Imagine an ordinary Tuesday morning, you are in Antarctica, looking up, the sky is so blue it is almost like lightning flashing across it, and it is so clear that you can "taste" it. Then imagine a sandstorm covering the Himalayas. You squint your eyes, trying to find a touch of the same clear blue in the sky, but you only see a blur of milky white. Why are the skies in some parts of the world exceptionally blue, while others are always gray and hazy?
For a long time, we have either been obsessed with the color of the sky, or taken it for granted, or even simply ignored it; however, scientists are discovering that the color of the sky is far more than just an aesthetic issue, but a "record book of the air visible when you look up" that faithfully reflects what is floating in the air around us.
The blue color of the sky comes from a physical phenomenon called "Rayleigh scattering": when sunlight passes through the atmosphere, the nitrogen and oxygen molecules in the air are "swayed" under the action of the oscillating electric field of the light. The electrons in the molecules oscillate and re-radiate light in all directions. The more violent the oscillation, the stronger the radiated light. In visible light, shorter wavelength and higher frequency light accelerates electrons more strongly, so blue and violet light are scattered more conspicuously.
Physically speaking, the sky is actually "purple" because purple light has a shorter wavelength and is more strongly scattered. However, part of the purple light is absorbed in the upper atmosphere, and the human eye is more sensitive to blue. In our visual perception, the sky appears blue as we are familiar with it.
However, the story changes when the air becomes filled with larger particles (i.e., aerosols) such as water vapor, dust, smoke, black carbon, and more. At this time, another type of scattering mechanism is dominant - "Mie scattering": when light encounters these particles that are much larger than the molecular scale, the particles no longer respond uniformly like a "point", but different parts produce complex, multi-directional responses to the same incident light, and the scattered light becomes more uniform at each wavelength. The result is that sunlight of different colors such as blue and red is scattered to a similar extent, and the sky changes from a single blue to a white "milk color". The reason why clouds (composed of tiny water droplets) appear white is essentially the same mechanism.
A new study, which has not yet been peer-reviewed, captures this process of change live. Scientists tracked and analyzed the optical properties of a cloud of dust during a dust storm that crossed the Western Himalayas, measuring its evolution over time and along its path. As dust moves through the air, it continues to mix with pollution particles emitted by human activities; by measuring the degree to which these mixed particles scatter, absorb, and deflect light, the team back-derived their "complex refractive index"—a key physical quantity that describes the intensity and manner in which particles interact with light. They found that when desert dust is mixed with black carbon, sulfate and other pollutants, these "pollution dust" will scatter light in a wider wavelength range and enhance the absorption of light, making the sky appear hazy white or even gray-white.
Amit Singh Chandel, the first author of the paper, explained to Refractor that in the Western Himalayas, people rarely see "pure" mineral dust, but more of a complex "pollution dust": natural mineral particles like "bases" with black carbon, sulfates and other pollutants produced by human activities attached to their surfaces. This mixed state changes the light scattering and absorption cross-sections of the particles, allowing them to both scatter more wavelengths of light and "eat" sunlight more intensely. The more pollutants are attached, the stronger the absorption of sunlight by the mixed particles, the less blue sky is left to the human eye, and the entire sky looks cloudier.
At first glance, this may seem like just a subtle change in the color of the sky, but the implications go far beyond the visual level. Frank Robinson, associate professor of physics at Sacred Heart University in the United States, pointed out that these same aerosol particles also act as cloud condensation nuclei and have an important impact on clouds and weather. This is precisely one of the biggest uncertainties in the current global climate model. In the lower atmosphere, cumulus clouds formed by "helping" the condensation of pollution particles reflect a large amount of sunlight back into space, cooling the earth's surface. On the contrary, cirrus clouds in the upper layers act more like a thermal blanket, enhancing the warming effect.
This effect is often called “masked cooling”: airborne pollutants pose health risks while acting as a parasol, masking in the short term the true magnitude of some of the warming driven by greenhouse gases. If humans were suddenly "enlightened" and drastically removed these pollution particles from the air in a short period of time - and there are good reasons for doing so from a public health perspective - then this "umbrella" would dissipate within a few decades, while greenhouse gases such as carbon dioxide would remain in the atmosphere for hundreds of years. The result is likely to be that in the short term, the rate of global warming will accelerate significantly, because the warming effect that was originally obscured will be "immediately shown off."
Therefore, the blue sky you see when you look up is not just "good weather" in the aesthetic sense, but is often a visual signal of the cleanliness of the air, which is the result of the combined efforts of invisible particles. Why the sky is sometimes so blue and so white at other times is a complex story intertwined with pollution, clouds, aerosols and climate change. Every change in color reminds us that the game between humans and the atmosphere is quietly rewriting the background of the sky above us.