Orange Sunsets: Pollution's Impact And Our Changing Skies

The vibrant hues of orange, pink, and red that grace the sky during sunrise and sunset are often attributed to air pollution. While pollution can indeed intensify these colours, the phenomenon primarily occurs due to the scattering of light by particles in the atmosphere. During sunrise and sunset, when the sun is closer to the horizon, its light travels through a greater portion of the atmosphere compared to midday, resulting in more scattering of shorter blue and green wavelengths. Consequently, the longer wavelengths, such as orange and red, endure and reach our eyes, painting the sky with rich, warm tones.

Natural vs. man-made aerosols

The vibrant hues of red, orange, and pink streaked across the evening sky during sunset are the result of a scientific phenomenon called light scattering, or Rayleigh scattering. This occurs when light rays from the sun hit particles in the air, changing the direction of the light. The colours we see are determined by the different wavelengths of light and the size of the particles in the atmosphere.

The molecules and small particles scatter in the same way as long as the particle is sufficiently small. If the particle is small compared to the wavelengths of visible light, it will scatter short wavelengths, such as blues and violets, more than long wavelengths, such as reds.

Aerosols are solid or liquid particles suspended in the air that originate from both natural processes and human activities. Natural aerosols, such as those from forest fires, mineral dust, sea spray, and volcanic eruptions, tend to block out only a single wavelength of light. Combined with the nitrogen and oxygen particles in the atmosphere, this often produces a vibrant sunset with a single dominant colour, like red.

On the other hand, man-made aerosols are composed of varying types and sizes of particles. These include factory and coal-power exhaust, car exhaust, leaking gas, and the byproducts of burning plastics and aerosol spray cans. These human-generated aerosols can enter the atmosphere directly, as is the case with soot emitted by internal combustion engines in cars and trucks. They can also enter the atmosphere when molecules in the gaseous state react with other chemicals, such as when burning fossil fuels release sulphur dioxide gas, which turns into sulphuric acid aerosols.

The number of aerosols produced by human activity far exceeds natural sources. While natural aerosols can create vibrant sunsets, man-made aerosols tend to scatter all colours indiscriminately, increasing the overall brightness of the sky but dampening colour contrast. This can result in a sunset with no colours at all, where the sky appears hazy, grey, or white.

Therefore, while both natural and man-made aerosols can contribute to colourful sunsets, an excess of man-made aerosols can lead to washed-out colours and a muted sunset.

The role of clouds

Clouds act as a theatre screen, catching the last rays of the setting sun, which are predominantly red-orange, and reflecting this light back to the ground. This reflection enhances the colours we perceive during sunsets. Elevated clouds, such as cumuliform clouds, create particularly memorable sunsets due to the gradations of lighting they produce. The vertical extent of these clouds and their trailing precipitation cascades result in a varied lighting display.

The presence of clouds at higher altitudes can also influence the colour of sunsets. As the sun dips below the horizon, its rays illuminate the clouds from below, creating a theatre-like effect. This illumination enhances the red and orange hues, reflecting them back to the ground and contributing to the vibrant colours often associated with sunsets.

Additionally, the absence of clouds can also impact sunset colours. In certain conditions, such as in the eastern half of the United States during late fall and winter, a clean and clear lower atmosphere can increase the likelihood of vibrant sunsets. In these cases, the absence of clouds allows for unobstructed viewing of the sunset, showcasing the full spectrum of colours produced by the scattering of light.

The effect of particle size

The colour of the sky during sunset results 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. All wavelengths of light are scattered, but they are not scattered equally. The shorter the wavelength, the more it is scattered. Violet, for instance, is scattered the most, followed by blue, then green, and so on.

However, when there is a high concentration of large aerosols, the sunset colours tend to be washed out. This is because large particles scatter all colours indiscriminately, increasing the overall brightness of the sky but dampening colour contrast.

Natural aerosols, such as smog from volcanic eruptions, tend to block out only a single wavelength of light. Combined with the nitrogen and oxygen particles in the atmosphere, this effect produces a vibrant sunset with a single dominant colour, often red. On the other hand, man-made aerosols are made up of varying types and sizes of particles. As a result, the scattering of light is inconsistent, and instead of allowing just one colour to shine through, we end up seeing a mix of greyish white and a handful of weak, muted colours.

Therefore, while particle size does play a role in determining sunset colours, other factors, such as the type of aerosol (natural or man-made), the concentration of particles, and the overall level of pollution, also come into play.

The impact of pollution on sunset colours

The colours of a sunset are determined by a scientific phenomenon called light scattering. This occurs when light rays hit particles in the air, changing the direction of the light. The different wavelengths of light, along with the size of the particles in the atmosphere, determine the colour we see.

During sunrise or sunset, when the sun is close to the horizon, its light must travel through more of the atmosphere than when it 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. When there are more particles in the air, more light is scattered, and the colours are enhanced.

Airborne pollutants, such as dust, smoke, and other particles, can indeed enhance the colours of sunsets, especially when the sun is low in the sky. This is because pollutants scatter light, particularly longer wavelengths like orange and red. Natural aerosols, such as those from forest fires, volcanic eruptions, or sandstorms, tend to block out a single wavelength of light, often resulting in a vibrant sunset with a dominant colour, often red.

However, it is important to note that the impact of pollution on sunset colours is complex. While some types of pollution can enhance sunset colours, excessive pollution can also dampen the overall sunset experience. Large particles in the air, including certain pollutants, can absorb more light than nitrogen and oxygen, scattering wavelengths of light mostly equally, which can mute the colours of a sunset. Additionally, when the sky is saturated with pollution, the sun may not be clearly visible, and the colours may appear washed out.

In conclusion, while pollution can indeed influence the colours of sunsets, it is not the sole factor. Other factors, such as the angle of sunlight, atmospheric conditions, and natural sources of particles, also play a significant role in the appearance of sunset colours.

The science of light scattering

The vibrant hues of red, orange, and pink streaked across the evening sky during sunset are the result of a scientific phenomenon called light scattering. This occurs when light rays from the sun hit particles in the air, changing the direction of the light. The phenomenon is named Rayleigh scattering, after the British physicist Lord Rayleigh, who wrote about it in 1871.

During sunrise or sunset, when the sun is close to the horizon, the light must travel through more of the atmosphere than it does when the sun is overhead. This results in more scattering of light, including longer wavelengths such as yellow, orange, and red, which creates colourful skies. When there are more particles in the air, more light is scattered, and the colours are enhanced.

The particles that cause light scattering may be solid or liquid and may result from both natural occurrences and human activities. Natural aerosols, such as smog from volcanic eruptions, tend to block out only a single wavelength of light. Combined with the nitrogen and oxygen particles in the atmosphere, this effect produces a vibrant sunset with a single dominant colour, often red.

On the other hand, man-made aerosols are made up of varying types and sizes of particles, resulting in inconsistent scattering of light. This leads to a mix of greyish-white and weak, muted colours. Additionally, when there are large numbers of big particles in the atmosphere, the sunset can appear bright but washed out, as these large particles scatter all colours indiscriminately, increasing overall brightness but dampening colour contrast.

The presence of clouds also plays a role in creating vibrant sunsets. Clouds reflect the last rays of the sun's waning light back towards the ground, contributing to a more vivid sky. Typically, the best sunsets are observed in skies with high and mid-level clouds like altocumulus and cirrus clouds.

Frequently asked questions