At day, the sky appears blue because light is scattered by air molecules; this occurs more with wavelengths of blue light than with any other hues.
Tyndall came up with this solution by shining a beam of white light through a tank filled with clear water containing small particles suspended in it. He found that its light appeared blue from a distance while reddish when seen directly from one end of the tank.
Rayleigh Scattering
The sky is blue because sunlight interacts with atmospheric gases in such a way that more of its shorter wavelengths (such as violet) are scattered towards our eyes, while longer ones pass through unscattered. This process is known as Rayleigh scattering.
Lord Rayleigh first described this phenomenon in the 1870s; as a physicist he also studied sunlight’s thermal distribution and discovered it peaking at specific wavelengths.
He realized the cause was the interaction of sunlight with particles in the atmosphere, such as water droplets, dust particles and nitrogen molecules which make up over 75% of air composition. Rayleigh scattering is the dominant process and produces a lightening effect on sunlight’s color spectrum by scattering blue wavelengths more strongly than red ones.
As such, most of the sunlight that reaches our eyes during the day appears predominantly blue to us; we perceive it as cloudless blue. At sunrise and sunset however, bulk attenuation effects cause this entire curve to shift towards red colors since less blue light is scattered during these times.
This occurs because photon energy inversely proportional to wavelength, meaning longer-wavelength photons have lower energy and are therefore less likely to scatter than shorter-wavelength violet photons (which have higher energies). When this happens, the sky appears duller blue because fewer blue wavelengths return toward the sun while higher-energy violet wavelengths get deflected away more readily.
Another factor is our eyes’ greater sensitivity to blue light; therefore, we see it more vividly than other wavelengths. Without this process in place, however, the sky would appear much duller blue due to all of its gases evenly dispersing light evenly and creating similar hues across its entire expanse.
Ozone Depletion
As soon as sunlight hits Earth’s atmosphere, it is scattered and absorbed by gases in our atmosphere, with shorter wavelengths (blue, violet, green and red) being scattered more intensely than longer ones (orange and yellow). This gives the sky its distinctive blue hue similar to when you switch on an unlit light bulb and see a ring around its circumference.
One of the primary contributors to the blue hue of our sky is ozone. Composed primarily of oxygen molecules, this gas boasts the highest scattering ability among all atmosphere gases. Furthermore, ozone acts as a powerful absorber of UV-B radiation and prevents much from reaching Earth’s surface and being absorbed by our skin cells.
Ozone is an extremely reactive gas, and as such its concentration in our atmosphere naturally fluctuates over time. Without ozone present, skies would appear much less blue! Ozone levels vary with season (lower concentration in tropical latitudes while higher at high latitudes), time of day (peaking during summer days), and cloud cover variations.
At small solar zenith angles, ozone contributes substantially to the blue hue of the sky; however, at larger zenith angles it gradually decreases due to scattered gas molecules from other gases being scattered by sun’s rays, more likely than at smaller ones interacting with stratospheric aerosols such as dust and smog.
At these angles, ozone’s contribution decreases but remains substantial; for instance, at a solar zenith angle of 100deg the contribution is approximately 12 per cent; yet even here its effect remains very substantial compared with that of nitrogen and oxygen due to their smaller atoms not scattering light as effectively.
The Sun’s Position in the Sky
As the Sun moves closer or farther from your position on Earth, its color gradually alters throughout the day due to sunlight’s interaction with our atmosphere. At dawn, sunlight is scattered in all directions by air molecules in Earth’s atmosphere – more so for shorter wavelengths like blue and violet light which tends to scatter more readily than longer ones such as red or orange light; our vision catches more of shorter ones than longer ones in what we perceive as blue sky.
When a rainbow appears in the sky, you’re witnessing its result of Rayleigh scattering and how our eyes perceive different colors. Isaac Newton, famous for his prism experiments in school experiments, made significant contributions towards our understanding of light’s composition and how it breaks up into rainbow hues.
John Tyndall demonstrated this further in 1859 by passing a beam of white light through a clear fluid with small particles suspended within, producing light that looked blue from one angle and red when seen directly from its end.
The blue hue of our skies comes from light’s interactions with nitrogen and oxygen molecules found in Earth’s atmosphere, particularly light waves with shorter wavelengths like blue, violet and white light being scattered more widely than longer-wavelength red, yellow or green light waves.
Our eyes respond most strongly to signals from cones for blue, green, and yellow colors – yet less so for red and violet hues – from cones located within cones that contain cones of cones that detect blue-green-yellow cones; red-violet hues have less of an effect. Combined together these signals create the blue hue we associate with sky. Violet and indigo hues result from reduced scattering; hence their appearance can appear similar but with either an apparent violet or green tinge than full blue.
The Moon
“Once in a blue moon” doesn’t refer to an event on the Moon; rather, it refers to an extremely rare occurrence when high altitude dust particles cause the Moon’s surface to turn blue – known as a blue Moon occurrence and believed to be its original source.
Over the centuries, numerous scientists have attempted to understand why the sky is blue. From expeditions to mountaintops for observation purposes to conducting intricate experiments with glass tubes called cyanometers – scientists from many fields of study attempted to understand why. 19th-century scientific society saw many efforts put forth as their understanding of light’s wave theory evolved rapidly.
Aristotle was among the earliest thinkers to address this question, writing in On Colours that air “appearing on Earth’s surface appears white but is actually dark blue in hue”.
This idea is quite accurate today: aircolour is produced by light passing through it and diffracted differently at different wavelengths; shorter ones like violet are scattered more than longer wavelengths like red; this explains why daytime skies appear blue while sunrise or sunset bring reddish hues.
Other factors contribute to the blueness of the sky as well, including atmospheric refraction that makes the Sun and Earth appear closer during daylight than at night, as well as being composed of lighter basalt instead of granite like on Earth.
All these observations contributed to our current understanding of why the sky is blue. At its core, its science is no more complex than that found elsewhere in nature; simply observe and learn from its colorations as with anything else we encounter in nature.
The beauty of studying the sky lies in its depth. Over time, our knowledge will expand and we may come to appreciate why its hue changes; nevertheless, now is never too late to start exploring!