Red Sky Alert: Pollution's Impact And Influence

The colour of the sky is determined by the scattering of sunlight by molecules in the atmosphere. This is known as Rayleigh scattering. Nitrogen and oxygen, the two main molecules in the air, scatter shorter blue and purple light more than red light, which is why the sky usually appears blue. However, during sunrise and sunset, when the sun is close to the horizon, the light must travel through more of the atmosphere, resulting in more scattering of longer wavelengths such as red and orange, creating colourful skies. This scattering of light is also influenced by the presence of particles in the air, such as those from pollution, which can enhance the intensity of colours in the sky.

The scattering of light by particles

The degree of scattering varies depending on the ratio of the particle diameter to the wavelength of the radiation, as well as other factors like polarization, angle, and coherence. For larger diameters, the problem of electromagnetic scattering by spheres was first solved by Gustav Mie, and this scattering is known as Mie scattering. Mie theory, also called Lorenz-Mie theory or Lorenz-Mie-Debye theory, is a complete analytical solution of Maxwell's equations for the scattering of electromagnetic radiation by spherical particles.

Rayleigh scattering, on the other hand, is the scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. It is the dominant form of light scattering in the atmosphere, where light rays hit small objects, such as molecules of nitrogen and oxygen, and bounce off in all directions. Blue light has shorter wavelengths and is scattered more efficiently by small objects, which is why the sky appears blue during the daytime.

When there are more particles in the air, more light is scattered, and the colours are enhanced. Intense red sunsets are often visible when there is an abundance of human-made aerosols in the air, such as from forest fires, volcanic eruptions, or industrial pollution. These particles scatter longer wavelengths of light, such as red, yellow, and orange, creating colourful skies at sunrise and sunset.

However, if there is too much pollution, the sunset's colours may be dampened. When large numbers of big particles accumulate in the troposphere, the sunset may appear bright but washed out. Additionally, aerosols that are close in size or larger than the wavelengths of visible light tend to scatter all colours indiscriminately, increasing the overall brightness of the sky but reducing colour contrast.

The effect of pollution on sunset colours

The colours of a sunset are determined by the way molecules and particles in the atmosphere interact with sunlight. This interaction is known as Rayleigh scattering, and it occurs when light rays bounce off molecules of nitrogen and oxygen, the two main molecules in the air, and other particles, scattering in all directions.

During sunrise and sunset, the sun is closer to the horizon, so its light must travel through more of the atmosphere than when the sun is overhead. This results in more scattering of light, including longer wavelengths such as yellow, orange, and red, which creates colourful sunrise and sunset skies.

The presence of certain particles in the atmosphere can enhance or dampen the colours of a sunset. Natural aerosols, such as those from forest fires, volcanic eruptions, mineral dust, sea spray, and human activities, can scatter light differently and create more vibrant sunsets. The most heavily polluted cities tend to have more orange and red sunsets due to an abundance of human-made aerosols. These small particles scatter shorter wavelengths of light, such as blue and violet, more than longer wavelengths like red, resulting in deeper crimson sunsets.

However, excess pollution can also reduce the overall colour saturation of a sunset. Large particles, such as those found in urban smog or summertime haze, can scatter all colours of light indiscriminately, increasing the brightness of the sky while muting the colour contrast. Additionally, airborne pollutants can enhance the attenuation of both direct and scattered light when the sun is low in the sky, reducing the intensity of sunrises and sunsets.

Therefore, while pollution can indeed enhance the colours of a sunset, particularly the reds and oranges, it can also diminish the overall sunset experience if the pollution levels are too high. The effect of pollution on sunset colours is a complex interplay between the types and concentrations of particles in the atmosphere, scattering of sunlight, and the sensitivity of the human eye.

The role of nitrogen and oxygen molecules

The colour of the sky is determined by the scattering of sunlight by molecules in the atmosphere. This phenomenon is known as Rayleigh scattering. The molecules that make up most of our atmosphere are nitrogen and oxygen. When sunlight enters the atmosphere, it travels as waves of energy, with different colours of light having different wavelengths.

Nitrogen and oxygen molecules scatter sunlight into separate wavelengths of light. Blue light, with its shorter, smaller waves, is scattered more than other colours of light. This is why the sky appears blue during the day. However, at sunrise or sunset, when the sun is closer to the horizon, the light must travel through more of the atmosphere. This results in more scattering of light, including longer wavelengths such as yellow, orange, and red, creating colourful skies.

The presence of certain particles in the atmosphere, such as aerosols, can enhance the scattering of light and intensify the colours of the sky. Aerosols are solid or liquid particles suspended in the air, originating from both natural processes and human activity. Human-made aerosols, in particular, can contribute to the deep crimson sunsets often observed in polluted cities.

