Dawn, the time marking the beginning of morning twilight, is the first light in the sky before the sun breaches the horizon. Sometimes it paints the sky in shades of orange and red thus the term “red dawn.” There are many adages concerning red dawns. Within them are generally correct references to the weather, based on years of observation. However, they do not explain why, if the sky is red in the morning, it means rain. Science has helped people to understand the phenomena that cause a red dawn, a harbinger of rain to come, according to the old weather predicting adages. Here is an interesting one found in “The Frank C. Brown Collection of North Carolina Folklore, p.225, 6123”
“Evening red and morning gray
Helps the traveler on his way.
Evening gray and morning red
Brings down rain upon his head.”
Colors that people perceive in the atmosphere result from the light of the sun. Sunlight contains all of the colors of the visible light spectrum known as ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo and Violet). According to Classroom Physics, when interacting with matter in the atmosphere such as moisture, dust particles and gases, specific wavelengths of light are often absorbed. The remaining frequencies are transmitted or reflected.
Rayleigh scattering, the process that causes the eye to sometimes see only red in the atmosphere at dawn, is also responsible for the blue color of the daytime sky. The light absorbed by the nitrogen and oxygen of the atmosphere is re-emitted in varying directions. Higher frequencies, such as blue and violet light, scatter more than the lower frequencies of red or orange. With the sun closer to the earth at noon than at sunset, the scattered violet and blue light waves have a short distance to travel to reach an observer on the ground. They also have fewer particles to pass through such as dust and pollutants. Therefore, a sky in shades of blue and violet is what the eye beholds.
In order for the eye to see a red dawn, Rayleigh scattering needs to be increased so that the only light waves remaining after scattering are the longer wavelengths of orange and red. The sun, before it peaks over the edge of the horizon, is at a greater distance from earth than at noon. At dawn, the light from the sun must pass through the thickest part of the atmosphere and through more particles in the air to reach the observer on earth. The scattering of the blue and violet frequencies is increased allowing the longer wavelengths of orange and red to pass through the atmosphere.
So why is dawn not viewed as red every morning? The type of matter that the light rays are interacting with is the answer. According to the proverb quoted above, a red sky morning forecasts rain and indeed all adages concerning red dawns foretell of foul weather. Most storms come from the west and move east. The sun, reflecting off the dust particles in the air after a weather system has moved west, casts an orange-red hue to the sky, suggesting that a storm may be coming from the west. When interacting with high moisture content in the atmosphere at dawn, the color reflected back is reminiscent of the deep red of a fire and heralds the coming of rain. Environmental pollutants are also particles or gases in the atmosphere and are sometimes responsible for the red color at dawn as well. Red sunsets however, are the opposite of red dawns. The dust particles that reflect red, orange and yellow at sunset are an indication of dry air in the west, hence the threat of rain is little to none.
Reference
The Library of Congress, Researchers, Science Reference Services, August 10, 2011 , (February 14, 2012)
