All About Space Astronomer Book - 2014 UK

Cloud Iridescence

Water droplets or small ice crystals in a layer in front of the Sun

or Moon can produce a palette of vivid pastel colours by an effect

known as cloud iridescence.

Corona

A thin cloud layer in front of the Sun or Moon can give rise to a set

of coloured rings known as a corona. More commonly seen around

the Moon, this effect can be produced by any small particles, such

as fine pollen grains suspended in the air. As pollen grains aren’t

necessarily spherical and are often aligned by the prevailing wind,

a corona created by light passing through a pollen cloud may be

elongated in shape and contain bright patches.

Crepuscular Rays

So-called “crepuscular rays” are rays of sunlight that appear to

radiate from a single point in the sky. They are caused by sunlight

streaming through gaps in the clouds or between other objects and

are basically columns of sunlit air separated by darker regions of

shadowed air. Even though crepuscular rays appear to radiate away

from the Sun, they are in fact parallel to one another in the sky.

Under rare circumstances, if you face in the opposite direction to

the Sun you can see rays converging to a point. These are known as

anti-crepuscular rays. Rays of light penetrating through holes in low

clouds are often termed Jacob’s ladders.

Mirages

Under normal circumstances, the temperature of the atmosphere

falls smoothly with height. This makes the Sun or Moon appear

flattened when close to the horizon. Where warmer layers of

air interrupt the otherwise smooth temperature fall, other

effects can occur, such as the image of a distant object appearing

above or below the real object. When the image appears below the

real object, this is called an inferior mirage; when above, it’s called

a superior mirage. The effect of a shimmering reflective pool in

the distance along a hot tarmac road is an excellent example of an

inferior mirage. A Fata Morgana is an example of a superior mirage

creating an image which sits above the real subject. These

are commonly seen in polar regions above large sheets

of flat ice.

A warm layer of air close to the surface of the sea can introduce

some rather interesting effects on the rising or setting Sun.

One particularly impressive display results when the Sun’s image

is reflected close to the horizon resulting in what looks like the

Greek letter omega. An alternative name for this effect is the

Etruscan vase.

[6] An Etruscan-vase sunset, also known as an "omega sun". [7] A glory, photographed from the window of an aircraft.

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