Bronte Wells
The colours we see in the sky are the result of sunlight being scattered by molecules in the Earth's atmosphere. This process, known as Rayleigh scattering, typically results in blue light being scattered more than other colours due to its shorter, smaller waves, giving the sky its characteristic blue appearance. However, when there are more particles in the air, such as human-made aerosols caused by pollution, the light is scattered differently, affecting the colour of the sky. This is particularly noticeable during sunrises and sunsets when the Sun is closer to the horizon, and its light must travel through more of the atmosphere, resulting in more scattering of longer wavelengths like yellow, orange, and red. Intense red sunsets, orange or yellow skies during the day, and fuzzy clouds can all indicate high levels of air pollution. While pollution can enhance the colours of sunsets, excess pollution can also dampen the overall experience by reducing the colour contrast and making the sky appear brighter.
| Characteristics | Values |
|---|---|
| Effect of pollution on sky color | Pollution can affect the color of the sky, making it appear more orange, yellow, or red, especially during sunrise or sunset. |
| Cause of color change | The presence of pollutants, such as aerosols and particles, in the atmosphere scatters sunlight, resulting in enhanced colors during sunrise or sunset. |
| Examples of pollution impact | The Kuwait oil well fires in 1991 and the Mt. Pinatubo eruption were noted to have changed the color of the sky in North America for months. |
| Visibility of pollution | While air pollution can sometimes be visible, such as in heavily polluted cities, it can also be challenging to detect, especially in cities with moderate pollution levels. |
| Impact on sunset perception | While pollution can enhance sunset colors, extremely high levels of pollution can dampen the overall sunset experience by reducing color contrast and making the sky brighter. |
| Indicators of pollution | Red sky at night, orange/yellow sky during the day, and fuzzy clouds can indicate high levels of air pollution. |
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What You'll Learn
How does pollution affect sky colour?
The colours we see in the sky are due to sunlight being scattered by molecules in the atmosphere. This phenomenon is called Rayleigh scattering. The sky usually appears blue because blue light is scattered more than other colours. However, during sunrise or sunset, when the sun is close to the horizon, the light has to travel through more of the atmosphere, resulting in more scattering of light, including longer wavelengths such as yellow, orange, and red.
Pollution can significantly impact the colour of the sky. When there are more particles in the air, more light is scattered, and the colours are enhanced. Intense red sunsets are often visible when forest fires are burning or during volcanic eruptions due to the abundance of aerosols in the atmosphere. The most heavily polluted cities in the world tend to have more orange and red sunsets, resulting from human-made aerosols. These aerosols can be solid or liquid particles suspended in the air, originating from various human activities.
Additionally, certain times of the year tend to have more aerosols in the air, leading to more colourful skies. For example, the air tends to be dustier during the summer months due to hot and dry weather, and in the fall, crop harvesting increases the amount of dust in the air. The Harvest Moon, which occurs in the fall, often appears orange due to the extra dust particles in the sky.
While the enhanced colours caused by pollution can be visually spectacular, they are also an indication of poor air quality. For instance, the sunset may appear bright but washed out when large numbers of big particles accumulate in the troposphere, dampening the colour contrast. Therefore, while pollution can intensify the colours of the sky, it can also detract from the overall visual experience.
Furthermore, the effects of pollution on sky colour can vary depending on the specific pollutants and their concentrations. In some cases, excess pollution may lead to a loss of colour purity in the sky, resulting in a washed-out appearance. Nevertheless, by paying close attention to sky colour, one can become adept at identifying even moderate levels of pollution in the atmosphere.
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The science behind Rayleigh scattering
The phenomenon of Rayleigh scattering is named after the 19th-century British physicist Lord Rayleigh (John William Strutt). In 1869, John Tyndall discovered that bright light scattering off nanoscopic particulates was faintly blue-tinted. He conjectured that a similar scattering of sunlight gave the sky its blue hue. However, he could not explain why this occurred. In 1871, Lord Rayleigh published two papers on the colour and polarization of skylight, thus quantifying the phenomenon now known as Rayleigh scattering.
Simply put, Rayleigh scattering is the scattering or deflection of light by particles with a size much smaller than the wavelength of the radiation. The particles may be individual atoms or molecules, and this occurs when light travels through transparent solids and liquids, but it is most prominent in gases. The scattering of light increases with frequency. Therefore, light with higher frequencies undergoes more lateral scattering than light with lower frequencies.
Sunlight is made up of all the colours of light. It appears as white light when all the colours are together. Sunlight travels as waves of energy, and different colours of light have different wavelengths. Red light has long wavelengths, while blue light has short wavelengths. When light from the sun interacts with our atmosphere, the lateral scattering of blue light is much greater than the lateral scattering of the rest of the visible spectrum, and this is why the sky appears blue.
The angle of sunlight as it enters the atmosphere also affects the colour of the sky. During sunrise or sunset, when the sun is close to the horizon, the 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.
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The impact of pollution on sunsets
However, pollution can significantly influence the colour of the sky, especially during sunsets. Sunsets occur when the Sun is close to the horizon, causing sunlight to travel through a greater portion of the atmosphere. This increased path length results in more scattering of light, particularly longer wavelengths such as yellow, orange, and red, leading to vibrant sunsets.
