Boston Sunrise Courtesy of Hurricane Irene

“Red sky at night,
Sailor's delight;
Red sky at morning,
Sailor's take warning.”

Taken at 5:50 AM looking due east with the buildings of Boston's Back Bay 4.5 miles away.
Notice the earliest indication of Irene's outer bands beginning to appear.
They represent the outermost periphery of the hurricane.

Operating under the premise of this old English rhyme used by shepherds, sailors and farmers in the days before barometers when you watched the sky to forecast the weather, I waited with camera in hand at the crack of dawn on the morning of Hurricane Irene’s New England arrival on August 27th, 2011. Anticipating a colorful red sunrise, I was duly rewarded. Who ever said Arizona has the most spectacular first light?

Taken at 6:00 AM

Interestingly, the precise origin of the saying is unknown, although a form of it appears as early as the 14th century in the Wyclif Bible (Matthew 16:2-3). Apparently, the English form of the rhyme first applied to shepherds. Later, British and American maritime necessities modified the rhyme for usage by sailors.

WHY ARE SUNRISES AND SUNSETS RED IF THE SUN IS YELLOW?

Light bounces (reflects) off small dust particles in the atmosphere (called scattering). Lord Rayleigh, an English physicist in 1871, determined that shorter wavelength light is scattered more efficiently than light at longer wavelengths. The result is that blue light with a small wavelength is scattered 10 times more efficiently than red light with a larger wavelength. Light from the sun contains all the colors of the spectrum from red to blue. Because blue light is scattered significantly, we see a sky that’s blue. When the sun is low in the sky, such as at sunrise and sunset, scattering is much greater. Much more of the blue light coming from the sun is scattered away from the direct path towards our eyes. As a result, the sun will appear very red when it is low in the sky. Not surprisingly, the same phenomenon happens when the moon it is low in the sky.

SO WHAT'S THE RELATIONSHIP OF RED SKIES TO THE WEATHER FORECAST?

The weather in Britain (where the saying arose) largely comes from the west. So if the sky was red when the sun set (a clear sight of the red-setting sun), there was a good chance of a clear night and morning ahead with no storms to the west. But if the sky was red when the sun rose, it was likely there was a day of rain in store (with the sun casting its rays on storm clouds approaching from the west). Obviously, this was not a fool proof method of predicting the weather, but was fairly reliable for the immediately foreseeable future. It works best in the mid-latitudes of the Northern Hemisphere, where storm systems generally follow the jet stream from west to east. 

When rhymes such as this arose in old England, the country had a primarily rural and maritime economy. Predicting the weather often was a matter of life and death. Before the invention of the barometer, there was no accurate means of anticipating changes in the weather other than reading the sky. Nowadays, with Doppler radar, satellite imagery, computer modeling and the Weather Channel, there doesn't seem to be much need for us to "look to the skies." Unless you see that it's raining when you walk out the door!

On a lighter note...
"Weather forecast for tonight: dark. Continued dark overnight, with widely scattered light by morning."
George Carlin