Searching for a Spectacular Summer Sunset

Erin Parcher wants it all.

My office mate, a frequent provider of column ideas, wants to know if we're being robbed of pretty summer sunsets because of our location on the globe.

One would think that near-constant sunshine would be enough to satisfy most Alaskans during our brief, beautiful summer. But Erin wonders if Alaska sunsets may be a bit less colorful in summer because we have less atmosphere between us and the sun when the sun sets to the north. She figures that because the northern hemisphere nods toward the sun this time of year, rays of sunlight penetrate less atmosphere to get here.

To answer her question, I researched a question she already knew the answer to: What makes sunsets colorful?

The sun transmits much of its energy as radiation waves of visible light, according to the textbook Meteorology Today. This light stimulates our eyes so we see colors ranging from purple to red. Purple has a wavelength of about 0.4 micrometers, which means light we see as the color purple measures four ten-thousandths of a meter from one wave crest to the next. Blue, and shades of blue, have a bit longer wavelength than purple, followed by green, yellow and red. Red has the longest visible wavelength of light, at about 0.7 micrometers.

At midday, when the sun is most intense, all wavelengths of visible light reach the eye with nearly equal intensity, and the sun appears white. When the sun is rising or setting, it sits on the horizon at a low angle, which causes it to be filtered through much more of the earth's atmosphere. When the sun is directly overhead, its rays travel 93 million miles through space and then penetrate the atmosphere, a 20-mile thick layer of air that coats the planet. When the sun is sitting just above the horizon, its rays penetrate about 12 times more atmosphere than at midday.

The atmosphere is primarily composed of nitrogen (78 percent) and oxygen (20 percent), along with much smaller portions of other gases. As sunlight passes through thick layers of atmosphere at sunrise and sunset, purple, blue and green light waves collide with air molecules and are scattered away before they reach the ground. Yellows, oranges and reds all have longer wavelengths, and are not affected as much by air molecules. Because of this, it's possible to see sunrises and sunsets of any variation of the three colors.

Intensely colorful sunsets are driven almost entirely by airborne microscopic particulates large enough to block yellow and orange light waves, according to Rich Benner, assistant professor of chemistry at the Geophysical Institute. Sulfur from man-made pollution and released from ocean organisms, ash from volcanic eruptions, and soot from wildfires are among the many particulates large enough to scatter away yellow and orange light waves, setting sunsets aflame by allowing only red light to reach the eye. The eruption of Mt. Pinatubo in the Philippines in 1991 was responsible for months of beautiful sunsets around the globe, and the same effect can be seen in Alaska when wildfires fill the air with smoke.

As for Erin's question, Benner and other atmospheric scientists at the Geophysical Institute agreed when the sun is setting at any time of year it's at the same low angle and is therefore going through approximately the same amount of atmosphere, regardless of season. Some suggested that the reason Alaskans don't often see beautiful summer sunrises and sunsets is because they occur when most of us are sleeping. On June 1, sunrise and sunset for interior Alaska was 3:33 a.m. and midnight, respectively.

So I guess Erin really can have it all--if she chooses not to sleep. That's what winter's for, anyway.