Sunsets

Last updated on September 16, 2023

The Fascinating Phenomenon of Sunsets

Sunsets are not just beautiful to behold; they are also a captivating atmospheric optics phenomenon. As the sun dips below the horizon, its rays take a long and meandering path through the lower atmosphere, creating a mesmerizing display of colors. The atmosphere acts as a colossal lens, bending and twisting the low sunset rays around the edge of the Earth. This bending of light results in the rays passing through up to 40 times more air than the rays from a high midday sun.

The Role of Scattering and Absorption

During their journey through the atmosphere, sunset rays encounter various obstacles. Air, dust, aerosols, and water droplets scatter and absorb the rays, giving rise to the stunning hues we associate with sunsets. The scattering of light by these particles depends on their size relative to the wavelengths of visible light.

• Rayleigh Scatterers: The air itself, along with small dust and aerosol particles smaller than the wavelengths of visible light, are known as Rayleigh scatterers. These particles predominantly scatter shorter wavelength blue and green rays more strongly than longer wavelength yellow and red rays. As a result, the remaining direct unscattered light appears dimmed but enriched in reds and yellows.
• Mie Scatterers: Larger dust particles and suspended water droplets, known as Mie scatterers, scatter light differently. They do not produce vivid red sunsets like Rayleigh scatterers; instead, they simply dim the sun.

The Colorful Palette of Sunsets

The interplay between scattering and absorption processes in the atmosphere gives sunsets their remarkable array of colors. The scattering of shorter wavelength blue and green rays by Rayleigh scatterers leads to the dominance of longer wavelength yellow and red rays in the unscattered light. Additionally, specific wavelengths of green and blue light are further absorbed by ozone and water vapor molecules, intensifying the redness of the remaining light.

Moreover, sunset rays can undergo multiple reflections between clouds and the ground, enhancing the complexity and richness of the colors observed. This interplay of scattering, absorption, and reflection creates a spectacle seemingly painted with every color and shade of the palette.

The Influence of Volcanic Dust

The presence of extra fine dust particles in the upper atmosphere, such as those emitted during a volcanic eruption, can further intensify the reddening of sunsets. These larger particles scatter light differently than Rayleigh scatterers, leading to a deepening of the red hues. When skies are laden with volcanic dust, sunsets become even more captivating, displaying an enhanced and otherworldly beauty.

Understanding Rayleigh and Mie Scattering

To delve deeper into the mechanisms behind the scattering of light in the atmosphere, it is essential to understand Rayleigh and Mie scattering.

• Rayleigh Scatterers: Particles smaller than the wavelengths of light scatter light in all directions. Their scattering is inversely proportional to the fourth power of the wavelength. Blue light, with a wavelength of approximately 450 nanometers, is scattered over four times more strongly than red light, which has a wavelength of around 650 nanometers. Very small dust particles and some smoke particles fall into the category of Rayleigh scatterers.
• Mie Scatterers: In contrast, particles larger than visible wavelengths predominantly scatter light forward in the direction of the original beam. Examples include water droplets, which not only scatter light forward but also exhibit strong scattering in specific directions, leading to the formation of rainbows, fogbows, glories, and coronae. Mie scatterers scatter light of different wavelengths more equally than Rayleigh scatterers.

In conclusion, sunsets are a breathtaking interplay of atmospheric optics phenomena. The bending and scattering of sunlight by the Earth's atmosphere, combined with the absorption and reflection processes, create the vibrant and ethereal colors we witness during this daily spectacle. Whether it's the gentle pinks and purples or the fiery oranges and reds, each sunset is a unique masterpiece painted by nature. So, next time you witness a sunset, take a moment to appreciate the wonders of atmospheric optics that lie behind its captivating beauty.

Sunset rays pass long and deep through the lower atmosphere. They are bent and twisted around the very rim of the Earth.

The atmosphere acts as a giant lens which refracts low sunset rays into long curved paths passing through up to 40 times as much air than the rays from a high midday sun.

Air, dust, aerosols and water drops scatter and absorb the rays throughout their long passage.

Reds, yellows and golds arise because the air itself, small dust and aerosol particles smaller than the wavelengths of visible light, Rayleigh scatterers., scatter short wavelength blue and green rays much more strongly than longer wavelength yellow and red. The remaining direct unscattered light is dimmed but relatively enriched in reds and yellows. Absorption of specific green and blue wavelengths by ozone and water vapour molecules redden the light further. The sunset rays are sometimes reflected back and forth between clouds and the ground. All this goes to makes a spectacle seemingly painted with every colour and shade of the palette.

When the upper atmosphere contains extra fine dust from a volcanic eruption skies are reddened further.

Large dust particles and suspended water droplets scatter light differently, they are Mie scatterers.. and do not produce vivid red sunsets, they merely dim the sun.

.Rayleigh scatterers

Particles much smaller than wavelengths of light scatter light in all directions. Their scattering is inversely proportional to the fourth power of the wavelength. Blue (~450 nanometer wavelength) is scattered over four times more strongly than red (~650 nm). Very small dust particles are Rayleigh scatterers. Some smoke particles are small enough also, watch smoke from a fire, it looks red or brown when viewed against a bright light but blue/white otherwise.

..Mie scatterers

Particles larger than visible wavelengths scatter light predominantly forwards in the direction of the original beam. Some, like water droplets, also scatter strongly in other quite specific directions to form rainbows, fogbows, glories and coronae. With the exception of these specific directions light of different wavelengths is scattered much more equally than by Rayleigh scatterers.

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Reference Atmospheric Optics

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