While nitrogen and oxygen molecules play a crucial role in scattering sunlight and determining the colour of the sky, they also have a significant impact on air quality. Nitrogen dioxide (NO2), a gaseous air pollutant composed of nitrogen and oxygen, is formed when fossil fuels, such as coal, oil, and diesel, are burned at high temperatures. NO2 contributes to particle pollution and the formation of ozone, with adverse effects on respiratory health.

Additionally, nitrogen oxides (NOx) are produced from the reaction between nitrogen and oxygen during the combustion of fuels, especially at high temperatures. NOx contributes to the formation of smog, acid rain, and the depletion of ozone. While nitrogen and oxygen molecules are essential for Rayleigh scattering and the vibrant colours of the sky, an excess of certain pollutants, such as NO2 and NOx, can negatively affect air quality and have detrimental health impacts.

The impact of forest fires and volcanic eruptions

Forest fires and volcanic eruptions can significantly impact the environment and human health. Forest fires spread air pollution over vast distances, including particle pollution, carbon monoxide, nitrogen oxides, and other hazardous air pollutants, causing respiratory issues even in healthy individuals. Particle pollution, composed of tiny solid and liquid particles suspended in the air, poses risks, especially to children, the elderly, and those with pre-existing health conditions such as asthma, COPD, bronchitis, and heart disease.

The smoke from forest fires can trigger asthma attacks, heart attacks, and strokes and can even be fatal. Research on children in California exposed to smoky air from wildfires revealed increased respiratory issues, including coughing, wheezing, bronchitis, and colds. Additionally, carbon monoxide inhalation can lead to reduced oxygen delivery to the body's organs and tissues, resulting in headaches, nausea, dizziness, and potential premature death in high concentrations.

Volcanic eruptions also contribute to air pollution by injecting volcanic gases, aerosol droplets, and ash into the stratosphere. Gases like sulfur dioxide can cause acid rain and air pollution downwind of a volcano, while volcanic carbon dioxide emissions contribute to global warming. The ash released during volcanic eruptions can travel thousands of miles, posing risks to human health, particularly for infants, the elderly, and those with respiratory conditions. It can also damage infrastructure, such as drinking water and wastewater treatment facilities.

Both forest fires and volcanic eruptions produce aerosols, which are solid or liquid particles suspended in the air. These aerosols can lead to more colorful sunsets, including intense red skies. The particles scatter sunlight, and when there are more particles, more light is scattered, enhancing the colors. While the resulting sunsets may be visually stunning, they are an indication of increased air pollution.

The influence of human activity and aerosols

The appearance of the sky is determined by the scattering of sunlight by molecules in the atmosphere. This process, known as Rayleigh scattering, occurs when light rays hit small objects, such as molecules of nitrogen and oxygen, and bounce off in different directions. As shorter wavelengths of light, such as blue light, are scattered more efficiently by small objects, we typically observe the sky as blue during the day.

However, at sunrise or sunset, when the sun is close to the horizon, the light must travel through a greater portion of the atmosphere, resulting in increased scattering of longer wavelengths such as yellow, orange, and red. This phenomenon gives rise to vibrant sunsets and sunrises.

Human activities, such as burning fossil fuels, deforestation, and industrial processes, release various pollutants into the atmosphere, including aerosols. Aerosols are solid or liquid particles suspended in the air, and they play a significant role in altering the appearance of the sky, particularly at sunrise and sunset. These human-made aerosols can be small enough to scatter longer wavelengths of light, enhancing the intensity of red hues during sunrise and sunset.

The most heavily polluted cities tend to experience more orange and red sunsets due to the abundance of these human-made aerosols. For example, the deep crimson sunsets observed in Los Angeles and other polluted cities worldwide can be attributed to the scattering effects of these pollutants.

Additionally, natural events, such as forest fires, volcanic eruptions, and sandstorms, also contribute to the presence of aerosols in the atmosphere. These natural sources release particles like mineral dust, sea spray, and sulfuric acid droplets, which can influence the colour of the sky.

While the increased presence of aerosols can intensify the redness of sunsets, excessive pollution can also dampen the overall experience. When large numbers of big particles accumulate in the lower atmosphere, the sunset may appear bright but washed out, and the colour contrast may be reduced.

In summary, human activities that release pollutants, including aerosols, into the atmosphere can significantly influence the appearance of the sky, particularly at sunrise and sunset. The scattering of light by these pollutants can enhance the intensity of red hues, creating striking sunsets in heavily polluted areas. However, it is important to note that excessive pollution can also diminish the overall aesthetic experience of a sunset.

Frequently asked questions