This early March, a barely believable spectacle took place off the English coast. Indeed, in the distance, an oil tanker seemed to fly over the azure waters of the Atlantic.
In this respect, the images captured by David Morris from the port city of Falmouth (England) are very impressive. The British photographer has himself admitted having been stunned by observing the phenomenon.
A very rare mirage outside the poles
No question here of scientists testing a revolutionary invention, the explanation actually stems from a form of optical illusion. As forecaster David Braine explains at the BBC, we usually observe this kind of mirage in the polar regions. In particular, in the Arctic. However, sometimes conditions become favorable for them to occur at much lower latitudes – including England. This type of mirage says superior nevertheless remains very rare outside the poles and occurs mainly in the cold season.
But what exactly are the right conditions? The appearance of the mirage is in fact due to the temperature difference between the air located at sea level and that above. Usually, the temperature decreases as one rises in altitude. But if the vertical profile describes a inversion, that is, if warm air overhangs a layer of colder surface air, the normal propagation of light rays is altered. This is what happened at the beginning of March in the south-west of England.
Deceive our perception system
Indeed, since cold air has a higher refractive index, an inversion situation prevents light from escaping in a straight line and causes the spokes to bend down – see the diagram above. Result: instead of being seen at a higher altitude, the image of the boat sailing offshore becomes visible to the passer-by located at the level of the coast. The latter then thinks he sees a ship flying because our brain works on the principle that the light rays are hardly bent. For the record, sailors use this optical effect to spot ships traveling beyond the horizon.
Let’s finish by mentioning that mirages often leave us stunned, and there are good reasons for that. Thus, just like optical illusions, these phenomena constitute a very good illustration of the biases associated with our system of perception.