As soon as daybreak hits and sunlight hits the atmosphere, light is scattered in all directions – shorter wavelengths of light tend to disperse more rapidly than longer ones and give off a blue hue in the sky.
The same process explains why sunrise and sunset skylines appear orange or red respectively.
The Sun
As soon as sunlight hits Earth’s atmosphere, its color rays are scattered by gas molecules in various directions, with blue hues being scattered more strongly than other colours – thus giving us our blue skies most often!
As soon as you look to the horizon, the sky becomes paler because sunlight had to travel further through the atmosphere before reaching your eyes – meaning its rays had more time to disperse, making the blues less intense while red and yellow hues come through more clearly.
People typically imagine the sky to be blue because light scatters across it in certain patterns; our eyes have also evolved to detect certain wavelengths of light.
As you ascend through the atmosphere, blues become increasingly diffuse as there are fewer molecules to diffuse the light and absorb blue hues into the atmosphere.
Scientists Tyndall and Rayleigh attempted to explain why the sky is blue in 1907; they assumed its color came from small particles of dust and droplets of water vapour in the atmosphere. Later research proved them wrong, leading them to suspect (correctly) that nitrogen and oxygen present in air were responsible.
At home, you can experiment with this theory by placing a piece of white paper on a window and shining sunlight through it. After doing this, look through the paper – you should observe that when there’s paper present on the window, the sky appears bluer than when no paper exists on it!
But that isn’t the entire reason, since our sun is yellow; had it been blue, the sky would likely have been orange or pink! Luckily there are other factors at work here.
The Earth’s Atmosphere
Not only can our atmosphere provide us with oxygen to breathe, water vapor to sustain life, carbon dioxide to warm the planet and a blanket of nitrogen giving the sky its color; but it’s also home to many other surprises–including an unifying blue hue found across altitudes, landscapes and climates.
Scientists first began to gain a better understanding of why the sky was blue at the end of the 19th century. Two British physicists named John Tyndall and Lord Rayleigh independently proposed their theory of light scattering by particles in our atmosphere as being responsible. Their reasoning suggested that when light passes through our atmosphere, gases or small particles smaller than wavelength of light scatter it differently, giving blue light its characteristic hue when directed back toward Earth through absorption by smaller particles that absorb some of its energy and redirecting it toward us all making our skies beautiful blue.
Light from the Sun travels through our atmosphere to reach us, reaching the eyes. Earth’s atmospheric composition consists of an intricate mix of gases dominated by molecular nitrogen (78 percent) and molecular oxygen (21). Other trace amounts include argon (1 percent), water vapor (1 percent), carbon dioxide (0.0395 percent and increasing), methane, and other volatile substances.
As sunlight enters our atmosphere, it mixes all colors in the visible spectrum; most waves have long wavelengths like red and orange waves while blue and violet wavelengths tend to scatter easily through our atmosphere; this phenomenon is called Rayleigh scattering, and accounts for why sky appears blue during the daytime.
The troposphere, or middle atmosphere, extends from approximately 375 miles up to 6,200 miles above Earth and is comprised of the air above this layer. It is the most dynamic portion of our atmosphere where weather occurs as well as most airborne vehicles such as helicopters and light planes flying. Furthermore, most of Earth’s blue hue is generated in this layer.
Due to this layer’s highly active state, the gases comprising it are constantly shifting shape and moving around, leading to turbulence within it and giving our skies their signature blue hue while simultaneously creating clouds we see everywhere we look.
Cyanobacteria
Cyanobacteria (commonly referred to as blue-green algae), one of the first photosynthetic microorganisms to produce oxygen, were among the earliest photosynthetic organisms to produce oxygen. Found everywhere from freshwater lakes and oceans to moist soil and moistened rocks, these unicellular microorganisms have an amazing tolerance to UV radiation, high salinity levels and extreme temperatures over long periods. Furthermore, cyanobacteria form unique relationships with plants, seagrasses, fungi and even sloths!
Cyanobacteria are capable of harnessing sunlight for energy production through photosynthesis, which is a natural process in which carbon dioxide and water convert into oxygen and other organic compounds through natural chemical reactions called photosynthesis. Cyanobacteria are similar to plants when it comes to producing energy via photosynthesis; however, their rapid energy production allowed for the Great Oxygenation Event which occurred approximately 2.5 billion years ago.
One reason the sky appears blue is due to cyanobacteria’s ability to absorb and reflect specific wavelengths of light, helping them more efficiently extract energy from the sun. Red and orange light waves have longer wavelengths while blue and violet have shorter ones; gas molecules in Earth’s atmosphere mainly composed of nitrogen and oxygen scatter these shorter wavelengths into all directions, creating the illusion of blue skies.
An additional factor contributing to the blue sky is cyanobacteria’s use of pigment called phycobilins to absorb red and blue light. This pigment allows them to use photosynthesis energy efficiently while protecting themselves from UV radiation – essential components in phototrophic processes that produce ATP for fueling life processes such as phototrophic reactions.
Cyanobacteria play an integral part in phototrophic processes, yet they’re also capable of being motile, making gliding and swimming movements possible due to a protein associated with flagellar bacteria. They form biofilms on surfaces which are useful for material protection, cleaning procedures, flow-through membranes in medical devices, water treatment systems and ship hulls; however, when created at inappropriate places or times they can lead to biofouling that causes significant issues.
The Great Oxygenation Event
Today’s atmospheric levels of oxygen result from an equilibrium between organisms producing it and those consuming it, but during Archean times things were very different – prior to the Great Oxygenation Event (GOE), oxygen was scarce – estimated as only few micrograms for all of Earth’s atmosphere!
At this point, everything changed thanks to the Greenhouse Effect (GEE). Cyanobacteria became more abundant, producing oxygen through photosynthesis and gradually turning the atmosphere bluer as they did so.
Cyanobacteria were known to produce significant quantities of oxygen, and were even capable of discharging some into the atmosphere due to eating their waste products, leading to an unexpected surge in atmospheric oxygen levels which is thought to have led to most anaerobic bacteria being killed off and ultimately precipitating Earth’s first mass extinction event.
As more oxygen leaked out from cyanobacteria, atmospheric levels continued to increase until they eventually reached what is considered their maximum level of atmospheric oxygen. At this point, anaerobic bacteria could no longer absorb enough oxygen to survive and died out, leaving behind new forms of life that thrived in this newly oxygenated environment.
Oxygen levels remained at this elevated level until more methane was replaced with oxygen, and the cycle began again. Although this phenomenon could have taken place for billions of years before being identified through studies on ancient shale deposits from Western Australia, only recently has its existence been made apparent through one such deposit.
Light traveling through the atmosphere can be scattered in various ways by different particle types present, and this process gives the sky its characteristic color. Shorter wavelengths like blue and violet tend to be scattered more readily than longer ones, creating its bluish hue; similarly this same phenomenon causes sunset or sunrise sunrise to look orange or pink in comparison.