We all know what a rainbow is, but have you ever heard of a “fogbow?” One of these rare, ghostly rainbow cousins was spotted this week over Martha's Vineyard and Nantucket. Before we describe what a fogbow is, let’s first talk about how a rainbow is formed. While rainbows and fogbows are very similar, they have some key differences.A rainbow forms when white light from the sun enters a raindrop. The difference in density between air and water bends, or refracts, the light. Some of the light is then reflected off the back of the raindrop back toward the viewer. When the light waves exit the raindrop after reflection, they are refracted again, into different wavelengths, producing the colors of the visible spectrum of light that we know as “Roy G. Biv” (red, orange, yellow, green, blue, indigo, violet). Basically, rainbows form when light is refracted and reflected. Sometimes, there can actually be two reflections within the drops, leading to a double rainbow.Fogbows are formed by much smaller water droplets suspended in the air or fog. These tiny droplets diffract sunlight, spreading it out into a wider arc. Because most of the light is being diffracted, the light is not separated into the visible spectrum, and thus, fogbows are seen as mainly white and fainter. The smaller the droplet, the larger and brighter the fogbow appears. For larger droplets, the fogbow appears smaller and less bright. Fogbows can also produce secondary bows called “supernumeraries,” which are formed by wavelength interference. Smaller droplets produce a wider space between the arcs, while larger droplets cause the supernumeraries to be closer together.In both cases, the light source — the sun — needs to be behind the viewer, and the droplets need to be in front of the viewer. Both types of bows are actually complete circles but are only visible above the horizon. The center of the arcs is actually directly opposite the sun’s position. Therefore, rainbows are much more common in the morning or evening when the sun is lower in the sky.