OPOD - Clean Air

Last updated on September 16, 2023

OPOD - Clean Air: Exploring the Phenomena of Atmospheric Optics

Atmospheric optics is a captivating field that unveils the hidden beauty of the sky. In this edition of the Optical Phenomena of the Day (OPOD), we delve into the concept of "clean air" and its impact on our perception of celestial objects. The images captured by Elmar Schmidt, soaring at an altitude of 12,000 meters (39,000 feet) over Southern Germany, offer a unique perspective on the interplay between light and our atmosphere.

The photographs are intriguing not only for what they reveal but also for what they conceal. The sun, partially eclipsed by the wing of the Air Namibia Airbus, is only faintly visible due to the diffraction and reflection of light on the curved wing surface. This phenomenon, rarely observed from lower altitudes within our dusty and aerosol-laden troposphere, highlights the significance of clean air in shaping our visual experiences.

In our everyday encounters with the sun, we often witness its radiance surrounded by a brilliant aureole. This halo-like feature is typically a few degrees in diameter and varies in size depending on the concentration and size of atmospheric dust and aerosols. These particles scatter and diffract incident light, redirecting it forward but slightly deviating its path. As a result, we observe a gentle glow encircling the sun. It is worth noting that even without the presence of external particles, air molecules themselves scatter light, contributing to the ever-present blue sky.

When nearly monosized particles are involved in the diffraction process, a sharply defined aureole emerges, accompanied by vibrant colored rings known as a corona. However, the absence of clean air can alter this phenomenon. For instance, high concentrations of stratospheric dust or sulfate aerosols, often originating from volcanic eruptions, can produce an intense and expansive aureole referred to as a Bishops Ring. This milky glow gradually fades at the edge, transitioning into hues of straw and reddish colors.

To fully appreciate the significance of clean air in atmospheric optics, it is essential to recognize its impact on our perception of celestial events. When the atmosphere is free from dust, aerosols, and other pollutants, our observations become clearer and more vivid. Clean air allows for a greater transmission of light, enabling us to witness the intricate details of astronomical phenomena.

In addition to its aesthetic appeal, clean air plays a crucial role in scientific research. Researchers rely on pristine atmospheric conditions to capture accurate data and images. By minimizing the interference caused by atmospheric particles, scientists can obtain clearer measurements, leading to more precise calculations and a deeper understanding of the cosmos.

It is important to note that clean air is not a constant feature of our atmosphere. Human activities, such as industrial emissions and vehicular pollution, contribute to the degradation of air quality. These pollutants introduce particulate matter and aerosols, which scatter and absorb light, ultimately affecting our ability to observe celestial objects with clarity.

In conclusion, the concept of clean air holds immense significance in the realm of atmospheric optics. It influences our visual experiences, shapes our understanding of celestial phenomena, and impacts scientific research. As we strive to protect and preserve the environment, we must recognize the value of clean air in unraveling the mysteries of the sky and appreciating its ethereal beauty.

Clean Air

Images by Elmar Schmidt 12000m (39,000ft) over Southern Germany on June 20, 2011 on his return from Namibia to research lunar eclipse darkness. ©Elmar Schmidt, shown with permission

The image is noteworthy for what is not visible.

The sun is eclipsed by the wing of the Air Namibia Airbus. Its presence is only weakly apparent by rimes of light reflected and/or diffracted by the curved wing surface.

We almost never see this sight from lower down in our dusty, water droplet and aerosol laden troposphere. The sun is usually surrounded by an extremely bright aureole.

Here the sun is not eclipsed by the wing and its light dazzles the camera CCD and scatters from window scratches.

The aureole is normally a few degrees in diameter, its apparent size depending on the size and concentration of atmospheric dust and aerosol.

The scattering particles diffract incident light to direct it forward but slightly deviated to give the glow around the sun. Air molecules themselves scatter light (hence our blue sky) and thus an aureole is never totally absent.

Diffraction by nearly monosized particles would give a sharply defined aureole surrounded by coloured rings – a corona.

High concentrations of stratospheric dust or sulfate aerosol from volcanic eruption produce a strong and large aureole known as a Bishops Ring, a milky glow fading at the edge to straw and reddish colours.

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