Think of the sun. What color is it in your mind’s eye? Most of us would say yellow without hesitation. That’s how appears in a clear blue sky. I looked at it this morning and thought pale gold was also a good fit. We’ve also seen red and orange suns around sunrise and sunset, but we know that’s not the intrinsic color of our star but soupy air and dust at work.
White light or sunlight is composed of every color in the rainbow from violet to green to yellow to red. When the sun is high in the sky, all these colors reach our eye with equal intensity and the sun appears pure white. But wait – the sun doesn’t look white. It’s yellow, right?
In truth, colors aren’t created equal when it comes to Earth’s atmosphere. Each is affected differently by everything ranging from air molecules to suspended dust to volcanic aerosols.
Air molecules scatter blue and violet rays away from the hidden rainbow of colors in white light and send them bouncing around the sky. That’s why the sky is blue. When you look up to admire a blue sky, you’re seeing the blue part of sunlight set free. Come to think of it, we’re literally inhaling blue sky every time we take a breath. And to go one step further, we spend most of our time with our heads in the sky, since our feet are the only part of us touching the ground.
This loss of blue and violet to the sky causes the sun to look look yellow or “warmer” than it should. If we could peer out the cupola windows of the space station at a sun unfiltered by the atmosphere it would appear its natural color – glaring white.
The size and concentration of particles in the atmosphere like smoke, dust, pollen and pollution affect what colors we see from the sun. When the sun’s high overhead, its light takes the short path through mostly rarefied air and then through the bottom 10 miles of atmosphere where the air is thickest. No great loss of light. But at sunset and sunrise, the sun is near the horizon and has to shine horizontally through hundreds of miles of the lowest, thickest layer of Earth’s atmosphere.
Air molecules and aerosols across that great distance scatter away not only the shorter wavelength violet and blue parts of sunlight but also the longer wavelength yellows, leaving the familiar rich oranges and reds of a beautiful sunset.
This is a weird analogy but imagine tossing a handful of rocks, some very tiny pebbles and others the size of golf balls, at a beaded curtain. The teeny-tiny rocks will be scattered back by the curtain but the big ones will sail right through. Blue light (tiny pebbles) consists of very short wavelengths easily scattered by air molecules; red light rays (big rocks) have longer wavelengths and move through the air unimpeded.
When particles are few, sunsets are a bright yellow, but when the air is laden with dust or salt (near the oceans) the sun looks like a big red ball of fire. What color will it be tonight?