Why does the sky at dawn look so different than the sky at sunset? Shouldn’t the colors be the same? asks a reader.
When the sun sets, the light streaming through our western windows turns a deep gold, and bright oranges and reds streak the sky. But when the sun rises, the light that first floods through our east-facing windows may be a beautiful blue, often followed by a delicate, rosy pink as the sun makes it appearance. In fact, we can sometimes tell by the sky colors whether a photo has captured a sunrise or a sunset.
The colors come courtesy of the Earth’s atmosphere, which is thickest nearer the ground. As white sunlight zips into the atmosphere, it is invisibly full of color — red, orange, yellow, green, blue, indigo, violet. Teased out by tiny particles in the atmosphere, it’s these colors that we see splashed across the sky at sunrise and sunset.
As sunlight encounters the atmosphere’s gas molecules (such as oxygen and nitrogen), it is broken up into its colors, and then scattered every which way. Longer-wavelength light, like red, is scattered less. Sixteen times as much shorter-wavelength light — blues and violets — is scattered out from gas molecules, flooding the sky. And we see blue.
At sunrise and sunset, with the Sun near the horizon and its rays traveling nearly parallel to the ground, sunlight must pass through a thick blanket of air before it reaches our eyes. As even more of the blue end of the spectrum is scattered out of the light beam and into the sky, we see the Sun’s face dim and redden. Clouds lit by the altered sunlight turn shades of red and orange and pink. The more blues that are deflected from the beam, the deeper red the sky around the setting or rising sun will appear.
So why do sunrises often display a sky in delicate shades of pink, while sunsets blaze crimson? Scientists say that the air light must travel through is usually thicker at sunset than at sunrise. Dust and pollutants are near their peak at sunset, after a day’s activity by human beings — digging in the ground, constructing buildings, operating machinery and driving cars.
In addition, the air at sunset is usually much warmer than the air at sunrise, since the Earth cools overnight, radiating daytime’s heat into space. And hotter air is more turbulent than cooler air.
So at sunset, we are looking through the thickest layer of gas molecules, dust, and other pollutants, such as soot. More blues are scattered out of incoming beams of sunlight, and we see red, orange, and deep yellow light. But at dawn, the lower atmosphere is calmer, cooler, and cleaner. Daytime’s dust has settled overnight. Air molecules still scatter blues out of the streaming sunlight. But since more blue is left in the beam, the rising sun itself appears less red, and the dawn sky wears a more delicate tint of blue-pinks, pale yellows, and peach.
Your sunrise may vary; sunrises over the sea may look different than sunrises over land. And summer smog, or the lofted dust from a volcano, can make a sunrise as vivid as a sunset.
