Alpenglow is simply redirected light. Think of the clouds and atmosphere as a giant strobe reflector. Think of the sun as a giant strobe (for which, there is no PocketWizard controls). In the image art right, the entire scene is lit from light bouncing off the atmosphere and hitting Seattle and Mt. Rainier. If we had no atmosphere, the light would just pass on by as the sun is not yet over the horizon. (If you don’t believe me, go to the dark side of the moon and see if alpenglow exists.)
Alpenglow often has a characteristic red to orange hue to it. This is because the light of the nearly rising or recently setting sun is passing through more of the atmosphere than when it is high overhead. The atmosphere strips out the blues first and leaves us with warm, red/orange light.
Some people call direct light hitting a distant peak alpenglow, while they stand in the shadows of a valley (i.e. image at right). While this isn’t technically correct, it’s important not to start an internet flame war about it (hint: that means not in the comments section of this post, either, although legitimate debate is always welcome). If you find yourself getting in a tizzy about someone mislabeling Alps glowing from direct sunlight, instead of indirect sunlight, it might be time to step away from the computer and go shoot some photos. It’s just not worth the headache, both situations make for excellent photos.
Shooting alpenglow presents no particular challenge to the photographer. Metering is done in the same fashion but post processing is where some magic can happen. Because of the intense colors, photos often have more saturation and are more vibrant than once the sun peeks over the horizon and lights the scene directly.
The Blue Hour, an hour before sunrise and after sunset, intermingles with alpenglow depending on location, atmospheric conditions and topography. Think of it as a continuum from sunset, to alpenglow (if there is enough atmosphere/clouds to reflect off of) into the Blue Hour when the red leaches from the scene.
Again, don’t get too caught up in terminology. Just remember that there is excellent light early and that’s why you should drag yourself out of bed when it is still dark in order to capture the blue turning to red turning to orange to direct sunlight as the sun races over the horizon. Then go back and catch a nap.