December 23, 2011
The eye-catching fogbow shown above was observed while trying to get a glimpse of Mt. Everest when I visited the Tibet Plateau in China. Fogbows, also known as cloudbows, are colorless (or nearly so) cousins of rainbows. It’s the small fog droplets rather than falling raindrops that are responsible for the wan color and broader arch of a fogbow compared to the more showy rainbows. Actually, as long as the drop size is greater than about one millimeter in diameter, faint coloration is usually present. In spite of the long shadows noticeable in the picture, the photo was taken just after noon, on October 29, 2011.
Is it possible to see a rainbow or fogbow near noontime in the middle latitudes? Well, evidently the short answer is “yes,” but the answer should be “no.” Because the minimum angle of deviation of light rays in a spherical drop of water is 42 degrees, a rainbow can only be observed when the Sun is higher than 42 degrees above the horizon. This is why these bows are typically appreciated in the late afternoon or early evening. However, this classic fogbow was captured 37 minutes past noon. China stretches from about 135 east longitude into central Asia at a longitude of about 75 east, a distance of nearly 3,000 mi (4,828 km). The U.S. extends about the same distance and carries four time zones. Nonetheless, there’s only one time zone for all of China: when it’s noon in Shanghai, it’s also noon in western Tibet. During fall and winter, while the Sun is relatively high in the sky at noon time in Shanghai, it’s still clutching the horizon in western China. Thus, it’s at least technically possible to see a mid-day bow in the mid-latitudes.
Note how well the red parka stands out in this foggy landscape. Is it any wonder Rudolph leads Santa’s sleigh on those gloomy Christmas Eves.