Inferior mirage: 'water on the road'

A floating road train on the Stuart Highway, Northern Territory, Australia

A commonly seen mirage is 'water on the road'. The density of air in the atmosphere often decreases monotonically with increasing altitude, due to its weight: the air above compresses the air below. Above a very hot road, however, there is often a layer of warm air whose density is lower than that of the less warm air above. The denser air slows light very slighly more than the less dense, so it has a slightly higher refractive index, so light can bend from the warm air into the cool: light travelling roughly parallel to the road is curved slightly, concave up. This allows the rays, one of which is sketched above, that come from the sky, pass close to the road, then bend back upwards to the eye of the observer. Because one sees sky where the road should be, we often interpret this as a reflection caused by water on the road. The 'water' magically evaporates as we approach.

This photo was taken durng the 2009 World Solar Challenge, a transcontinental race for solar cars between Darwin and Adelaide. The 'truck' in question was a road train: a prime mover followed by three large trailers. It was interesting to see it appear to drive into the sky.

The green flash

When the horizon is distant, the setting sun often looks very red, because light from the sun passes through a long section of the atmosphere, so nearly all of the blue light and most of the green is scattered out. Sometimes, for just a second or two, the very last visible part of the sun appears bright green. This is the green flash, which is due to both refaction (as are the mirages shown above) and scattering, which is responsible for the colours of the sky.

For the observer at a, the red image of the sun is no longer visible. Of the remaining colours, the blue is scattered out.

The light from the sun bends towards the earth when it enters the atmosphere (at point c in the diagram), so that it is possible to see the sun even when the earth lies between the sun and the observer. All the light is bent, but blue (b in the diagram) is bent most and red (r) is bent least. For observer at point a, the red light from the sun does not bend enough to be seen, so his/her image of the sun will have no red light—for this observer, the red image of the sun has already set. Blue and green light do bend enough (the blue and green suns have not yet set!), but the blue light is scattered out by the large thickness of atmosphere traversed by the light. So the image is green for a short time—typically a second or two. The effect requires very clear air, so that not too much green is scattered, and a distant horizon. The duration is variable. Some atmospheric effects probably magnify the angle of the green limb, so that the effect lasts longer.

The effect is real—photographs, including a study by the Vatican's observatory, show it. It is however made more spectacular by the fact that, while you are looking at the red sun, your red photoreceptors suffer retinal fatigue so that, when the red light disappears, the remaining green appears all the more vivid. This is one of the reasons why it is easier to see the green flash in the evening. The others are that it is easier to predict where and when the sun will set than where and when it will rise, and that some of us see more sunsets than sunrises!

I've only seen it several times. The first was when a friend and I watched a sunset from the top of the Dune du Pila on the French Atlantic coast under propitious conditions. The last limb of the sun seemed to disappear and we both thought that we had been unlucky. Then suddenly we saw a bright, clear green line where the sun had disappeared. We were too astonished to take a photo. I live on an East coast and look for it at dawn, but have never yet seen it over the Pacific. When eventually I catch it with a camera in hand, the photo will go here!

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