While the Lake Tahoe area is known widely among photographers for its blazingly colorful sunsets, those of us that live here year round know that the summer months are dominated by high pressure, and high pressure means no clouds. No clouds often equals a “boring” sunset.
What is a landscape photographer to do in Tahoe in the summer? Other than hop in the car and drive to the edge of the high pressure, there are several tactics that can be taken to hunt down some color in the sky – hope for a volcanic eruption, pray for the jetstream to bring some of that awesome pollution and/or Gobi desert dust from China, or just head high up into the mountains and get ready to capture some alpenglow. This week, I’m going to talk about the impact of all of the above on bringing us those vibrant, colorful sunrises and sunsets.
What exactly is alpenglow? Alpenglow is a narrow band of reddish or pinkish coloring in the sky or on the tips of mountains opposite the rising or setting sun. Since the sun has already set to the west, or has yet to rise in the east, its rays are not directly illuminating and coloring the mountaintops or any clouds above them. True alpenglow results from rays of sunlight that come from the sun below the horizon and travel through the atmosphere above the viewer, where they reflect downwards off of particles in the atmosphere and shine onto clouds or mountaintops.
Why are sunsets and sunrises colorful, as opposed to the yellow-white light of the sun that we are used to seeing in the sky the rest of the day? As easy way to begin visualizing the answer is to think of the cover of the seminal album “The Dark Side of the Moon” by Pink Floyd. If you own this album now would be a great time to put it on and turn your stereo up to 11.
Sunlight appears “white” or colorless, but it is actually comprised of a full rainbow of different colors of light, each of which has its own unique wavelength. You can see this when white light passes through a prism – it breaks into different colors just like on the Pink Floyd cover. Back on earth, in the middle of the day, the sun is high in the sky and its light shines directly down through a relatively thin section of atmosphere. As the earth rotates in space and the sun moves lower in the sky relative to the viewer, sunlight has to shine through a thicker section of atmosphere. Because there are more molecules of oxygen, nitrogen, carbon dioxide, and other atmospheric gases in this thicker parcel of atmosphere, the different component colors of sunlight are scattered to differing degrees as the atmosphere acts like a prism and holds back the light rays with shorter wavelengths.
Visible light ranges from short wavelenghts of blue and violet, to medium wavelengths like yellow and green, and on to longer wavelengths, like orange and red. In the thicker atmospheric section, blue and violet light rays are scattered or bounced back out into space due to their shorter wavelengths. The longer wavelength colors, like red and orange, make it through the thicker atmosphere at sunrise and sunset, and color the sky, the ground, and clouds with their hues.
Sunrise and sunset color can be further amplified by particulate matter in the atmosphere. Many people alive in the Western US will recall vividly bright red sunsets after the eruptions of Mt. St. Helens in 1980, and more recently the eruption of Mt. Pinatubo in the Phillipines in 1991. The enormous amounts of pulverized rock (ash) ejected into the upper atmosphere by the latter eruption quickly circled the globe, and not only gave us vivid sunrises and sunsets, but it also reflected higher than normal levels of the sun’s infrared rays, and caused cooler weather. If you see Scott Gaffney in Safeway, after you tell him you can’t believe he’s a professional cinematographer and you’re so much better than him, ask him and he will tell you that the winter of 1992-1993 in Squaw was one of the deepest on record.
Particles in the atmosphere are larger than the gas molecules, and thus only the very longest wavelengths of sunlight make it through at sunrise and sunset, explaining the red coloration to the sky immediately following eruptions, or prevalent in heavily polluted areas like China or Los Angeles. Check out this photo taken a few weeks ago in Chile after the eruption of Puyehue volcano.
Got an earth-science question for me to answer? Post your queries in the comments and I’ll pick one to write about in next week’s column.