Red Sky: Pollution Or Natural Wonder?

The red sky is a well-known phenomenon that has been observed and discussed for centuries, with references in the Bible and Shakespeare's works. While red skies during sunrise and sunset are common, the night sky in urban areas sometimes exhibits this hue due to light pollution, a phenomenon known as skyglow. This occurs when light from various artificial sources reflects off cloud cover, creating a noticeable red glow. During the day, a red sky is unusual and often indicates high levels of particles in the atmosphere, such as smoke from forest fires or volcanic eruptions. While not always indicative of pollution, a red sky can signify its presence, especially when caused by human-made aerosols.

Red skies can be caused by smoke particles from forest fires

Red skies are usually observed during sunrises and sunsets. However, in recent times, the night sky in urban areas has also been observed to turn red. This phenomenon, known as "skyglow", is caused by light pollution.

While red skies are naturally occurring, they can be intensified by air pollution. The presence of dust, moisture, and other particles in the atmosphere scatters sunlight, allowing only the longer wavelengths of red light to reach our eyes. These particles come from various sources, including human-made aerosols, forest fires, and volcanic eruptions.

Forest fires, in particular, have been known to cause red skies. In 2019, widespread forest fires in Indonesia resulted in a thick haze that turned the sky blood-red, even during the day. This phenomenon was caused by the illegal practice of slash-and-burn farming, where fires are set to clear forest areas for plantations. The smoke from these fires contained large particles that effectively scattered the long wavelengths of red light, a process known as Mie scattering.

Similarly, in the U.S. Pacific Northwest, smoke from fires in Siberia caused reddish sunsets. The smoke particles allowed the longer wavelength colors of red and orange to pass through while blocking the shorter wavelengths of blue, green, and yellow. This resulted in a red or orange tint in the sky.

It's important to note that red skies caused by forest fires can have significant impacts on human health and the environment. The smoke from these fires can lead to reduced air quality, respiratory illnesses, and even premature deaths.

Air pollution from human-made aerosols scatters red light

The colour red in the sky is a captivating yet relatively uncommon phenomenon. While red skies can occur naturally, air pollution from human-made aerosols increases their likelihood and brightness.

During the day, the sun is at the perfect distance from Earth for the sky to appear blue. However, at sunrise and sunset, the sun sits low on the horizon, causing light to travel through a more significant portion of the atmosphere before reaching us. The atmosphere is replete with particles that scatter light, and the longer distance travelled by sunlight results in the early scattering of blue wavelengths. Consequently, by the time the light reaches us, only the longer wavelengths of red, yellow, and orange light remain.

The presence of more particles in the atmosphere intensifies the redness of sunrises and sunsets. Smoke and other large particles are adept at scattering red light. Notably, air pollution from human-made aerosols, such as those produced by the combustion of fossil fuels, wood, and biomass, also scatters red light very effectively. In heavily polluted cities, sunsets and sunrises often exhibit a higher proportion of red light due to the increased scattering of shorter wavelengths before they become visible.

The adage, "Red sky at night, sailor's delight; red sky in the morning, sailor's warning," encapsulates the association between red skies and weather conditions. In mid-latitudes, storms and winds typically move from west to east. A red sky at night indicates that the setting sun is sending its light through a high concentration of dust particles, signifying stable air and good weather from the west. Conversely, a red sky in the morning suggests that a storm system with low pressure may be moving eastward.

While red skies during sunrise and sunset are well-known, light pollution in urban areas can also cause the night sky to glow red. This phenomenon, known as "skyglow," is more noticeable on cloudy nights due to the reflection of light from various sources, such as streetlights and outdoor lighting.

The saying red sky at night, sailor's delight has biblical roots

The sky turns red due to the scattering of light by particles in the Earth's atmosphere. During sunrise and sunset, the sun sits low on the horizon, causing light to travel through more of the atmosphere. The atmosphere is full of particles that scatter light, and during this longer journey, the shorter blue wavelengths of light scatter away, leaving the longer red, yellow, and orange wavelengths.

