Many people have gazed up at the night sky and noticed that it often appears to have a purple or violet hue, especially on the horizon. This phenomenon has sparked questions about why the sky takes on this colorful appearance after sunset. The reasons behind the purple night sky have to do with the physics of light and the way our atmosphere scatters different wavelengths of visible light.
Rayleigh Scattering
The primary reason that the night sky looks purple is due to a phenomenon called Rayleigh scattering. This refers to the scattering of light off of particles in the atmosphere that are smaller than the wavelengths of visible light. During the day, sunlight enters the atmosphere and its component colors with shorter wavelengths, like blue and violet light, are scattered more strongly than colors with longer wavelengths, like red and orange. The extra scattering of blue/violet gives the daytime sky its familiar bright blue appearance.
But this scattering process doesn’t stop when the sun sets. Even without direct sunlight, there is scattering of ambient light in the atmosphere. At night, there is very little light at the red end of the spectrum coming from sunlight. However, artificial lighting on the ground emits some longer wavelength orange and red light that manages to bounce around and contribute to the night sky. But this long wavelength light is not scattered very much compared to the remaining blue/violet light coming from scattering of moonlight and starlight. With less red light to balance it out, the predominance of scattered blue and violet light makes the night sky take on a purplish tone.
Altitude Effects
The purple hue of the night sky is most pronounced closer to the horizon. Overhead, the sky tends to transition to a darker blue color. This effect occurs because there are more air molecules lower in the atmosphere to scatter the light at altitudes near the horizon. Light coming in at a shallow angle travels through more atmosphere than light coming from directly overhead, resulting in stronger scattering at the blue and violet end of the spectrum that gives a purple tint when looking out towards the horizon.
Overhead, there is less scattering between the viewer and the upper reaches of the atmosphere. Without as much blue/violet light removal, the sky takes on a darker blue or nearly black appearance rather than purple when looking straight up on a clear night.
Particles and Pollution
The composition of the atmosphere can also have some influence on the purple night sky effect. Besides nitrogen and oxygen, natural atmospheric particulates like dust, soot, and water droplets can contribute to scattering. These natural particles tend to be on the smaller side and thus scatter shorter wavelengths more efficiently, adding to the violet/blue light distribution.
In polluted urban areas, artificial particulate matter from vehicle emissions and industry can greatly increase the amount of scattering particles. Just as polluted daytime air appears hazier and milkier, increased nocuturnal scattering from pollution exaggerates the purple hue, occasionally making city night skies appear intensely violet. Cleaner country air means fewer scattering particles and slightly darker skies.
Cloud Color and Cover
In addition to air molecules and particles, clouds can also scatter light at night. Being composed of tiny water and ice droplets, clouds can efficiently scatter short wavelength blue light coming from the moon and stars just like the clear air. This provides another source of scattered violet light that contributes to the typical purple coloring.
Thick or dense cloud cover acts like an overhead diffuse light source, creating a more uniform night sky color. Whereas clear skies allow the light at the horizon to pass through more atmosphere than the zenith, heavy cloud cover creates more uniform scattering across the viewable sky for a consistent purple hue.
On partly cloudy nights, variations in cloud cover and density will result in a mottled sky with patches of darker purple and black visible in clearer areas. Backlit clouds near the moon or city lights often appear purple-tinged as well due to the same scattering principles.
Altitude Effects
The purple hue of the night sky is most pronounced closer to the horizon. Overhead, the sky tends to transition to a darker blue color. This effect occurs because there are more air molecules lower in the atmosphere to scatter the light at altitudes near the horizon. Light coming in at a shallow angle travels through more atmosphere than light coming from directly overhead, resulting in stronger scattering at the blue and violet end of the spectrum that gives a purple tint when looking out towards the horizon.
Overhead, there is less scattering between the viewer and the upper reaches of the atmosphere. Without as much blue/violet light removal, the sky takes on a darker blue or nearly black appearance rather than purple when looking straight up on a clear night.
Purple Sunset Colors
| Wavelength (nm) | Color |
| 700-635 | Red |
| 635-590 | Orange |
| 590-560 | Yellow |
| 560-490 | Green |
| 490-450 | Blue |
| 450-400 | Violet |
The various sunset hues of yellow, orange, and red occur for the same reasons that the night sky turns purple. During twilight around sunrise and sunset, the sun’s light passes through more atmosphere than during the day when the sun is higher overhead. With sunlight taking a longer path and scattering at a shallow angle, the blue and violet wavelengths are scattered away, leaving predominantly longer wavelengths of yellow, orange, and red light to reach our eyes. The table above shows the visible color spectrum and corresponding wavelengths.
As the sun continues to set, the long wavelength sunset colors eventually fade. This leaves behind the shorter wavelength blues and violets to dominate the growing darkness, giving rise to the purple night sky. So in essence, the colorful sunrises and sunsets and the purplish night sky arise from opposite ends of the same scattering phenomenon.
Moon and Starlight
The moon and stars contribute some of the visible light at night. Moonlight, being reflected sunlight, contains a full spectrum of visible wavelengths. Starlight, on the other hand, originates from hot glowing gases that emit light in narrow bands or lines. But combined, their light still shows a distribution favoring shorter visible wavelengths. When this celestial light is scattered by the atmosphere, the violet components remain to tint the night sky purple.
Without moonlight and starlight, the night sky would appear much darker. But the total illumination level is still far lower than daytime sunlight. So nighttime scattering is much less intense overall, requiring dark adjusted eyesight to perceive the delicate purple hues. The varying phase and position of the moon can influence the purple sky’s brightness and hue night to night.
Light Pollution
Increasingly, artificial light pollution from expanding urbanization competes with and overwhelms natural nocturnal illumination. Street lights, signs, and interior lighting escape upward into the night sky, altering its natural coloration. Light pollution tends to favor longer wavelengths like orange sodium vapor lighting. When this artificial light scatters together with remaining moonlight and starlight, the balance shifts toward redder hues, sometimes eliminating the natural purple coloring of unpolluted night skies.
To experience the phenomenon of the Earth’s shadowed atmosphere scattering violet hues, the best viewing conditions are far from city lights on clear, moonless nights. The pronounced purple coloration can serve as a reminder of nature’s omnipresent scattering processes and the delicate interplay of light and sky.
Conclusion
The muted purplish tones that often grace the night sky arise primarily from selective scattering of short wavelength blue and violet light. This occurs due to the Rayleigh scattering process as natural light illuminating the nocturnal atmosphere encounters gas molecules and particulates. The effect is heightened near the horizon, where light passes through more air mass and scattering particles. Cleaner air allows the process to manifest more vividly, while pollution dampens the effect.
So the next time you admire a dazzling sunset or a beautifully clear twilight full of oranges and reds, remember that the other end of the spectrum is still at play. As the sun disappears below the horizon, keep gazing into the growing darkness. Let your eyes adjust, and enjoy the delicate and intriguing phenomenon of the purple night sky.
