Atmospheric
Optics
Fogbow & Supernumeraries
A remarkable fogbow with at least four inner supernumeraries imaged by Ramie Liddle mid afternoon at St. Augustine Beach, Florida.
Images ©Ramie Liddle, shown with permission
This fogbow was from droplets with a narrow size distribution in thin fog. Fogs tend to evolve from a narrow size distribution to a broad one. The St Augustine Beach was there all day. In the afternoon the number of supernumeraries actually increased with time indicating a narrowing size spread. Perhaps fresh fog was continually forming and burning away.
Below: What happens when the fog droplets are not monosize. At right the droplets have a 20% (standard deviation) distribution about the mean diameter of 50 micron. Different size droplets give different supernumerary fringe spacings and the resulting overlap blurs away the fringes. Only the first supernumerary is well seen.
At left, an approximate fit to the fogbow by an IRIS Mie theory calculation. Monosized droplets with a diameter of 50 micron (1/20 mm) were used. Notice how the supernumeraries in the image and simulation have stronger colour than the main bow. Red is on the inside of the supernumeraries while reds and straw colours are on the outside of the main primary bow.