A red sky can be caused by natural factors, but it can also be the result of air pollution, which can brighten the colour. Sources of air pollution, such as smoke and human-made aerosols, scatter red light effectively, leading to a reddish sky. Light pollution in urban areas can also cause the night sky to appear red, a phenomenon known as "skyglow".

The saying, "Red sky at night, sailors delight; red sky in the morning, sailors take warning", is an old adage with biblical roots. In the Bible, specifically Matthew 16:2-3, Jesus is quoted as saying:

> When it is evening, ye say, fair weather: for the heaven is red. And in the morning, foul weather today for the heaven is red and lowering.

This saying holds some truth, especially in the mid-latitudes where storms tend to travel from west to east. A red sky at night indicates that the setting sun is sending its light through a high concentration of dust particles, suggesting stable air and good weather approaching from the west. Conversely, a red sky in the morning means that a high-pressure system has already passed, indicating that a storm system is likely moving eastward.

Thus, the saying "Red sky at night, sailors delight" has biblical roots and provides a simple method for sailors and farmers to predict upcoming weather conditions.

Light pollution makes the night sky appear red in urban areas

The phenomenon of a red sky is usually observed during sunrises and sunsets. This occurs because the sun sits low on the horizon, and light travels through more of the atmosphere before reaching us. The atmosphere is filled with particles that scatter light, and the blue wavelengths of light scatter early on, leaving longer wavelengths of red, yellow, and orange light.

A red sky can also be the result of excess dust, moisture, or pollution in the atmosphere. The presence of these particles scatters the shorter wavelengths of light, causing the sky to appear more red. While not every red sky is caused by air pollution, it is a contributing factor that makes red skies more likely and brighter.

In addition to natural causes, light pollution in urban areas can also contribute to the redness of the night sky. Light pollution refers to the brightening of the night sky, primarily over urban regions, due to artificial light sources such as electric lights from cars, streetlamps, offices, factories, and buildings. This phenomenon, known as "skyglow," has been observed to alter the color of the night sky, with clouds enhancing the redness.

The sources of light pollution in urban areas include streetlights, outdoor lighting around homes and businesses, lights from shopping malls, billboards, and other artificial light sources. Light pollution can have adverse effects on human health, disrupting natural body rhythms and sleep patterns, and causing potential safety issues. It also impacts the environment, affecting wildlife and ecosystems, wasting energy, and contributing to climate change.

To address light pollution, efforts are being made to reduce its impact. These include adopting legislation to control outdoor lighting, designing more efficient light sources, and encouraging individuals to use outdoor lighting only when necessary, shield lights to reduce glare, and minimize light trespass into unwanted areas.

Red skies can indicate high water content in the atmosphere

Red skies are most commonly observed during sunrises and sunsets. At these times, the sun is low in the sky, and its light must travel through the thickest part of the atmosphere. The atmosphere is full of particles, including water vapour and dust, that scatter light. During sunrise and sunset, the shorter wavelengths of light, such as blue, are scattered and broken up, while the longer wavelengths of red, orange, and yellow light remain. This is why we perceive the sky as reddish during sunrises and sunsets.

The amount of water vapour and dust particles in the atmosphere influences the colours we observe in the sky. A red sky during sunrise or sunset suggests an atmosphere loaded with dust and moisture particles. This is because the red wavelengths, the longest in the colour spectrum, are breaking through the atmosphere.

The presence of dust and moisture particles in the atmosphere is also linked to weather conditions. The old adage, "Red sky at night, sailor's delight. Red sky in the morning, sailor's warning," suggests that a red sky at night indicates good weather, while a red sky in the morning foretells incoming storms. This saying holds some truth, especially in mid-latitudes where storms tend to travel from west to east. A red sky at sunrise can indicate that a high-pressure system (good weather) has already passed, suggesting that a low-pressure system (storms) may be approaching from the west.

Additionally, air pollution can contribute to the appearance of red skies. Intense red sunsets can be observed in heavily polluted cities due to the abundance of human-made aerosols in the atmosphere. Forest fires, volcanic eruptions, and certain seasons can also lead to an increased presence of particles in the air, resulting in more colourful sunsets.

Therefore, while red skies can have multiple causes, including natural variations in atmospheric conditions, they can indeed indicate high water content in the atmosphere, particularly during sunrises.

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