1. Which tiny particles make the sky blue?
When particles that scatter light are much smaller than the wavelength of light, there is a rule to how they scatter it. Light with shorter wavelengths is scattered more strongly, and the strength is inversely proportional to the fourth power of the wavelength. If you compare blue light (about 0.425 μm) and red light (about 0.650 μm), blue is scattered about 5.5 times more strongly than red[1]. The molecules in the air are these "much smaller particles," which is why the daytime sky looks blue.
In the evening or morning, sunlight travels through a longer path in the atmosphere. During this time, blue light is scattered sideways and diminishes, while red light reaches your eyes[2]. So, what happens to the color if larger particles than air molecules increase in the sky?
2. What grows more common in the air when the sky turns whitish?
When particles become about the same size as the wavelength of light, or larger, the difference in how colors are scattered becomes smaller. Because all colors are scattered almost equally, the contrast between colors weakens, and the sky looks bright, whitish, or grayish[3].
In Japan, the yellow dust common in spring is a familiar example. This is sand blown up by strong winds from deserts and loess areas in East Asia, carried by upper-level winds, and falling. It can make the sky look hazy and yellow-brown. Fine particles smaller than a few micrometers are carried far away, and there have been cases where they crossed the Pacific Ocean[5]. If there are many particles, the sky becomes white. So, if particles increase, does the sunset always fade?
3. Does polluted air make sunsets redder?
Meteorologist Bohren states that small particle pollutants make sunsets redder. However, if pollution is too severe, the sun itself becomes unclear, and the colors are no longer vivid[3].
This means that not just the "size" of the particles but also the "amount" matters. A moderate amount of fine particles strengthens red, while too many particles change the color to a colorless brightness. Polluted air does not always result in a red sunset. Of course, polluted air is not good for health, and it is not something that should exist just for red sunsets. The color of the sky is a clue to understanding the particles floating there. There is also a natural force that carries many particles to high places: volcanoes.
4. What did the sky in faraway Norway show after the Krakatau eruption?
Krakatau erupted on August 27, 1883. One to three months later, observers at the observatory in Kristiania (now Oslo), the capital of Norway, were surprised by an "extremely strong red glow." This change in twilight was said to be seen in Norway from late November 1883 to mid-February 1884[9]. This is a place quite far from the eruption.
There is a theory that this sky is connected to the painter Munch’s "The Scream" (1893). The theory suggests that the blood-red sky was based on the experience of the twilight after Krakatau, and the place said to have seen the same sky is a southwest-facing road. However, this is not a confirmed fact. There are other theories that suggest mother-of-pearl clouds or other causes[9]. So, why did the particles from the eruption reach faraway Norway and dye the sky?
5. Why do volcanic particles color the sky for years high above the ground?
Volcanoes send sulfur dioxide gas into the stratosphere. In the stratosphere, this gas changes into sulfuric acid particles through chemical reactions[8]. Unlike the lower sky where rain falls, particles in the stratosphere do not fall immediately and spread around the world.
In the 1991 eruption of Mount Pinatubo in the Philippines, about 20 million tons of sulfur dioxide entered the stratosphere. It spread worldwide and temporarily lowered the Earth's temperature by about 0.5°C from 1991 to 1993[6]. Photos taken by astronauts within two months of the eruption show two dark layers of aerosols at the horizon. The altitude is about 20–25 km, and these aerosols caused many phenomena, from "spectacular sunsets" to global cooling[7].
Even after sunset on the ground, particles in the stratosphere are still exposed to sunlight. The western sky is dyed after the sun has set and the ground is dark because of this[8]. On September 5, 1883, in Honolulu, Hawaii, a minister named Bishop noted a ring of light spreading around the sun. This is said to be the first observation record of the phenomenon later called "Bishop’s Ring"[10]. Observations after Pinatubo showed that particles of about 0.8 μm created this ring[8]. After the Krakatau eruption, it came to be thought that this haze of sulfuric acid particles caused the spectacular sunsets and Bishop’s Ring[10]. The size of the particles does not seem to only create red skies.
6. Can volcanic particles make the sky look greenish instead of red?
A report by the Royal Society in the UK (1888) after the Krakatau eruption contains records of blue and green suns in the tropics and greenish twilight in the UK. A 2024 study tried to answer this mystery. Model calculations show that greenish twilight occurs when the radius of sulfuric acid particles in the stratosphere is about 500–700 nm, the particle sizes are uniform, and the amount is sufficient. The calculation reproduced this under conditions where the optical thickness (a measure of particle amount) exceeded about 0.2, but this is not a strict boundary value[11].
Small particles scatter a lot of blue and leave red. However, with particles close to the wavelength of light, blue is not scattered strongly; instead, red is scattered strongly, and green remains (anomalous scattering)[12]. Even with the same sulfuric acid particles, if the conditions are right, the color reverses. However, this is a phenomenon that rarely happens after major eruptions when conditions are right; it is not always seen. University news introducing the research explains that the frequency is about once every 100 years[12].
In the January 2022 Hunga Tonga eruption, the plume reached an altitude of about 58 km, and the amount of stratospheric aerosols was the largest since Pinatubo. Within about two weeks, the main substances had circled the Earth at an altitude of 20–30 km[13]. It is reported that vivid sunsets and sunrises were seen almost every day in the Southern Hemisphere, such as New Zealand. This description of vividness is from an astronomy magazine article, while the paper provides the basis for the amount of particles and altitude.
Safety Note: The description of vivid sunrises and sunsets seen almost daily in the Southern Hemisphere, such as New Zealand, comes from astronomy magazines.
7. How can you investigate the color of the sky yourself?
Things you can do today: On a clear day, look up at the sky and compare the intensity of the blue for two or three days. The difference between days with deep blue and whitish days may be related to the size and amount of particles in the air. However, the color of the sky also changes with clouds and the height of the sun, so do not assume it is only due to particles. It is dangerous to look directly at the sun, so look in directions away from the sun.
Things you can do when you go out: Keep watching the western sky in the evening after sunset. If you note the difference between days with deep red sunsets and pale days along with the weather and air conditions, you can review them later.
Official information: The Japan Meteorological Agency has explanations of yellow dust in its materials[5].