Civil Dawn San Fransico River Valley at Alma, New Mexico un-edited

This photo takes advantage of the coloring of the civil dawn, which painted this western valley with these enhanced purple-blue- red colors. (Photo taken with my back to the sunrise)

Civil dawn begins when the geometric center of the Sun is 6 degrees below the horizon and ends with sunrise, which is the moment when the Sun's upper edge touches the horizon.
Color development of the twilight arch
When the sun reaches -6o altitude, roughly 30 minutes before sunrise (again, depending on your latitude, but true for middle latitudes), civil twilight starts, and it is at this point that things start developing faster and it gets really interesting. The zodiacal light will fade, to make place for a deep orange twilight arch stretching almost from N to S.
Purple red
20 minutes before sunrise: I noted how the twilight arch at the sun's azimuth starts to glow with a purple hue. This purple color is at first roughly 5o to 10o altitude and stretches about 40o to 60o in azimuth. This is called the purple red, and is caused by the deeply reddened sun shining from below the horizon onto a high stratum of aerosols (typically dust, at very high altitudes of up to 16 km). At times of volcanic eruptions, this purple red may be incredible, and it may look like the eastern horizon is on fire.
The twilight arch can display many different hues of colors. Most common is a red-orange-yellow-purple-blue ordering of colors from the horizon upward. The purple is called the purple red and only occurs for 5 to 10 minutes, during civil twilight.

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