Light in the Sky, from Noon to Night

Start with the combined direct and indirect sunlight of the day. Its intensity runs from around 120,000 lux under a bright midday sun to under 5 lux beneath thick storm clouds, so the same word covers a huge range of light.

This is the reason the daytime sky is blue. Shorter blue wavelengths scatter far more strongly than red ones, in proportion to the inverse fourth power of wavelength, and so reach the eye from every direction.

Thin clouds can turn the scattered light into color. Very uniform small droplets or ice crystals diffract sunlight into soap-bubble colors within about 10 degrees of the Sun, most vividly at the edges of newly forming clouds.

Ice crystals in high cirrus act as small prisms and mirrors. The result is a family of rings, arcs and spots, and because the cloud often precedes a front, a halo has long been read as a sign of rain within a day.

The most familiar halo is a ring about 22 degrees from the Sun or Moon, roughly an outstretched hand at arm length. Hexagonal ice crystals refract the light, which leaves the inner edge reddish and the sky inside the ring darker.

Flat, horizontally oriented ice crystals can paint a wide spectrum band parallel to the horizon, red on top. It needs the Sun at 58 degrees or higher, so it cannot be seen north of about 55 degrees north or south of 55 degrees south.

As the Sun drops, its light crosses more air and Rayleigh scattering removes the blues and greens, leaving oranges and reds. Evening air usually holds more particulates than morning air, which is why sunset colors tend to be more vivid than sunrise colors.

After the Sun is gone, a broad arch of whitish or pinkish light can linger, with a purple light and a bright segment beneath it. Fine particles from volcanic eruptions can intensify it into especially vivid displays.

Twilight is the sunless but still lit period covering both dawn and dusk, produced by diffuse sky radiation. It is split into civil, nautical and astronomical phases at 6, 12 and 18 degrees of the Sun below the horizon.

Dusk is the darkest stage of twilight, just before full night. Its length varies with latitude and season, lasting longest at high latitudes in summer and shortest near the tropics.

Look away from the sunset and a pink band can rise 10 to 20 degrees above the horizon, made of reddened sunlight backscattered by fine particles. Beneath it the darker shadow of the Earth itself appears as a twilight wedge.

Alpenglow is a reddish glow near the horizon opposite the Sun, seen only once the solar disk is below the horizon. It comes from indirect sunlight reflected or diffracted by airborne precipitation, ice crystals or other particulates.

Far above ordinary weather, at roughly 76 to 85 kilometres, ice crystals up to 100 nanometres across stay lit by the Sun after the ground is in shadow. They are the highest clouds and are seen only in twilight.

The trail ends with the light that never fully goes out. Self-illuminated gases hundreds of kilometres up keep the night sky from being completely dark, a separate effect from the aurora and brightest about 10 degrees above the horizon.