I can hear you asking, “Then shouldn’t the last speck of the setting Sun be a blue flash?” When the atmosphere is exceptionally clear, yes. But our atmosphere scatters (redirects) short-wavelength light very well — very little of the Sun’s blue light ever reaches our eyes directly. The proof of this is all around you on a clear day: the blue sky. Scattering is usually enough to remove blue from beams of sunlight and leave green light to predominate at the Sun’s top edge.

In addition, when the Sun hangs low, water vapor in the atmosphere absorbs most of the yellow and orange light, leaving only some green and a whole lot of red — the typical red Sun of sunrise or sunset. But suppose you see the Sun turn deeply red and noticeably dimmed many minutes before sunset? If that happens, the atmosphere must be humid (or hazy) enough to absorb even the green light. On such a day, the green flash will not occur.

So when the Sun stays bright and yellow-white until it is very low, that’s your cue to look for a green flash. But will it be visible on any such day? According to experts, yes. There is, however, a catch.

Looking at Sunset Safely

It turns out that on most clear, unhazy days the green flash is such a tiny sliver of light that you’d have to use optical aid to detect it. Yet looking through binoculars or a telescope at the Sun without a proper solar filter is normally extremely dangerous — you are likely to suffer eye damage from doing so! An observer absolutely should not look at the Sun with even small binoculars unless two conditions are met:

· First, the bottom of the Sun must already touch a very distant horizon, not just be passing behind a mountain or building;

· Second, the Sun must be dimmed enough for the naked eye to look at the solar disk quite comfortably.

Most beginners seeking the green flash would be better off relying on eyes alone (no binoculars or telescope) and waiting until the Sun is right on a distant horizon.
An obvious, eyes-only green flash happens when the atmosphere has significant temperature variations and produces anomalous, or greater than normal, refraction. This can occur over land, but it’s more likely over water. In fact, the most common kind of green flash takes place when the surface is distinctly warmer than the layer of air just above it. This happens most often over a body of water, and that’s why green flashes are often associated with seashore sightings.

That condition — air colder than water at sunset — is common in winter and after the passage of cold fronts in late summer and autumn. But it can occur at any time of year in most climates, and when it does you should be alert for a green flash visible to the unaided eye.

Such temperature conditions often produce mirages, and seeing them is a good predictor of a green flash. Have you ever seen a mirage? You may think you haven’t, but there are many types of mirages besides the classic imaginary oasis that fools a weary traveler in the desert. Almost all of us have driven down a road on a warm, sunny day and noticed what seem to be puddles of water on the street ahead — puddles that disappear as we approach them. The “water” is really a view of the sky refracted to us because the air just above the road is so much cooler than the very hot road surface. This is an example of an inferior mirage — “inferior” means that the phantom image appears below the normal image.