Fine volcanic aerosols in the stratosphere scatter blue light which, when mixed with ordinary sunset red, produces a violet hue. The purple color is often preceded by a yellow arch hugging the horizon. As the sun sets, violet beams emerge from the yellow, overlapping to fill the western sky with a soft purple glow.
While it's not uncommon to spot purple skies during sunsets or sunrises, we can't help but wonder what causes them.
In the case of a purple sunrise or sunset, cloud cover needs to be present. Pink wavelengths, caused by the sunrise or sunset, create an optical illusion as they reflect off the base of clouds. However, when these clouds are dark or the sky behind the cloud is dark, this pink can seem like purple.
Besides atmospheric gases, water droplets, and dust particles, air pollutants also determine the sky's color at sunrise and sunrise. Aerosols suspended in the air scatter sunlight into a band of colors. When there are more aerosols or smog, more sunlight is scattered, resulting in purple or pink sunsets.
The purple sky can also be seen when sunset or sunrise. When the sun sets or rises, it casts a pink and orange glow on the clouds. This glow reflects the water droplets and ice crystals in the clouds. This reflection causes the sky to appear purple.
If we judge by the most prominent color, the sky is violet. But the sky appears blue due to the limitations of our eyes. Our sensitivity to light decreases as we reach the shortest wavelengths of the visible spectrum. The violet is there, but our eyes detect it only weakly.
A purple sky is a sight to behold, because it looks magical and is quite rare. They can be caused by a whole range of different reasons, which you now know about.
According to SpaceWeather.com, almost 3 months after the Kirul Islands' Raikoke volcano blasted a plume of sulfurous gas into the stratosphere last summer, sunsets around the northern hemisphere were turning purple. Raikoke volcano eruption, June 22, 2019; NASA photo.
Similarly, the vibrant oranges and reds of "clean" sunsets give way to pale yellows and pinks when dust and haze fill the air. But airborne pollutants do more than soften sky colors. They also enhance the attenuation of both direct and scattered light, especially when the sun is low in the sky.
This is due, in part, to the large amount of dust in the atmosphere which helps to refract light from the sun and paint the sky in those brilliant orange, reds and yellows. Here is a compilation of some of the more notable sunsets that we've seen in Tsavo recently.
An afterglow in meteorology consists of several atmospheric optical phenomena, with a general definition as a broad arch of whitish or pinkish sunlight in the twilight sky, consisting of the bright segment and the purple light.
The other colours pass through the Earth's atmosphere to reach us, but because of the great abundance of blue light wavelengths, our eyes see the sky as blue. Technically, the short wavelengths that scatter across the sky correspond to the colours blue and violet, making the real colour of the sky a bluish purple.
Sunlight is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
No color has arguably done so more than the color purple. Specifically, Tyrian purple, the production of which was a closely guarded secret for millennia, making the dye the rarest and most expensive color in history.
Blue is one of the rarest of colors in nature. Even the few animals and plants that appear blue don't actually contain the color. These vibrant blue organisms have developed some unique features that use the physics of light.
An exotic colour at the far end of our visible spectrum and often associated with royalty, purple is relatively rare in nature. But some vibrant plants, animals and fungi do show off a regal purple, using it to warn predators, attract pollinators and protect themselves from the Sun.
You may have heard that air pollution, dust and even smoke from wildfires far afield can make our sunsets more vivid, but it's actually the opposite that is true, according to National Geographic.
On the other hand, more dust and smog (at sunset) can have the effect of scattering light across a greater region of the sky, creating a larger drape of colors, whereas sunrise colors tend to be more focused around the sun.
According to urban legend, air pollution enhances the beauty of a sunset. And pollution does indeed change the appearance of sundown, but whether it tips it in the direction of beauty is a matter of personal taste—and the overall amount of that pollution in the air.
On Mars, the atmosphere is mostly CO2 and filled with iron-rich dust, which scatters the red light, turning the sky there… red. Except at sunset, when the longer trip through the Martian atmosphere scatters and filters so much red light that blue light passes through—making the sunset… blue!
But when you limit yourself to “natural” sunset colors, violet and/or indigo are the most uncommon.
Different shades of purple have different spiritual meanings. For instance, light purples are associated with light-hearted, romantic energies, while darker shades can represent sadness and frustration. In some parts of Europe, purple is associated with death and mourning.
While a green sky is often an indicator of a severe storm that can produce tornadoes and damaging hail, a green sky does not guarantee severe weather, just as tornadoes can appear from a sky without a hint of green. So, the reason for green skies before a storm isn't entirely known.
Given the number of factors involved it isn't unreasonable to suppose that exoplanet skies could be any colour at all – from blue or cyan, through green and yellow to red, orange and purple – even brown and white are possible.
In some cases, a purple sky may simply be the result of light refraction. When the sun is low on the horizon, its light has to travel through more atmospheric pollutants, which can cause the sky to take on a purple hue. However, a purple sky can also be caused by hurricanes, wildfires, or dust storms.