Sunsets And Pollution: What's The Link?

Sunsets are a natural phenomenon that occurs when the sun is low on the horizon, and its light passes through more air in the atmosphere than it does when the sun is higher in the sky. The colours of the sky during sunset result from sunlight interacting with molecules in the air, primarily nitrogen and oxygen, which cause it to be deflected in all directions, a phenomenon called Rayleigh scattering. While sunsets are often associated with vibrant colours, the presence of pollution can alter their appearance.

So, does pollution cause sunsets?

Sunsets are caused by a phenomenon called Raleigh scattering

Sunsets are a result of a phenomenon called Raleigh scattering, which is the scattering of light by small molecules in the atmosphere. This scattering effect is caused by particles that are about one-tenth the size of the wavelength of light. The particles may be individual atoms or molecules, and the phenomenon is most prominently visible in gases.

During sunrise and sunset, when the sun is lower in the sky, its light must travel through a greater thickness of the atmosphere. This results in more scattering of light, particularly the shorter wavelengths of blue and green, which are scattered more easily. The longer wavelengths, such as yellow, orange, and red, are able to make it through, creating the warm hues often associated with sunrise and sunset.

The type of clouds present can also affect the quality of a sunset. Typically, the best sunsets are seen in skies with high and mid-level clouds, while low-lying clouds rarely yield noteworthy sunsets. This is because the higher-level clouds intercept more sunlight that has not been muted by passing through the boundary layer, which is the space between the upper atmosphere and the Earth.

While Raleigh scattering is a natural phenomenon, human activity can also contribute to the intensity of sunset colours. Human-generated aerosols, such as soot from internal combustion engines and sulfuric acid from burning fossil fuels, can enter the atmosphere and scatter light, enhancing the sunset's colours. Wildfires, volcanic eruptions, and industrial activities can also release particles into the atmosphere that contribute to more vibrant sunsets.

However, it is important to note that an excess of pollution will dampen the overall sunset experience. Large particles in the air, such as dust, smoke, or pollutants, absorb more light than nitrogen and oxygen, scattering wavelengths of light mostly equally and muting the colours of a sunset. Therefore, while some pollution can enhance sunset colours, too much pollution will reduce the vibrancy and purity of colours.

Human-generated aerosols can cause sunsets to appear redder

The vibrant colours we see at sunset are the result of a phenomenon called Raleigh scattering. During sunset, light must travel through more atmospheric particles to reach us, and shorter blue and green wavelengths scatter more easily, so they have a harder time making it through. On the other hand, the longer wavelengths, which produce orange and red colours, do make it through, giving us those rich, vibrant colours.

The number of aerosols produced by human activity far exceeds natural sources. In major cities, smog is a human-made mixture of chemicals, including chlorofluorocarbons (greenhouse gases), hydrocarbons, sulphuric acid, and other unpleasant pollutants. While volcanic smog and forest fire smog are composed of fairly uniform aerosols, man-made smog is composed of countless different molecules and different types of matter, and all of these aerosol particles vary in size.

The size of the particles in the atmosphere determines the colour we see. When something like a volcanic eruption sends particles into the air, it produces smog that blocks only certain wavelengths of light. However, when there are too many large particles in the air, they scatter all colours indiscriminately, increasing the overall brightness of the sky but dampening colour contrast. Therefore, while human-generated aerosols can cause sunsets to appear redder, excess pollution will also dampen the overall sunset experience.

Natural causes of smog are negligible in big cities

The vibrant colours of a sunset are the result of a phenomenon called Raleigh scattering. During sunrise and sunset, light must travel through more atmospheric particles to reach us, and shorter blue and green wavelengths scatter more easily, while longer wavelengths, which produce orange and red colours, make it through, giving us those rich, vibrant colours.

While natural sources like forest fires, mineral dust, sea spray, and volcanic eruptions produce aerosols, in big cities, the number of aerosols produced by human activity far exceeds natural sources. Human-generated aerosols are released into the atmosphere through car exhaust, coal power plants, industrial emissions, and the burning of fossil fuels. These activities are more common in big cities, and as a result, the natural causes of smog are negligible in such places.

Photochemical smog, also known as "Los Angeles smog," occurs in urban areas with a high number of automobiles. It is caused by nitrogen oxides and hydrocarbon vapours emitted by automobiles and other sources, which then undergo photochemical reactions in the lower atmosphere. The resulting smog causes a brownish discolouration of the atmosphere, reduced visibility, plant damage, eye irritation, and respiratory distress.

