and violet light away from your eyes more than any other color; this is why green flashes are more common than blue ones. Blue flashes are more visible at higher elevations, where the atmosphere is thinner and contaminants fewer.

The “flash” terminology is misleading, so you might expect to see an intense flaring of blue light. Generally, however, the opposite is true, and the last bit of sunlight gets progressively smaller. Only on rare occasions, under specific atmospheric conditions, does a true flash appear. The best blue flashes occur under mirage conditions, which can magnify refraction and enhance the view for a couple of seconds before the phenomenon vanishes.

Of course, you must be very careful when looking for the blue flash, keeping your direct gaze away from the Sun until only the last segment of the upper rim is visible. Staring directly at the Sun’s entire disk, even when it lies low in the sky, may cause eye damage.
Stephen James O’Meara
Contributing Editor

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