Monsoon Sunset and Red Rainbow

August 17, 2012



Photographer: Dave Lynch
Summary Author: Dave Lynch

Monsoons are seasonal reversals or changes in prevailing wind direction bringing increased and often heavy rainfall to a region. In North America, monsoons arrive in June and last until September, primarily affecting the southern tier of states. Summer's dry prevailing winds in southern California are from the southwest. When the monsoons arrive, they temporarily interrupt the sunny weather and bring rain and thunderstorms in from the east. These last a few days until the dryer summer winds return.

On July 12 and 13, 2012, monsoon weather ruled southern California. The sunset viewed near Topanga was gorgeous on the 12th (top photo) with clouds lit by the low red Sun and a blue-sky background. Looking in the opposite direction a red rainbow could be detected (bottom photo). The rainbow was red because the raindrops were illuminated by a reddened Sun. The Sun, in turn, was red because absorption by atmospheric particles and water vapor extinguished the blues and greens (shorter wavelengths of light), leaving only the orange and red colors to penetrate the air. When the Sun is on the horizon its path length is considerably greater than when it’s overhead causing the sunlight to pass through almost 40 times as much atmosphere and aerosols.