Sunsets, Aurorae, Colorful Clouds - Optical Phenomena

The sky appears blue during the day because of small particles Earth's atmosphere: gases, volatile oils, dust, smoke, pollen and water droplets. These small molecules scatter the sun's light to blue wavelengths preferentially (Rayleigh scattering).

If Earth lacked an atmosphere, there would be no particles to absorb or scatter sunlight and the sky would look black.

The sky is bluest in the middle of the day when the path of sunlight through the atmosphere is most direct. It also appears bluer after a rainstorm when rain has washed away larger dust particles from the air.  The blue of the sky is usually graduated because of particles like dust.

If there is more dust particles in the air, more scattering occurs and the different wavelengths of light recombine to form white light, so the sky looks paler.

When the sun is low on the horizon, sunlight travels a longer distance to travel through the atmosphere and green as well as blue wavelengths are scattered by particles. Only yellow and red wavelengths reach the observer, so the sun appears redder.

Clouds (water droplets or ice crystals) usually result in a more colourful sunset/sunrise when light is reflected from the sun.

Sunsets in coastal areas are usually less intense, as water droplets and salt mist result in more scattering, which recreates white light. If the water particles are very small, good sunsets may be seen, particularly if there are clouds in the sky as well.

Violet hues arise when the sun is no longer visible and Earth's shadow rises in the sky. The night sky is actually dark blue, but there is not enough light for our eyes to see colour.

Sunsets in deserts, during dust-storms and after bush-fires or volcanic eruptions can be very red.

Distant mountain ranges in areas with extensive vegetation, good air circulation and little human pollution often have a blue-green haze. Volatile components in plants such as terpenes combine with ozone in the upper atmosphere to produce tiny particles that scatter blue light.

Larger particles in air like water droplets or salt (in coastal regions) result in a whiter haze. Clouds, mists and fogs usually appear white, even though they contain colourless water droplets. This is because scattered light is recombined to form white light.

Clouds appear whiter if they are 'fresher' and therefore contain more numerous, smaller water droplets and when then sun's light is reflected directly back to the observer. Seen from the other side, the same cloud will appear grey, because it is in shadow.

Tiny smoke particles appear blue. Larger smoke particles can appear whiter. Black smoke both absorbs and scatters light, so appears grey.

Pollution haze is brown and is a familiar sight over large cities. Nitrogen dioxide, a toxic brown gas turns smog brown. Nitrogen dioxide is a major pollutant resulting from car exhaust gases. Larger particles such as smoke, dust and toxins scatter all wavelengths of light resulting in an opaque haze which would be white if it wasn't for the brown nitrogen dioxide. Pollutant particles are attracted to water droplets, preventing them from evaporating. This adds to the glare of pollution haze.

Crepuscular rays are frequently seen at dawn or sunset (when there is more contrast in the sky). They are columns of light that appear to radiate from a single point in the sky. They stream through gaps in clouds and objects such as trees and even buildings. The rays are near-parallel, but appear to diverge because of linear perspective, like rows of a plowed field seem to be narrower with distance. Particles in the atmosphere make the rays visible.

Iridescent clouds (also called mother-of-pearl clouds or nacreous clouds) are seldom seen but can be observed after sunset or before dawn. They shimmer with vivid and slowly shifting sheets of colour, including silvery blue and gold. They have a pearly effect, like the inside of a mother-of-pearl shell. They are formed by diffraction of sunlight through water droplets or ice crystals of uniform size high in the atmosphere. They are most often observed in winter in in polar regions such as Alaska, Scandinavia and Antarctica.

Although pretty, they accelerate damage in the ozone layer by promoting destructive chemical reactions of pollutants such as chlorofluorocarbons (CFCs).

An aurora is a luminous effect seen in the polar sky, a phenomenon known as the aurora australis (or southern lights) in the southern hemisphere and aurora borealis (or northern lights) in the northern hemisphere.

Aurorae (or auroras) range from faint glows to vivid, flickering, pulsating, kaleidoscopes of colour and movement. Prime viewing locations include Alaska and northern Scandinavia and the most dramatic displays are seen on clear nights with dark skies.

Aurora occur when the sun emits energy in the form of charged particles in the solar wind. A few days after a sun flare or sunspot activity, a portion of the solar wind reaches Earth's atmosphere. The solar wind is directed to regions where Earth's magnetic field bends inwards ( where the Earth's magnetic field is strongest).

Charged particles, mostly electrons accelerate along the magnetic field and collide with atmospheric gas molecules, which are ripped apart into electronically charged ('excited') atoms. Light is emitted when these excited atoms reform molecules.

The colour depends on the element: oxygen produces green (most common colour) in the lower atmosphere and red in the higher atmosphere. Nitrogen produces blue and violet.