Cool Mirage

by don - 2021-03-07 03:26:31 ( in education, research) [php version] rebuild

Default The weirdest mirage you'll ever see: a ship floating high in

the air

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https://theguardian.com/science/2021...tical-illusion

*Walker 'stunned' to see ship hovering high above sea off Cornwall *

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David Morris encounters rare optical illusion known as superior

mirage while out on coastal stroll*/

There are only so many polite words that come to mind when one spots

a ship apparently hovering above the ocean during a stroll along the

English coastline.

David Morris, who captured the extraordinary sight on camera,

declared himself "stunned" when he noticed a giant tanker floating

above the water as he looked out to sea from a hamlet near Falmouth

in Cornwall, SW England.

The effect is an example of an optical illusion known as a superior

mirage

.

Such illusions are reasonably common in the Arctic but can also

happen in UK winters when the atmospheric conditions are right,

though they are very rare.

The illusion is caused by a meteorological phenomenon called a

temperature inversion. Normally, the air temperature drops with

increasing altitude, making mountaintops colder than the foothills.

But in a temperature inversion, warm air sits on top of a band of

colder air, playing havoc with our visual perception. The inversion

in Cornwall was caused by chilly air lying over the relatively cold

sea with warmer air above.

Because cold air is denser than warm air, it has a higher refractive

index. In the case of the "hovering ship", this means light rays

coming from the ship are bent downwards as it passes through the

colder air, to observers on the shoreline. This makes the ship

appear in a higher position than it really is – in this instance,

above the sea surface.

"Superior mirages occur because of the weather condition known as a

temperature inversion, where cold air lies close to the sea with

warmer air above it," said David Braine, a BBC meteorologist. "Since

cold air is denser than warm air, it bends light towards the eyes of

someone standing on the ground or on the coast, changing how a

distant object appears."

Photographers around the world have captured striking images of

ships, yachts and other vessels apparently hovering in mid-air

thanks to superior mirages. One potential clue that the sight is a

mirage is the lack of any detail below the vessel's waterline – for

example a mirage of a "hovering" yacht lacked the lower hull and keel.

The latter effect is well known to sailors who can sometimes rely on

refraction to spot ships that are geometrically beyond the horizon.

Sailors say such ships are "looming" over the horizon and sometimes

report distortions that stretch or compress the images, making them

"towering" or "stooping" mirages, respectively.

More familiar optical illusions are the "inferior mirages" that give

rise to apparent oases in the desert and puddles on hot summer

roads. These mirages happen when cooler air sits on a layer of hot

air, directly above a road, for example. When sunlight coming down

from the sky approaches the air near the hot surface, it is bent

back upwards to the observer's eye, making the sky appear to be

reflected on the road.