Not many can resist admiring a picturesque blue sky, yet few understand why its hue comes about. No, it doesn’t come from oxygen levels in the atmosphere or because sunlight emits blue rays; there’s another explanation.
Light that reaches Earth’s atmosphere is scattered by gases and particles in the air, with blue wavelength light being scattered the most by gases and particles – this is why skies appear bluer than others.
Light
On a clear day, almost everyone enjoys taking in the sky’s striking blue hue – but have you ever considered why it appears this way? The answer lies within the physics of sunlight passing through our atmosphere – as sunlight bounces off oxygen and nitrogen molecules it scatters light in various directions; blue wavelengths in particular tend to scatter more than other colors and hence make us perceive a sky with blue hues.
Sunlight contains all of the colors of the rainbow. When sunlight enters Earth’s atmosphere, its hue changes dramatically due to the different wavelengths emitted by each color; red has long wavelengths while blue and violet light have shorter ones; long wavelengths tend to get absorbed by atmospheric gases and particles while shorter ones are likely scattered more easily, hence why skies appear bluer.
As the sun sets and rises, you may have noticed its hue dwindle as it nears the horizon. This is due to sunlight having had to travel farther through atmosphere and becoming scattered; consequently, blue part of spectrum dominates over reddish orange of horizon.
When the Sun is high in the sky, its color remains more intense because its more evenly-distributed blue light overwhelms any weakening reds and yellows from its rays.
Weather and chemistry also influence sky color changes; for instance, forest fire smoke or volcanic eruptions could result in bright red skies while pollution or humidity could produce grey ones. No matter the cause, however, any such changes are usually temporary and won’t last too long.
However, even beyond Earth’s atmosphere lies space where skies remain blue. This is because due to Sun being so much bigger than us and taking longer for its light to reach us; more blue light gets scattered before reaching us while less red and violet does; combined together these effects contribute to making the sky appear bluer than it actually is.
Rayleigh Scattering
Light rays that reach Earth’s atmosphere are scattered by molecules primarily composed of nitrogen and oxygen atoms, giving the sky its distinctive blue hue. The intensity of scattering depends on wavelength of the light; shorter wavelengths (such as blue or violet light ) tend to be scattered more heavily than longer ones, giving rise to its distinctive hue.
Physics scientists have a special term for scattering: Rayleigh Scattering. It was named for Englishman John William Strutt, 3rd Baron Rayleigh who first researched it. Rayleigh scattering occurs when light bounces off tiny particles such as gas atoms or water droplets in the air; elastically, these particles can ricochet off each other many times before coming to rest again. As its energy increases, its rate of scattering increases exponentially as does its speed through space.
Scientists have gained a greater understanding of this process over recent years. Now they recognize that particles in the air such as water vapor, dust particles, pollutants and volcanic ash all influence how intensely sunlight scatters around and changes to appearance of sky due to changes in humidity or haze conditions. Additionally, humidity changes also influence its hue as can color shifts as a result of changes to hue.
The sky appears blue because sun rays are scattered by air molecules before reaching our eyes, with shorter wavelengths from blue and violet end of spectrum being more widely scattered than red/orange end of spectrum. On clear days this gives rise to what seems to be an overall blue hue to the atmosphere.
On cloudy days, light rays tend to reflect off clouds rather than scatter. This allows more of the rays to come directly into our eyes and appear brighter; alternatively it may appear darker due to reduced exposure from scattered rays.
As we observe a dark sky, we witness Rayleigh Scattering at work. This phenomenon explains why our skies don’t appear black like in space where there are no molecules present to scatter light back onto Earth.
Humidity
As sunlight reaches Earth’s atmosphere, it is dispersed across space by air molecules and scattered. This effect is especially strong with shorter-wavelength lights such as blue and violet tones; our eyes detect this scattered light as blue sky – this phenomenon was first identified and described by Lord Rayleigh back in the 1870’s.
Humidity refers to the amount of water vapor present in an atmosphere. More moisture holds more heat, creating warmer conditions overall and blurring night skies more noticeably when humidity levels increase significantly. When humidity levels decrease substantially this effect is less pronounced.
The blue color of the sky can be traced to many factors, but humidity plays a large part. Most major cities worldwide are situated near bodies of water or near the equator, making them very humid environments. Climate, pollution and dust also play a factor; for instance, polluted cities might feature skylines with redder hues than nonpolluted ones.
Weather factors play a part in whether or not the sky appears blue or red; clouds and fog can reduce visibility, while strong sunlight can make it appear bluer. Furthermore, its color can change throughout the year: sunsets in humid cities tend to have more orange-red hues due to atmospheric particles absorbing short wavelengths from sunlight that has penetrated more deeply.
Astronauts have reported that even as they fly through clouds and atmospheric particles, the stars appear clear from a space shuttle window. This may be because Mars’ atmosphere is much thinner than that of Earth, so its effects do not have as dramatic an effect on sky color as does Venus’, due to carbon dioxide absorption from visible light absorption gases such as methane.
Reflection
From a physical viewpoint, color refers to the wavelengths of light that leave an object and hit your eyes’ sensors. Sunlight contains all wavelengths within its visible spectrum, so when it hits an environment it illuminates grass, flowers, and wood with color. However, when we view the sky it often appears blue due to shorter wavelengths (blue and violet) being scattered more widely than longer ones (red).
Rayleigh scattering is a process by which light waves collide with air and gas molecules in our planet’s atmosphere – mostly nitrogen and oxygen atoms – and are then dispersed across space by collisions between blue and violet wavelengths and air and gas molecules, and red, orange and yellow wavelengths which pass straight through. Our eyes are more sensitive to shorter blue-violet wavelengths which give rise to our perception of blue skies.
Light passes through water in much the same way, producing similar effects when sunlight strikes it. We see clear or blue waters depending on their chemical composition and size of suspended particles; large bodies of clean water like lakes and oceans often appear blue due to dissolved minerals; however this isn’t the sole factor responsible for their appearance – there may also be other natural processes at work in their appearance.
Humidity also plays a factor in defining the sky’s blue hue; humid skies tend to contain more moisture, leading to clouds condensing water vapor and giving off its signature blue hue. This effect is most notable at sunrise and sunset when light reflects off of this watery cloud cover and back into space.
Chemical composition and particulate matter also influence the color of the sky, such as mountainous regions where aerosols of terpenes in vegetation react with ozone to form blue haze in the air. Sand and dust in the atmosphere may create white or golden haze that scatters all wavelengths equally (Mie scattering), while other types will scatter blue wavelengths more than red ones.