Coronae

Coronae: A Phenomenon of Light and Color in the Sky

Coronae, also known as a corona, are captivating atmospheric optics phenomena that occur when thin clouds partially veil the sun or moon. These ethereal displays of light and color can be observed when the conditions are just right. In this article, we will delve into the fascinating world of coronae, exploring their appearance, formation, and the mesmerizing interplay of colors they exhibit.

The Appearance of Coronae

A corona is characterized by a central blindingly bright disk, known as the aureole, which is surrounded by one or more successively fainter and gently colored rings. The aureole itself is almost white and often fringed with hues of yellow and red. The first ring closest to the aureole exhibits a bluish color on the inside, gradually transitioning through shades of green and yellow, and finally reaching a red outermost edge. These colors are not as vivid as those found in a rainbow but instead offer subtle mixtures that create a captivating visual spectacle.

Observing Coronae

To witness a corona, one must look for it when thin clouds partially obstruct the sun or moon. When searching for a solar corona, it is crucial to shield the eyes from direct sunlight, as staring directly at or near the sun can permanently damage eyesight. Instead, it is safer to reduce the intensity of light by observing the sky reflected in a pool of water or a plain glass mirror. For lunar coronae, keep an eye out when the moon is near its full phase and the sky is dark.

The Interplay of Cloud Droplets and Ice Crystals

Coronae are formed through the diffraction of light by tiny cloud droplets or, at times, small ice crystals. When light passes through these minuscule particles, it undergoes bending and scattering, resulting in the creation of the intricate patterns and colors observed in a corona. The size and gradation of these droplets or ice crystals play a role in the appearance of the rings within the corona. In some cases, the rings may appear slightly off-center due to variations in droplet sizes within the cloud.

Contrasting with Other Atmospheric Optics Phenomena

It is essential to differentiate coronae from other atmospheric optics phenomena to avoid confusion. While coronae may appear similar to the 22° halo, they are considerably smaller in size. The 22° halo, which can also encircle the sun and moon, is caused by the refraction and reflection of light within ice crystals. Additionally, coronae should not be mistaken for the corona visible during a total solar eclipse, which refers to the Sun's outer atmosphere.

The Dynamic Nature of Coronae

Coronae are not static phenomena but rather dynamic displays that can change in size and shape. As different clouds move across the sun or moon, the corona may shrink or swell, adding to its mesmerizing allure. These fluctuations in size and shape contribute to the ever-changing beauty of coronae, making each observation a unique experience.

Appreciating the Wonders of Coronae

Witnessing a corona can be a truly awe-inspiring experience. The intricate interplay of light and color, the delicate rings surrounding the central aureole, and the ethereal nature of these atmospheric optics phenomena create a spectacle that captivates observers. Whether observed around the sun or moon, coronae offer a glimpse into the enchanting world of atmospheric optics, reminding us of the beauty and complexity of our natural surroundings.

In conclusion, coronae are captivating displays of light and color that occur when thin clouds partially veil the sun or moon. These phenomena, created by the diffraction of light by tiny cloud droplets or ice crystals, manifest as a central aureole surrounded by one or more successively fainter and gently colored rings. Observing coronae requires caution to protect the eyes from direct sunlight, and they should not be confused with other atmospheric optics phenomena such as the 22° halo or the corona visible during a total solar eclipse. The dynamic nature of coronae, with their ever-changing size and shape, adds to their allure. Appreciating the wonders of coronae allows us to marvel at the intricate beauty of our natural world.

Corona around the sun.

The central blindingly bright disk fringed with red is the aureole. Two rings surround it. The rings in this corona are slightly off centre because of a gradation in droplets sizes in the cloud.

Imaged by Richard Fleet (Glows, Bows & Haloes) in Wiltshire, England during the summer of 2003.

Image © Richard Fleet, shown with permission.

A corona may be seen when thin clouds partially veil the sun or moon. Look for one around the moon when it is near to full and the sky is dark. When searching for a solar corona, shield the sun and reduce the light intensity to safer levels by looking at the sky reflected in a pool of water or a mirror of plain glass. Staring directly at or near to the sun can permanently damage eyesight. Tips on eye care.

Coronae have an intensely bright central aureole which is almost white and fringed with yellows and reds. Sometimes that is all to be seen but the better coronas have one or more successively fainter and gently coloured soft rings surrounding the aureole. The first ring is bluish on the inside grading through greens and yellows to red outermost. The colours are subtle mixtures rather than the more direct hues of the rainbow. The corona can be 15º or so in diameter and often it shrinks and swells as different clouds scud across the moon.

The coronae is much smaller than the 22° halo which can also ring the sun and moon. The corona also has nothing to do with the Sun's outer atmosphere visible during a total eclipse and confusingly given the same name.

Coronae are produced by the diffraction of light by tiny cloud droplets or sometimes small ice crystals.

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Reference Atmospheric Optics

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  • "Coronae". Atmospheric Optics. Accessed on June 17, 2024. https://atoptics.co.uk/blog/coronae/.

  • "Coronae". Atmospheric Optics, https://atoptics.co.uk/blog/coronae/. Accessed 17 June, 2024

  • Coronae. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/coronae/.