In addition to vehicle emissions, industrial pollution and the solvents used in manufacturing plastics, textiles, and agricultural products contribute to smog. The burning of sugarcane as a harvesting method, for example, is a significant cause of air pollution in Bangkok. Similarly, Mexico City, Mexico, which was once known for its clean air, is now considered one of the smoggiest places globally due to industrial pollution.

To reduce smog and improve air quality, individuals can make behavioural changes such as driving less, using electric appliances, and avoiding products that release high levels of VOCs. Additionally, governments can implement policies to address sources of smog and reduce pollution, although these measures may be politically unpopular or challenging to enforce.

Sunsets are more vivid in the eastern US during late fall and winter

Sunsets are more vivid and colourful in the eastern US during late fall and winter. This phenomenon is caused by a combination of factors, including changes in wind patterns, the angle of the sun, and the presence of clean air.

During late fall and winter in the eastern US, winds from the northern part of the continent blow south, clearing haze particles from the summer months and replacing them with cleaner, drier air. This cleaner air contributes to the increased colour vibrancy observed during sunsets. Additionally, as the seasons transition from summer to fall and winter, the sun hits the Earth at a lower angle, resulting in more atmospheric scattering before the light reaches us. This scattering effect is responsible for the vivid colours typically associated with sunsets, with longer wavelengths producing vibrant orange and red hues.

Contrary to popular belief, pollution does not enhance sunsets. While it is true that certain particles in the atmosphere, such as those from forest fires, dust storms, or pollution, can contribute to the scattering of light and the creation of redder sunsets, excessive pollution can diminish the overall sunset experience. Large particles, including dust, smoke, or pollutants, tend to absorb more light and scatter wavelengths more equally, muting the colours of a sunset rather than enhancing them. Therefore, it is the clean air and specific atmospheric conditions of late fall and winter in the eastern US that contribute to the vivid sunsets observed during this time.

The type of clouds present in the sky can also influence the quality of a sunset. High and mid-level clouds, such as altocumulus and cirrus, tend to yield more impressive sunsets compared to low-lying clouds. This is because the higher-level clouds intercept more sunlight that has not been filtered or muted by the lower atmosphere. Additionally, the presence of interesting geographical features, such as mountains or bodies of water, can enhance the overall sunset viewing experience by providing a captivating foreground or reflective surface for the vibrant colours.

In summary, sunsets in the eastern US during late fall and winter are more vivid due to a combination of factors, including cleaner air, favourable wind patterns, and the angle of the sun. These conditions come together to create the perfect environment for the scattering of light, resulting in the vibrant orange and red hues that characterise a breathtaking sunset. So, for sunset enthusiasts, late fall and winter in the eastern US offer the ideal opportunity to witness nature's breathtaking light show.

Excessive air pollution does not result in more beautiful sunsets

The vibrant colours of a sunset are the result of a phenomenon called Raleigh scattering. During sunrise and sunset, light must travel through more atmospheric particles to reach us, and most of the shorter blue and green wavelengths scatter more easily and do not make it through. The longer wavelengths, which produce orange and red colours, do make it through, giving us those rich, vibrant colours.

While it is true that particles in the atmosphere can enhance sunset colours, excessive air pollution does not result in more beautiful sunsets. Large particles in the air, such as dust, smoke, or pollutants, absorb more light than nitrogen and oxygen, and they scatter the wavelengths of light mostly equally, which mutes the colour of a sunset.

As the air pollution gets worse, the sky becomes so saturated that the sun is no longer visible. At this point, the sunset can appear bright but washed out when large numbers of big particles accumulate in the troposphere. 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 dampening colour contrast.

In addition, the type of clouds present can also affect the quality of a sunset. Typically, the best sunsets are seen in skies with high and mid-level clouds like altocumulus and cirrus clouds. Conversely, low-lying clouds like stratus and stratocumulus clouds rarely yield noteworthy sunsets. This is because the higher-level clouds intercept more sunlight that has not been muted by passing through the boundary layer, the space between the upper or "free" atmosphere and Earth.

Therefore, while some pollution can enhance sunset colours, excessive air pollution does not result in more beautiful sunsets.

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