Of Aureoles, Sunsets and Ash

Last updated on September 16, 2023

Of Aureoles, Sunsets, and Ash: Unveiling the Atmospheric Optics Phenomena

Have you ever marveled at the captivating beauty of a sunset? The vibrant colors that paint the sky as the sun bids farewell for the day can be truly mesmerizing. But have you ever stopped to wonder about the science behind these awe-inspiring displays? In this article, we will delve into the fascinating world of atmospheric optics, specifically focusing on aureoles, sunsets, and their intriguing connection to volcanic ash.

Unraveling the Mystery of Aureoles

Let's start by exploring the concept of aureoles. Aureoles are phenomena that occur when light is scattered by air molecules, aerosols, small water droplets, and even dust particles, including volcanic ash. They manifest as a halo-like glow around the sun and are often mistaken for corona. However, while a corona is only fully visible when the scattering particles are of uniform size, an aureole represents the central part of a corona.

Interestingly, during periods of volcanic ash episodes, such as the one experienced in Europe, faint halos become more noticeable. The absence of contrails in the sky during these episodes may render these halos more visible to the naked eye. This phenomenon sheds light on how atmospheric conditions can influence the perception and visibility of aureoles.

Exploring Sunset Spectacles

Now, let's shift our focus to sunsets and their relationship with atmospheric optics. Sunsets have long captivated artists and photographers due to their ethereal beauty. The hazy and subtly hued sunsets observed over Europe during the volcanic ash episode provide a stark contrast to the vivid colors produced by major explosive eruptions. These eruptions disperse fine ash and sulfate aerosols high up in the stratosphere, resulting in more intense and dispersed hues.

Glimpses into Europe's Sunset Spectacles

To further illustrate the diverse manifestations of atmospheric optics during the volcanic ash episode, let's examine some captivating images captured by talented photographers:

• Saclay Pond near Paris on April 18: Yves Gandolphe's photograph showcases a hazy rusty brown sunset adorned with a faint sun pillar. Notably, faint halos are also noticeable, possibly due to the absence of contrails in the ash-free skies.
• St. Petersburg, Russia: Nikita Kulanov's image reveals a bright aureole encircling the sun. The presence of an aureole indicates the scattering of light by air molecules, aerosols, and dust particles, including volcanic ash. Interestingly, the Bishop's rings, caused by extremely fine volcanic material, appear significantly larger than a 22° halo.
• Belgium, April 18: Koen van Gorp captured a stunning aureole, adding to the visual tapestry of atmospheric optics during the volcanic ash episode.
• Near Paris, April 18: Denis Joye's photograph showcases a peculiar red upper tangent arc. This phenomenon occurs when the low sun continues to shine at high altitudes after setting at ground level. The sun's rays, reddened by their passage through the dusty atmosphere, contribute to the formation of this halo.

The Limitations of Identifying Volcanic Ash Effects

While these images offer glimpses into the atmospheric optics spectacle during the volcanic ash episode, it is important to note that it is not always possible to definitively attribute these effects solely to volcanic ash. The skies themselves exhibit a wide range of appearances, making it challenging to discern the exact cause behind certain optical phenomena.

In conclusion, atmospheric optics presents us with a captivating blend of science and artistry. Aureoles and sunsets, with their enchanting displays of light and color, provide a window into the intricate workings of our atmosphere. The interplay between air molecules, aerosols, water droplets, and dust particles, including volcanic ash, contributes to the diverse and mesmerizing visual experiences we witness in the sky. So, the next time you find yourself gazing at a breathtaking sunset, take a moment to appreciate the hidden wonders of atmospheric optics that make it all possible.

Aureoles, Sunsets & Volcanoes

Several recent images where Europe's tropospheric ash clouds might have had an influence. More in earlier OPODs (1,2)

(1) Saclay Pond near Paris on April 18. Yves Gandolphe (site) imaged this hazy rusty brown sunset with yet another faint sun pillar. Faint halos have been noticeable during the ash episode. It could be that the contrail free skies have rendered them more visible!

(2) St Petersburg, Russia. Nikita Kulanov imaged this bright aureole surrounding the sun. Aureoles are light scattered by air molecules themselves, by aerosol or small water droplets and by dust including volcanic ash. An aureole is essentially the central part of a corona but the complete corona is only seen when the scattering droplets or particles all have the same size. The Bishop�s rings produced by very fine volcanic material are huge � much larger than a 22� halo for example.

(3) Belgium April 18. An aureole seen by Koen van Gorp.

(4) Near Paris April 18. A strange red upper tangent arc imaged by Denis Joye. The sun had already set at ground level and the low sun still shining at high altitude was producing the halo. Its rays were reddened by passage through the dusty atmosphere.

(5) Sunset at Zabik�w, Poland by Dariusz Dorosz. The scene is typical of the hazy and subtly hued sunsets seen over Europe � Not at all the blazing colours from major explosive eruptions that produce widely dispersed fine ash and sulfate aerosol high in the stratosphere.

(6) Sunset 17th April over the Severn Estuary looking from near Bristol towards Wales. Imaged by Joanna Baron.

It is not possible to be certain that these are volcanic ash effects because 'ordinary' skies have themselves a tremendous variety of appearances.

All images ©the photographers

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