Atmospheric Refraction and Mirages - Bending Sunlight

Photo Credit: Robert Bradshaw

Look at the beautiful sunset above. In the photo, the sun is above the horizon, right? Right? Wrong.

The sun is below the horizon.

Surely you’re joking?

No, it’s true. As is the opposite: at dawn, we see the sun before it has physically crossed the horizon.

Why? Because the atmosphere ‘bends’ light.

Since the density of the atmosphere varies at different altitudes, light, and other electromagnetic waves, are refracted (denser air has a higher refractive index). In fact, at the horizon, light is bent about 0.5 degrees, which also happens to be very slightly more than the size of the sun in the sky. So, when you see the sun just ‘touching’ the horizon, it is actually already completely below it. Crazy.

Atmospheric refraction is zero at the zenith, and increases towards the horizon, as shown in the chart below. On the horizontal axis you have degrees from the horizon and on the vertical axis you have refraction in degrees.

Source: NOAA

Because of this, astronomers schedule their observations so that the object they want to look at is as close to the zenith as possible, when it is the highest in the sky. Sailors, when navigating with stars, don’t use stars below 20 degrees to the horizon. There are ways to compensate for this refraction, but since it is also a function of humidity and temperature and pressure, it can be very complicated to do so.

Mirages

Inferior mirage in Nevada

Atmospheric refraction is responsible for mirages as well, where objects at the horizon appear to be shimmering, almost as if they were puddles of water. In this case, light is refracted when it passes through a sudden transition between cold and hot air. Hot air has a lower refractive index (since it is less dense) than cold air and if, for example, the air is hotter next to the ground, the light bends upwards. These are called inferior mirages, and are common in very hot areas, like deserts, and can occasionally appear on hot asphalt during a hot day. Since the brain interprets the light as travelling in a straight line, an ‘inferior image’ of the sky appears, which is commonly mistaken for a puddle, since that would actually make sense to our brain.

The opposite situation, cold air below hot air, is quite uncommon (it’s the opposite of the usual arrangement in our atmosphere, cold air is denser therefore sinks to the bottom) unless you are at the poles. In this case, the light bends downwards and the phenomenon is called a superior mirage.

Red Sun

Sunrise at North Point Park, Milwaukee, Wisconsin – Photo Credit: Dori

Refraction affects blue and green light more than red and orange light, since blue and green light have shorter wavelength, and that is why the sun is redder during sunset and sunrise.

-Francesco

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Relevant Video

References

cuthbert51

It has no relevancy whatsoever, but the ship in the sunset picture is the Peter Iredale. nnYeah, I’m actually thinking the same thing as you right now; Really? That is what you felt could add to this conversation?
bobbobitybob

The idea of the horizon being the edge of what you can see, only you can see past it, is not a very helpful concept. On the QI episode in the video, and in the blog above, the explanation is confused. With refracted light you’re looking around a corner, as if you were looking through an endoscope. It’s not a mirage, as Frye says it is. You’re seeing a bit further because your vision goes around a slight corner, but there’s no way to measure that effect by looking at it, any more than you can tell the orientation of an endoscope by looking at the image. Expressing that in terms of the horizon doesn’t help, when “horizon” is a visual concept. How can you see past what you can’t see past? Everyone on the QI panel was miserable and confused, and Frye couldn’t really help them. What they’re trying to say is that the visual horizon is a little wider than the geometric horizon.
Roger Llamas

I witnessed a superior mirage of the sun at sunset and turned to see a very bright rainbow column 75′ away sitting on the hill above! The RBC exhibited strange phenomena of the primary & secondary RBC’s aligning on & off, like the 2 focusing in a wave motion!! this made it appear to be alive & breathing. I began to walk towards it slowly & instead of moving away it narrowed until I was within arm’s length of 3 large drops falling. The 3 had an inverted pyramid of color radiating in their correct order of color, red on right. The rain event was inside a 50 yd. radius. Somehow the extreme bending of sunlight seemed to make a look of converging light in a 3D cone shape, maybe like my eye’s structure!!! I stepped past the 3 drops into a landscape of of red, the afterglow of the secondary RBC!!!! I walked up to the red forest where 3′ from a madrone tree the color changed along with a deluge rain. splashing, sparkling, forming a glory around the shadow of my head for 3 seconds. The golden rain receded into said tree & vanished, altogether. please contact me @ 707 887 2190

Atmospheric Refraction and Mirages – Bending Sunlight