Why the Sky Changes Colour Throughout the Day!
Why does the sky change shades of blue throughout the day? Why does it transform into vivid colours during dusk and dawn? The research paper “The Color of the Sky, Atmospheric and Climate Sciences” [1] explores this fascinating question.
Why is The Sky Blue?
The atmosphere, primarily composed of nitrogen and oxygen, scatters sunlight through Rayleigh scattering [1]. Shorter wavelengths of light, such as those of blue and violet, are scattered more effectively than longer wavelengths, such as those of red and yellow [1].
As a result, when the sun is high in the sky, the blue and violet wavelengths are more visible as they scatter across the whole atmosphere. This is the primary reason why the sky appears blue during the day.
What Causes Sunrise and Sunset?
As the sun approaches the horizon during dusk and dawn, sunlight travels through a thicker layer of the atmosphere [1]. This amplifies Rayleigh scattering, which diminishes shorter wavelengths (blue and green) and allows longer wavelengths (red and orange) to dominate the sky's colour [1].
In summary, as the sun approaches the horizon, it scatters the blue and violet light out of our view, leaving the less scattered red and orange light that we characterise as the colours of sunrise and sunset.
Ozone absorption is when ozone molecules and absorption bands absorb ultraviolet radiation and wavelengths from the sun. One of these bands is the Chappuis band, which plays a significant role at sunrise and sunset [1].
This effect is most pronounced when the sun is low on the horizon; at this angle, sunlight travels through a thicker portion of the atmosphere, including a greater amount of the ozone layer [1].
Therefore, intensifies the absorption of light in the green-orange range of the spectrum, isolating the red wavelengths, and enhancing the red hues of the horizon [1].
As the sun continues to set or rise, ozone absorption becomes more pronounced, shifting the red spectrum to make the sky appear pinkish [1].
A Green Sky!
Without the ozone layer absorbing the orange-green colours, the horizon could be orange to green [1]. However, this would depend on the intensity of the Rayleigh function [1] (a mathematical expression used for probability in theories and statistics).
Influence of Natural Events
During events such as sandstorms, fires, and volcanic eruptions, the sky may turn reddish. This is because tiny particles such as dust, sand, and volcanic ash scatter and absorb sunlight, altering its colour and intensity and creating a reddish hue in the sky [1].
Similarly, aerosols (tiny particles floating in the sky, such as volcanic ash) in the atmosphere during cloudy skies can intensify the red colours seen at sunrise or sunset [1].
Still Learning
The paper calls for further exploration of sky colours. One intriguing area for future research is the green flash [1], a rare phenomenon observed during sunrise or sunset that causes a green light above the sun. Advanced spectral observations could provide deeper insights into this event and other atmospheric phenomena [1].
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Understanding the complexities of the processes that create bright blue skies and warm red sunrises and sunsets gives one a greater appreciation for the natural beauty we witness.
References
Reference paper: "The color of the sky. Atmospheric and Climate Sciences"