If you spent a minute or two on the web last week, you probably recall seeing the mesmerizing photo of a tanker ship appearing to levitate above the sea off the coast of England. And if you've been boating for a while, it's probably not the first time you've witnessed such a phenomenon. Turns out, that optical illusion really is a thing. It's a form of mirage, known as a "superior mirage."
Mirages are created when light passes through air of different temperatures. Two types of mirages are "inferior" and "superior."
An inferior mirage occurs when you have a dense layer of cold air sitting above your line of sight, with a layer of less dense warmer air below your line of sight. When rays of light coming down from the sky get close to the hot air near the ground, the light rays are bent (refracted) back up toward your eyes.
This is your typical "oasis in the desert" mirage, where you look down toward the desert sand (or down a highway, say) and see what appears as a pool of blue water. In reality, it's just light from the blue sky bending back up toward your eyes.
Inferior mirages happen less frequently over the ocean because the ocean surface usually doesn't get extremely hot. However, it can occur over the ocean when very cold arctic air blows over unfrozen water.
A superior mirage is the opposite. A layer of warm air sits above your line of sight with a cool layer beneath it. (Also known as a temperature inversion.) Light bends down towards the denser air, but because our eyes assume the light we see travels straight, the object we are seeing appears higher than it actually is. Therefore we see the image, or inversions of it, above where it actually is.
If another ship sails away from you, eventually they will disappear below the horizon. However, if conditions create superior mirages, then you can still the other ship further away from you even though the ship is a short distance below the horizon. Similarly, with the refraction of a superior mirage you can see mountain tops sooner as you sail toward a mountainous island or coastline, even though the mountain top might still be slightly below the geometric horizon. Also, boats on the distant horizon sometimes appear to be floating in the air above the water, as in this case of the mysterious floating tanker.
Research credit: UBC ATSC 113 - Weather for Sailing, Flying & Snow Sports
- Inferior mirage: Rays of light are bent upward. Looking down below the horizon, you see objects (or the sky) that are actually above the horizon.
- Superior mirage: Rays of light are bent downward. Looking up above the horizon, you see distant objects that might be lower or even below the horizon.
So, no, aliens haven't hijacked a tanker off the coast of England. Nor has Elon Musk recently purchased majority stake in Maersk (that we know of). What broke the internet last week was just a cool scientific phenomenon known as a superior mirage. The next time you see a flying Fleming on the horizon, don't panic—they haven't gone to foiling. Yet.
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