Saturday, April 2, 2016

Qualities of Sunlight

Chromaticities of black-body light
of various temperatures
source: User:PAR
The hue of Sunlight changes according to its angle relative to terrestrial observers.

The blue hour, the golden hour, alpenglow and the belt of venus are subjective phrases we use to describe our sensory experiences of the color of Sunlight.

Color temperature is an objective reading of the hue of a light source in units of Kelvin (K).

Warmer colors, yellowish white through red, or about 2,700 to 3,000 K are seen shortly after Sunrise or shortly before Sunset.

Sunset at Newport Beach
by Bert Kaufmann

A rising or setting Sun produces light described by photographers as the golden hour. Daylight is softer and much warmer during the golden hour than during the middle of the day when the Sun is higher in the sky. The softer quality of light during the golden hour is because highlights are muted and shadows are less dark.
Keep your face to the sun and you will never see the shadows.
― Helen Keller
During twilight, shortly after the Sun has set or shortly before the Sun has risen, indirect Sunlight produces cooler colors over 5,000 K (Kelvin).

View south from the observation deck on Rockefeller Center
by Daniel Schwen

The cooler bluish quality of light is called the blue hour.

Unlike the golden hour or the blue hour, alpenglow is caused by a projection of backscattered Sunlight onto topographic features like mountains.

Alpenglow projected on Mount Everest
source: NASA Goddard Space Flight Center

Alpenglow occurs when a topographic high like Mount Everest, positioned opposite the Sun, receives a band of light that has reflected off airborne moisture particles in the lower atmosphere causing a reddish glow.

An atmospheric phenomena observed long after a Sunset or long before a Sunrise is called the belt of venus.

Pinkish glow called the belt of venus
source: ESO

The belt of venus is a glowing pinkish arch visible 10°-20° above the horizon. Like alpenglow, the belt of venus is due to backscattered Sunlight. The belt of venus is caused by the refraction of light through dust particles high in the atmosphere.
She turned to the sunlight
And shook her yellow head,
And whispered to her neighbor:
"Winter is dead."

A.A. Milne, When We Were Very Young