The presence of pollutants in the atmosphere, such as human-made aerosols, can further enhance the scattering of light. Aerosols are solid or liquid particles suspended in the air, originating from both natural processes and human activities. Natural sources of aerosols include forest fires, mineral dust from sandstorms, sea spray, and volcanic eruptions. Human activities, such as burning fossil fuels and industrial emissions, also contribute to aerosol production.
When there is an abundance of these particles in the atmosphere, they scatter sunlight, intensifying the colours of sunsets. This is why heavily polluted cities often experience more orange and red sunsets. The most notable example of pollution's impact on sunsets was observed in 1991 when the Kuwait oil wells were burning or during the eruption of Mount Pinatubo, resulting in strikingly altered sky colours.
However, it is important to note that excessive pollution can also diminish the beauty of sunsets. When large numbers of big particles, such as excess aerosols, accumulate in the troposphere, the closest layer of the atmosphere to the ground, they can scatter all colours of light indiscriminately. While this increases the overall brightness of the sky, it can wash out the colours, reducing the contrast and vibrancy of sunsets. Therefore, while some pollution can intensify sunset colours, extremely high levels of pollution can have the opposite effect, resulting in a dull and washed-out appearance.
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How to identify air pollution in the sky
The colour of the sky can be an indication of air pollution. When there are more particles in the air, more light is scattered, and the colours are enhanced.
- Sunset colours: The most heavily polluted cities tend to have more orange and red sunsets, resulting from an abundance of human-made aerosols. Intense red sunsets can also be visible when forest fires are burning nearby or during volcanic eruptions.
- Moon colour: The Harvest Moon in autumn often appears orange due to the extra dust particles in the sky, indicating poor air quality.
- Haze: Haze is usually grey-white and uniform in texture. It can come from local traffic, industry (e.g. coal-burning power plants), or sources farther away.
- Dust: Dust tends to be tan, although the colour varies depending on the type of soil. It is common during the summer months due to hot, dry weather and during the fall harvest season.
- Smoke: Smoke ranges from brown to grey-white. It can come from forest fires, volcanic activity, or human activities such as industrial combustion or residential cooking and heating with wood or coal.
- Satellite images: Satellite images can help identify large areas of pollution caused by fires, dust or sandstorms, volcanic eruptions, industrial sources, or the transport of pollution from other regions.
While it can be challenging to identify specific types of pollution, observing the sky's colour and content (such as haze, dust, or smoke) can provide valuable information about air quality.
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The effect of pollution on the moon's colour
The Moon is often considered a source of natural light pollution, as its brightness can wash out the night sky and hinder the observation of stars and planets. However, the Moon itself can also be affected by light pollution, particularly when there are high levels of air pollution in the Earth's atmosphere.
The Moon does not emit its own light; instead, it reflects sunlight towards the Earth. As these reflected rays pass through the Earth's atmosphere, they interact with a mixture of gases, including nitrogen and oxygen, as well as any pollutants present, such as dust particles, smoke, and aerosol particles. This interaction causes the scattering of light, where different wavelengths of light are redirected and filtered.
The Moon typically appears white or yellow, but when there is significant air pollution, the shorter wavelength colours such as blue, violet, green, and indigo are scattered and filtered out to a greater extent. This is because shorter wavelengths have smaller waves, making it harder for them to pass through the dense and polluted atmosphere. As a result, only the longer wavelength colours like red and orange are able to pass through the atmosphere and reach our eyes, resulting in the Moon appearing reddish or orange.
This phenomenon is more commonly observed when the Moon is near the horizon during moonrise or moonset. The Earth's atmosphere is thickest at the horizon, which further contributes to the scattering and filtering of shorter wavelength colours. Events such as forest fires, volcanic eruptions, and industrial pollution can also increase the amount of pollutants in the atmosphere, enhancing the scattering effect and making the Moon appear even more reddish or orange.
While the colourful Moon created by this scattering effect can be aesthetically pleasing, it serves as a reminder of the negative impact of air pollution on our environment and our ability to observe the night sky in its natural state.
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Frequently asked questions
Yes, pollution can affect the colour of the sky. The sky's colour is determined by sunlight scattering off molecules in the atmosphere. This is known as Rayleigh scattering. When there are more particles in the air, more light is scattered, enhancing the colours we see. This is why sunsets are often more vibrant in heavily polluted cities, due to the abundance of human-made aerosols in the atmosphere.
While it can be difficult to discern with the naked eye, there are some tell-tale signs. A red sky at night, orange or yellow sky during the day, or fuzzy clouds can indicate high levels of air pollution.
Sunsets in polluted areas tend to be more colourful due to the increased number of particles in the air, which scatter sunlight. The most vibrant sunsets occur when there is a high concentration of large aerosols in the atmosphere.
Yes, while pollution often enhances the colours in the sky, too much pollution can dampen the colours. When large numbers of big particles accumulate in the atmosphere, the sunset may appear washed out, and the sun may not be clearly visible.
