Skys that change color from deep blue near the horizon to white near it are often due to dust or suspended water particles in the air, or indicate higher humidity.
As sunlight travels through our atmosphere, it interacts with air molecules which scatter its light like billiard balls – blue and violet wavelengths being scattered more than red ones.
Rayleigh Scattering
As sunlight passes through Earth’s atmosphere it becomes scattered in all directions, producing what appears to be blue skies or red ones depending on which wavelengths of light are being scattered; blue and violet light have shorter wavelengths than the longer red and orange lights and therefore are more likely to be scattered by tiny molecules within our atmosphere – this process is known as Rayleigh Scattering; hence why Earthly skies appear bluer.
But this is only part of the explanation; our eyes are more sensitive to blue than other colors, while Earth’s atmosphere tends to absorb blue-violet light more efficiently due to having less oxygen than on other planets – as such we see more blue-violet light being scattered than red and orange light from it being scattered by it.
At higher altitudes and during periods like September’s autumn equinox and December’s winter solstice (when conditions are perfect for viewing stars), skies appear bluer; this also explains why our skies appear so vivid on clear days prior to sunset or sunrise.
As the sun sets or rises throughout the day, its intensity of scattered blue light decreases due to it moving lower into the sky and our atmosphere absorbing less of its energy. As such, more colors scatter into their natural states in the sky, further intensifying scattering effects and making other colors stand out more prominently than before.
Our Earth has many atmospheric gases, most notably Nitrogen and Oxygen, which contribute to its atmosphere and can deflect or scatter light rays. The wavelength of light that is being scattered depends on both frequency of scattered light as well as size of gas or particle in question; Lord Rayleigh first discovered this law during the 1800s; it is known as Rayleigh Scattering Law because light scattering decreases with wavelength (inversely proportional). As such, shorter wavelengths such as blue or violet light are more likely to be scattered than longer wavelengths (physics behind this law). As per his law’s theory, light scattering increases exponentially with wavelength; consequently short wavelengths (blue/violet light) scatter more often than longer wavelengths (such as long).
Blue Light
From a physical viewpoint, color refers to the wavelengths of visible light emitted by objects and striking sensors such as your eye. Isaac Newton demonstrated this with a prism and we know that all colors exist within this spectrum – although as sunlight travels through our atmosphere to reach us through your eyes it loses many shorter wavelengths that scatter more easily, such as blue and violet which become scattered by dust, pollution and water vapor in the air – leaving only red, orange and yellow which form part of any sunset or sunrise scene.
The color of the sky on any given day depends on the concentration of dust and water particles suspended in the atmosphere. Since blue wavelengths scatter more easily than red ones, when air is clean and dry it appears as vibrant hues of blue, while when high concentrations of particles exist they appear paler pinkish or gray-tinged and may even appear more hazy than clear.
Additionally, our eyes are less sensitive to blue light due to the cornea and lens’ ability to filter out ultraviolet rays from sunlight that would damage them. Any shortwavelength blue light that makes its way through can sometimes cause the eyes to appear slightly defocused.
Blue light on Earth is more easily scattered than other colors due to oxygen and nitrogen molecules absorbing it and redirecting its path with higher frequency and energy, leading to its redistribution in another direction (possibly with higher frequency and energy). As such, re-emitted blue wavelengths appear much brighter to us compared with shorter-wavelength visible light spectrum colors with lower frequency emissions.
Why does the sky seem bluer during autumn and winter than spring or summer? Our northern hemisphere tilts away from direct solar radiation during these two seasons. As we approach autumn equinox in September and then winter solstice in December, that tilt allows fewer shorter wavelengths from sunlight to reach our eyes.
Red Light
On any given day, the color of the sky depends on how many dust and water droplets are lingering in the atmosphere. Dust particles scatter light, altering its hue to give an altogether lighter or darker shade of blue hue in the sky. On clear days with minimal dust or water droplets in suspension, its hue appears richer and deeper.
Also, our atmosphere’s gases and molecules scatter sunlight in all directions through Rayleigh Scattering; light waves with longer wavelengths like red and orange pass unimpeded through our atmosphere, but shorter wavelengths like blue and violet scatter more intensely and our eyes respond most strongly to them, leading us to perceive the sky as blue.
However, the science is more intricate. Without an atmosphere, we would likely never witness a blue sky due to light rays hitting Earth and reflecting off its surface without reaching our eyes – or being absorbed by retinal pigments inside our eyeballs and producing that characteristic blue hue that we recognize today.
Sky colors vary significantly depending on where you are in the world, for instance in mountainous regions with blue haze due to the interaction of terpenes found in vegetation with ozone to form fine particles that scatter blue light rays.
Other clouds may appear white or gray depending on the nature and concentration of their particles, which could either scatter all wavelengths equally or scatter only blue light (Tyndall effect). The latter process also accounts for other natural phenomena like jay wings being colored blue or the opalescence of gemstones.
One reason the sky may appear redder during sunrise and sunset is that the angle at which sunlight enters our atmosphere changes, refocusing longer wavelengths of red and orange into our atmosphere more effectively, giving our eyes the impression of seeing brighter and more vibrant skies.
Green Light
What gives the sky its signature hue is Rayleigh Scattering, where light waves interact with air molecules, where shorter wavelengths (such as violet and blue hues) are absorbed and scattered more than longer ones (red and orange hues). Our eyes’ greater sensitivity to blues means we perceive it as sky color.
As the sun rises and sets, you may observe that its impact changes the sky color with each hour that passes. This occurs because when sunlight passes through more of the atmosphere at low elevations nearer to the horizon, more short blue wavelengths may be scattered than when overhead; giving an appearance of a blue hue.
Depending on atmospheric conditions, bluer skies will result. This is because air molecules tend to be smaller, intensifying Rayleigh scattering effects and making for even brighter skies.
Moisture in the atmosphere, such as clouds, can dramatically change the hue of the sky. On a cloudy day, for instance, airborne particles such as dust or droplets absorb or block much of the sunlight that would normally reach our eyes and produce grey skies as a result.
Clouds also help us see stars more easily when viewing them at night compared to clear days; their filters make stars appear either more or less bright, depending on the amount of sunlight they block out.
The color of the sky can also depend on how dry or humid the air is. Dry air tends to have lower barometric pressures, meaning less water vapor is held by atmosphere and Rayleigh scattering is reduced on shorter wavelengths, making the sky appear bluer as more of these shorter wavelengths reach our eyes and less of them scattered off by Rayleigh scattering reaches them – also contributing to why clear winter days appear brighter than warm summer ones!