There are a few possible reasons why the sky may appear purple at night. The main factors that can contribute to a purple night sky are:
- dust and haze particles scattered in the atmosphere
- certain types of clouds that reflect light in the purple wavelength
- airglow – a phenomenon caused by chemical reactions in the upper atmosphere
- proximity to city lights which can create a purple glow
- the Purkinje effect – changes in color perception in low light conditions
Let’s explore each of these factors in more detail:
Dust and Haze Particles
One of the most common reasons for a purple tinted night sky is the presence of fine dust, smoke, and haze particles in the atmosphere. During the day, these particles scatter blue light which makes the sky appear blue. At night, when the sun is below the horizon, longer wavelength red light passes through the atmosphere unchecked while shorter wavelength blue light is scattered. The combination of the dominant reddish light combined with some scattered blue light can create a purple hue.
Regions with high levels of air pollution, wildfire smoke, sandstorms, and volcanic ash are more likely to experience this phenomenon and see vivid purple sunsets and night skies. The small particles selectively scatter the blue end of the visible spectrum, allowing more long wavelength red light to pass through.
High Altitude Clouds
Certain types of high altitude clouds can also contribute to a purple night sky. Noctilucent clouds are icy clouds that form in the upper atmosphere around 50-80 km above the Earth’s surface. They consist of tiny ice crystal particles that can refract and scatter light. These clouds tend to have a blue or purple hue and are most visible and brightly colored when illuminated by sunlight from below the horizon after dusk. The lights of cities and towns can also illuminate noctilucent clouds at night.
Other high altitude clouds like cirrus, cirrostratus, and cirrocumulus clouds may also scatter light at night and create purple fringing due to their composition of ice crystals. These thin, wispy clouds contain millions of hexagonal ice crystals that can produce halos and colorful effects around the moon and stars at night.
Airglow
Airglow is light emitted naturally by the Earth’s atmosphere. It is caused by various chemical reactions in the ionosphere and thermosphere regions of the upper atmosphere approximately 80-500 km above the Earth’s surface. The dominant airglow emission occurs at a deep red wavelength of 630 nm but secondary emissions also occur at shorter blue and green wavelengths. The combination of the red airglow blended with shorter wavelength emissions can lead to the faint purple glow to the night sky.
Airglow is more noticeable and vivid at high altitudes. When observed from sea level it manifests as a very faint homogeneous glow to the night sky with possible hints of purple. Airglow can become more pronounced and colorful in regions with stable air such as over the southern oceans.
Proximity to City Lights
The illumination produced in urban areas can also impact the visible color of the night sky. The orange sodium glow from street lights blended with other artificial lights can create a purple hue in the atmosphere. This is most noticeable on cloudy nights when the city lights are reflected back down from the cloud base. The effect is amplified when cloud density increases as light scatters through multiple layers of cloud droplets and ice crystals.
Light interaction with air pollution like ozone and nitrogen oxides from car exhaust can also add to the purple glow over cities at night. This urban airglow tends to be most obvious when observing the night sky from vantage points just outside of major cities rather than from downtown areas.
The Purkinje Effect
The way our eyes perceive color at night also plays a role in the phenomenon of purple night skies. As light levels decrease at nighttime, the eye’s color-sensitive cones become less effective. The dim light is detected mainly by the rod cells which do not perceive color very well. This adaptation of the eye to low light conditions is called the Purkinje effect.
What this means is that our color vision and ability to discriminate between different wavelengths is impaired. Reds will appear darker and blues/purples can dominate. Rather than seeing the night sky as shades of blacks and blues, the insufficient lighting can cause an apparent purple hue. The sky still contains the whole visible color spectrum but our night vision tricks us into seeing just the purple portion.
When is the Purple Sky Most Visible?
Certain conditions make the purple sky more likely to appear and be most vivid:
- At latitudes above 50° North – Where noctilucent clouds are more prevalent in summer
- In dry, dusty regions – More particles to scatter light
- After volcanic eruptions – tons of particles injected into the stratosphere
- Near large population centers – Urban light pollution and smog
- High elevation locations – Less atmosphere to look through, airglow more apparent
- During seasons with more high-level clouds
The purple sky usually first becomes noticeable at dusk as twilight fades after sunset. This is when the sky is still bright enough for our color perception and we can observe the scattering effects of clouds and haze most clearly.
Let’s compare how the prevalence and clarity of a purple night sky changes with different amounts of natural light:
| Time of Night | Visibility of Purple Sky |
|---|---|
| Early dusk | Apparent but muted by brightness |
| Mid-twilight | Vivid and pronounced |
| Full darkness | Still visible but very faint without sunlight |
| Moonlit night | Enhanced slightly by moonlight illumination |
| Astronomical twilight | No longer visible as sun 18+ degrees below horizon |
The purple night sky is at its most brilliant in the deeper twilight stages around the end of civil twilight when the sun reaches 6 degrees below the horizon.
Historical References to the Purple Sky Phenomenon
The striking sight of the purple sky at night has been documented for thousands of years:
- In ancient times, the purple glow was attributed to the presence of dragons in the sky
- Homer referenced the “purple sky” in his epic works the Iliad and the Odyssey
- Pliny the Elder described a purple glow permeating the night sky during the Roman Empire
- In 1883, the eruption of Krakatoa led to vivid red sunsets and purple night skies worldwide
- During the winter of 1884, newspapers widely reported the unusual prevalence of purple nighttime skies
- Edvard Munch depicted a purple sky in his famous 1893 painting The Scream
The natural phenomena that color the night sky often evoked wonder, fear, and even superstition in ancient observers without the scientific knowledge to understand what they were witnessing.
Conclusion
In summary, several different mechanisms can lead to the occasional phenomenon of purple-hued night skies. Tiny airborne particulates, high altitude cloud conditions, airglow emissions, light pollution, and the limitations of human color vision after sunset all intersect to produce this unusual sight. While a vibrant violet sky at night may seem mysterious or unnatural, it is a result of natural atmospheric phenomena interacting with normal physics and perception. Paying attention to the color of the sky offers clues into the atmospheric makeup and visibility each night.
