Blue skies form when violet and blue light wavelengths are scattered more effectively by oxygen and nitrogen molecules in the atmosphere than longer wavelengths such as red and orange ones, which pass directly through.
Rayleigh scattering is the name given to this process and it has its roots in 1871 when Lord Rayleigh first identified it. While it’s easy to comprehend why the sky appears blue during sunrise and sunset, explaining its red tint can be more complicated.
Why is the sky blue?
On a perfect summer day, the sky looks blue, with white puffy clouds drifting across its surface like waves in an ocean you could dive into. Scientists have long been confused as to the cause of such beautiful hues; most air we breathe contains only nitrogen and oxygen which do not have their own color.
Light from the Sun travels through our atmosphere and is scattered by molecules of gases in the air, striking at angles to reflect off them in different directions. Light with shorter wavelengths such as blue is more susceptible to being scattered off than longer ones such as red – this process creates what gives our skies their vibrant hue.
But it is more complex than that; several factors play a part, including sunlight’s color itself, how our air scatters different wavelengths of light and our eyes being particularly sensitive to blue light than other colors.
As sunlight travels through Earth’s atmosphere, its longer wavelengths of red and orange get absorbed by air molecules, leaving only shorter blue-violet wavelengths reaching our eyes.
But that is only half the story; other colors also absorb into their environments. For example, when shining a flashlight into a dim room and looking directly at its walls with your flashlight beam, they appear black because the light from its source has been absorbed by them directly. But by moving your torch around slightly and changing its orientation slightly within the room, suddenly brighter walls emerge because more light from your source is now reflecting off of ceiling surfaces instead of simply being absorbed by walls themselves.
Light hitting Earth scatters much in the same manner, except it is usually due to smaller particles in our atmosphere that cause this scattering effect – this phenomenon accounts for why skies tend to be blue; it can sometimes change color due to excessive dust or pollution levels in the air.
The Sun’s light
White sunlight that reaches Earth contains all the colors of the visible spectrum, striking air molecules in our atmosphere and being scattered back in many different directions; shorter wavelengths like violet and blue tend to be scattered more strongly than longer ones, thus giving rise to our familiar sky blue hue. Because we are so used to it, human eyes tend to blend these beams of blue-tinted direct sunlight together into what we know as sky blue – hence making sky blue what it is today!
Isaac Newton demonstrated this point in the 17th century with a prism to break apart light into its individual wavelengths. Sunlight comprises all colors in a rainbow spectrum and each wavelength corresponds to one color of its spectrum; violet and blue wavelengths tend to scatter more easily so they’re removed from what reaches your eyes, leaving mostly red and yellow light behind.
At midday, the Sun would appear yellow with some hint of orange or red; but during sunset or sunrise when light reaches our eyes from deeper within the atmosphere it disperses further, leaving only red and yellow wavelengths reaching directly our eyes.
Refraction refers to this bending of light; the higher up the Sun is in the sky, the more its rays bend (refract) over the horizon; this phenomenon creates the distinct lines on either side of sunrise and sunset horizons.
At night, this same phenomenon explains the differing hues of blue in the sky. As the Sun drops below the horizon, its rays must pass through a much denser layer of atmosphere than when high above. This causes red and yellow wavelengths to get scattered more readily, leaving violet and blue wavelengths more directly reach your eyes – giving rise to darker shades of blue than during the daytime skyscapes. This process produces deeper shades of blue at night than during daytime skies.
Rayleigh scattering
Rayleigh scattering is responsible for giving the sky its blue hue, where light is scattered by particles in the atmosphere much smaller than its wavelength and scattered into an inverse fourth power relationship with wavelength. Short wavelengths (like blue) tend to be scattered more easily than long ones due to air molecules having frequencies closer to that of blue’s natural resonant frequency than they do of red light’s.
When sunlight strikes molecules in the atmosphere, it is scattered in many directions including back toward the Sun, creating what seems like one uniformly blue hue to your eyes. A similar thing occurs with open water bodies such as oceans. Blue wavelengths reflect off them while longer ones get absorbed by their molecules and so appear bluer overall.
Rayleigh scattering also explains why the sky appears gray during a lunar eclipse or when the Sun is lower in the sky at sunset or sunrise. With more atmosphere between it and you and its light source, even more blue light gets scattered and scattered by this process; any remaining light passing through consists of violet, green and red that makes up what your eyes perceive as white light.
The same principles that make Earth’s skies blue also apply to other planets with atmospheres – though their effects may be less apparent than on ours. Jupiter, for instance, boasts an atmosphere similar to ours but with less density. Therefore, its effect is diminished and appears as more of a tan hue compared to Earth.
The atmosphere
The color of the sky depends on how much of its light is scattered by gases and particles in our atmosphere, where long wavelengths such as red, yellow and green light pass directly through to our eyes while shorter blue and violet waves are scattered by gas molecules in the air, creating the appearance of a blue sky.
Atmospheric conditions vary considerably and determine which gases and particles make up its composition, with nitrogen (78%), oxygen (21%), argon gas, water vapor emitted by clouds, raindrops and ice crystals and air being present as clouds, raindrops or ice crystals being the predominant constituents. There may also be smaller amounts of dust, soot, ash or pollen present that contributes to changing its makeup in response to forest fires, volcanic eruptions or pollution events.
As sunlight enters our atmosphere, it travels in an uninterrupted path until something interrupts it – such as dust particles or gas molecules; or perhaps an obstruction like smoke or smog causing gray haze; however if produced by plants such as sulfates or nitrates producing blue hues.
As the Sun rises and sets, its light must travel through vast quantities of atmosphere on its journey; the closer to the horizon it travels through, the more atmosphere must be traversed; this causes blue and violet wavelengths from its light to be scattered by gas molecules in our atmosphere, leaving us with a blue-tinged sky.
When the Sun is higher in the sky, its light has to travel through less of our atmosphere before reaching our eyes, making its wavelengths easier for our eyes to perceive – which explains why eclipses often look red or yellow due to Rayleigh scattering.