.
Rainbow Encircling
An Airplane Shadow
'Pilot's Glory': Why a
Rainbow Encircles an Airplane Shadow on Clouds
If you've ever flown in an
airplane and gazed out the window for long enough, chances are that you've
noticed what appears to be a halo encircling the plane's shadow against the
clouds.
It might seem spiritually
inspiring — or maybe a little spooky — but what you saw was nothing paranormal.
A glory, also known as a
glory of the pilot, a pilot's glory or a pilot's halo, is actually an optical
phenomenon, first observed by mountain climbers in the days before air travel
became common.
It's not created by the
shadow of the plane, but it can appear concurrently and in the same place as
one.
According to this 2012
Scientific American article by Brazilian physicist H. Moyses Nussenzveig,
the first reported observation of a glory was back in the mid-1700’s.
Members of a French
scientific expedition climbed Pambamarca, a mountain in Ecuador, and the
mountaineers described seeing the sun emerge from behind a cloud and illuminate
them, casting each man's shadow and surrounding their heads with what looked
like halos.
It wasn't until the early
1900s that German physicist Gustav Mie came up with a mathematical formula to
explain how water droplets suspended in air can scatter light.
As this article from
the meteorological journal Bulletin of the American Meteorological Society
details, glories are created by the backscattering — that is, the deflection at
an angle — of sunlight by tiny droplets of water in the atmosphere.
(And by tiny, we mean
droplets so small that they're only tens of wavelengths across.)
The size of the rings for
different wavelengths of light varies, according to the average diameter of the
droplets and their distribution; to see a glory, the viewer has to be directly
in between the source of light and the water droplets, which is why glories
frequently occur with shadows.
But even Mie's math didn't completely explain
how glories worked.
In the 1980’s, Nussenzveig and NASA scientist
Warren Wiscombe figured out that much of the light that forms a glory doesn't
actually pass through the droplets.
Instead, as this 2014 piece in the journal Nature explains, the main cause of a
glory is a process called wave tunneling, in which sunlight passes near enough
to a droplet to create electromagnetic waves within it.
Those waves bounce around inside the droplet
and eventually get out, sending out light rays that make up most of the glory
that we see.
NOW
THAT'S INTERESTING
To a pilot, a glory can be a warning that
the cloud layer below holds enough liquid moisture that flying into it can
create ice formation on the aircraft's exterior.
Patrick J. Kiger has written for HowStuffWorks since 2008 covering a wide array of topics, from history and politics to pop culture and technology. He worked as a newspaper reporter for the Pittsburgh Press, and the Orange County Register in California, where he covered one of the biggest serial murder cases in U.S. history, and also as a staff writer at Baltimore Magazine. As a freelancer, Patrick has written for print publications such as GQ, Mother Jones and the Los Angeles Times, and on the web for National Geographic Channel, Discovery News, Science Channel and Fast Company, among others. In recent years, he's become increasingly interested in how technological advances are altering urban life and the design of cities, and has written extensively on that subject for Urban Land magazine. In his spare time, Patrick is a longtime martial arts student and a fan of crime fiction, punk rock and classic Hollywood films.
Patrick J. Kiger has written for HowStuffWorks since 2008 covering a wide array of topics, from history and politics to pop culture and technology. He worked as a newspaper reporter for the Pittsburgh Press, and the Orange County Register in California, where he covered one of the biggest serial murder cases in U.S. history, and also as a staff writer at Baltimore Magazine. As a freelancer, Patrick has written for print publications such as GQ, Mother Jones and the Los Angeles Times, and on the web for National Geographic Channel, Discovery News, Science Channel and Fast Company, among others. In recent years, he's become increasingly interested in how technological advances are altering urban life and the design of cities, and has written extensively on that subject for Urban Land magazine. In his spare time, Patrick is a longtime martial arts student and a fan of crime fiction, punk rock and classic Hollywood films.
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