Fogbows, also known as white rainbows or ghost rainbows, are a captivating atmospheric phenomenon that occurs when sunlight interacts with tiny water droplets suspended in fog. While fogbows may resemble rainbows in appearance, there are subtle differences that make them a unique spectacle to behold. One such distinction is the effect of droplet size on the characteristics of fogbows.
As the diameter of the droplets increases, the fogbow narrows and the inner supernumerary bows move closer together. This narrowing is a result of diffraction, which causes the light waves to bend and interfere with each other as they pass through the droplets. In the case of larger raindrops exceeding 1 mm in diameter, diffraction effects become minimal, resulting in a rainbow with vibrant colors and barely noticeable supernumerary bows.
Cloud and fog droplets typically have diameters that rarely exceed 100 microns (1/1000 mm). These small droplets contribute to the unique appearance of fogbows, where the bow is wider and the supernumerary bows are more pronounced. However, when droplets ranging from 100 to 500 microns in diameter are present, such as those found in the sprays of waterfalls, the appearance of the fogbow may vary, displaying an intermediate form between that of cloud/fog droplets and larger raindrops.
To better understand the impact of droplet size on fogbows, scientists have conducted simulations using a lognormal size distribution with a standard deviation of 15%. These simulations allow researchers to explore how different droplet sizes contribute to the formation and characteristics of fogbows. By varying the droplet sizes within this distribution, they can observe the changes in bow width and the arrangement of supernumerary bows.
The relationship between droplet size and fogbow appearance provides valuable insights into the physics of light scattering and diffraction. It demonstrates the intricate interplay between droplet size, diffraction effects, and the resulting optical phenomena. Exploring the impact of droplet size on fogbows not only enhances our understanding of atmospheric optics but also adds to the allure of these mesmerizing natural displays.
In addition to their scientific significance, fogbows have also captured the imagination of artists, photographers, and nature enthusiasts. The delicate, ethereal beauty of fogbows serves as a source of inspiration and a subject for creative endeavors. By considering the effects of droplet size on fogbows, photographers can manipulate their camera settings and composition to capture the most captivating aspects of this atmospheric phenomenon.
In conclusion, the size of water droplets suspended in fog has a profound effect on the appearance of fogbows. Smaller cloud and fog droplets contribute to wider bows with more prominent supernumerary bows, while larger raindrops result in narrower bows and less noticeable supernumeraries. Through simulations and observations, scientists have uncovered the intricate relationship between droplet size and fogbow characteristics, shedding light on the physics behind these captivating atmospheric optics phenomena. Whether it is for scientific study or artistic appreciation, understanding the impact of droplet size on fogbows allows us to fully appreciate the enchanting beauty of these natural wonders.
As the droplet diameter (shown above in micron, 1/1000 mm) increases, the bow narrows and the inner supernumerary bows move closer together. Eventually, for raindrops larger than 1 mm diameter, diffraction effects become small and we have a rainbow with bright colours and hardly noticeable supernumeraries.
Cloud and fog droplets rarely exceed 100 micron diameter. The bows intermediate in appearance from 100 - 500 micron dia. droplets are occasionally seen in the sprays of waterfalls.
The simulations used a lognormal size distribution with a standard deviation of 15%
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"Fogbow droplet size effect". Atmospheric Optics. Accessed on December 10, 2023. https://atoptics.co.uk/blog/fogbow-droplet-size-effect/.
"Fogbow droplet size effect". Atmospheric Optics, https://atoptics.co.uk/blog/fogbow-droplet-size-effect/. Accessed 10 December, 2023
Fogbow droplet size effect. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/fogbow-droplet-size-effect/.