How Pollution Affects Sky Colors

The colours we see in the sky are due to a process called Rayleigh scattering, where molecules in the atmosphere scatter sunlight into different wavelengths of light. Blue light, for example, has shorter wavelengths and is scattered more than red light, which has longer wavelengths. When there are more particles in the air, more light is scattered, and the colours are enhanced. This is why sunsets are often more vivid when there is nearby pollution from forest fires or volcanic eruptions. Human-made aerosols can also cause the sky to appear more orange or red, and heavily polluted cities tend to have more colourful sunsets.

The sky is blue due to Rayleigh scattering

The phenomenon of Raleigh scattering, also known as Rayleigh scattering, is responsible for the blue colour of the sky. Named after the 19th-century British physicist Lord Rayleigh (John William Strutt), Rayleigh scattering is the scattering or deflection of light by particles with a much smaller wavelength than the light itself.

Sunlight, composed of all colours of light, appears as white light when all these colours are together. When sunlight enters the Earth's atmosphere, it is scattered by the particles in the atmosphere, which are mostly nitrogen and oxygen molecules. This scattering of light by particles is what we perceive as the sky's colour.

Blue light is scattered more than other colours because it has shorter, smaller waves. Therefore, when we look at the sky, we predominantly see the blue hues, as the blue light has been scattered more easily to our eyes. This is why the sky appears blue during the day.

However, the sky's colour can change due to various factors, including pollution. At sunrise and sunset, the angle at which sunlight enters the atmosphere changes, and more of the blue and green (shorter) wavelengths of light are scattered before reaching the lower atmosphere. As a result, we observe more of the orange and red colours in the sky during these times.

In addition, the presence of certain particles in the atmosphere, such as those from air pollution, can further enhance the scattering of light and alter the sky's colour. For example, the most heavily polluted cities tend to have more orange and red sunsets due to the abundance of human-made aerosols in the atmosphere. These aerosols, or solid and liquid particles suspended in the air, can originate from both natural processes and human activities. Natural aerosols include forest fires, mineral dust, sea spray, and volcanic eruptions. While pollution can contribute to more vibrant sunsets, it is important to note that excessively polluted skies can also appear washed out, reducing the overall colour contrast and brightness of the sky.

Nitrogen and oxygen scatter light into orange and reddish colours

The colour of the sky is influenced by the scattering of sunlight by molecules in the Earth's atmosphere. This process is known as Rayleigh scattering, and it occurs when sunlight reaches the Earth's atmosphere and is scattered in all directions by gases and particles in the air.

Nitrogen and oxygen constitute the majority of the molecules in our atmosphere, and they play a crucial role in scattering light. While blue light is typically scattered more than other colours due to its shorter and smaller waves, nitrogen and oxygen can scatter light into orange and reddish colours during sunrise or sunset. This is because, during these times, the sun is closer to the horizon, and its light must travel through a greater portion of the atmosphere, resulting in more scattering of longer wavelengths such as yellow, orange, and red.

The presence of additional particles in the air, such as human-made aerosols, can further enhance the scattering of light and intensify the colours of sunrise and sunset. These aerosols can originate from various human activities and natural processes, including forest fires, volcanic eruptions, and sandstorms.

However, it is important to note that while pollution can contribute to more vibrant sunsets, excessive pollution can also diminish the overall experience. When large numbers of big particles accumulate in the troposphere, the closest layer of the atmosphere to the ground, the sunset may appear bright but washed out. This is because aerosols that are similar in size or larger than the wavelengths of visible light tend to scatter all colours indiscriminately, increasing the sky's brightness but reducing colour contrast.

Aerosols are needed for red skies

The sky changes colour due to the scattering of light by molecules in the atmosphere, known as Rayleigh scattering. The most abundant molecules in our atmosphere are nitrogen and oxygen, which scatter light into different wavelengths, with blue light being scattered more than other colours due to its shorter, smaller waves.

However, other molecules in the atmosphere, such as aerosols, can also scatter light. Aerosols are small solid or liquid particles suspended in the air, which can originate from both natural processes and human activity. Natural aerosols include mineral dust, sea spray, smoke from wildfires, and volcanic ash. Human activities, especially since the Industrial Revolution, have also significantly contributed to the abundance of aerosols in the atmosphere. Examples of human-made aerosols include sulphates from the burning of coal and oil, nitrates from combustion engines, and organic and black carbon from combustion.

The presence of these aerosols in the atmosphere can lead to more colourful skies, especially during sunrise or sunset when the Sun is close to the horizon, and light must travel through more of the atmosphere. The scattering of light by these particles, especially when there are a high number of large aerosols, can result in more colourful skies, with enhanced oranges and reds. This is why heavily polluted cities tend to have more orange and red sunsets, and why forest fires or volcanic eruptions are often accompanied by intense red sunsets.

Therefore, while nitrogen and oxygen scattering can explain orange or reddish sunsets, the presence of aerosols is necessary to create the vivid red skies that are often observed in certain conditions.

Excess pollution dampens the sunset experience

The sky's colour is determined by the scattering of sunlight by molecules in the atmosphere. This process is called Rayleigh scattering, and it is influenced by the size of the particles in the atmosphere. Gases and aerosols suspended in the air scatter sunlight into different wavelengths, resulting in colourful skies.

During sunrise and sunset, when the sun is close to the horizon, its light must travel through more of the atmosphere than when it is overhead. This increased distance results in more scattering of light, particularly longer wavelengths such as yellow, orange, and red, creating vibrant sunsets.

While pollution can lead to more colourful sunsets, excess pollution can also dampen the overall experience. Large numbers of big particles in the atmosphere can scatter all colours of light indiscriminately, increasing the sky's overall brightness but reducing colour contrast. This effect is more noticeable when there is a high concentration of large aerosols in the air.

Craig Bohren, professor emeritus of meteorology at Pennsylvania State University, explains that while "pollution" results in redder sunsets, at some point, the air pollution becomes so severe that the sun becomes obscured. The sunset may appear bright but washed out, and the colours become less distinct.

Therefore, while pollution can enhance sunset colours, too much pollution will negatively impact the sunset experience by reducing colour contrast and obscuring the sun.

The sky colour can indicate air pollution levels

When there are more particles in the air, more light is scattered, and the colours are enhanced. These particles can be natural, such as those from forest fires, mineral dust, sea spray, and volcanic eruptions, or they can be human-made, such as aerosols. The most heavily polluted cities tend to have more orange and red sunsets due to the abundance of these human-made aerosols.

However, it is important to note that the effect of pollution on sky colour is complex and depends on various factors, including the size and concentration of the particles. While certain pollutants can enhance colours, excess pollution can also dampen the overall experience, making the sky brighter but less vibrant.

Additionally, different seasons and climates can affect the amount of dust and aerosols in the air, leading to more colourful skies during certain times of the year. For example, in some places, the air tends to be dustier during the summer months due to hot and dry weather, while in the fall, crop harvesting can increase the amount of dust in the air.

By training our eyes to notice these colour changes, we can start to identify even moderate levels of pollution in the sky. For example, a red sky at night, an orange or yellow sky during the day, or fuzzy clouds can all indicate high levels of air pollution.

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