Mountain Shadows: What Are They?

Mountain shadows are shadows of mountains. They are formed at sunrise or sunset when the sun is at a position lower than the mountain's summit. Les Cowley explains on the UK Web site Atmospheric Optics that mountain shadows can stretch for 100 to 300 miles. They are three dimensional columns of lightless air and have height, breadth, and length. They are actually a form of crepuscular ray.

They may be viewed from a variety of different points but when they are viewed from the mountain's summit they are always seen in perspective. The shadow extends and tapers to the furthest distance which ends in a point. This is true even when the mountain has a flat top. As explained on Atmospheric Optics a cross section of the shadow would reveal the true shape of the mountain, but because a mountain shadow is so long the shape is lost in perspective.

When you see a mountain shadow from the summit, Atmospheric Optics says you're looking along the top of the length of a "long tunnel of shadowed air...a three dimensional void." The umbra is the fully darkened portion of the shadow. The penumbra is the more faded edges. The shape of the sun, which is a curved disk and not a single flat point of light, creates multiple umbral parts and causes the "umbra parts to converge and taper." The tapering limits the "umbral length of shadows." Objects that are smaller than mountains do not experience tapering from perspective because their shadows are not long like mountain shadows.

Perspective

The appearance of long columnar or plane objects growing wider when approaching near to you and receding to a point when extending far from you is perspective. Perspective can be seen in redwood trees if you look up from the bottom, as shown at Atmospheric Optics. Redwoods are long and columnar and extend straight up and away. They look wide at the bottom and recede to finer points at the top. Perspective is involved in our perception of crepuscular rays.

Crepuscular Rays

Crepuscular rays are columns of dark and light air radiating out in near-parallel lines from the sun, as Atmospheric Optics explains. The sun from Earth appears 0.5 degrees across. This width causes crepuscular rays to spread 0.25 degrees"each side of the mean direction." This is why crepuscular rays are near-parallel to each other.

Dark crepuscular rays are produced when the sun shines from behind an object, like a mountain, and casts a shadow. Light crepuscular rays are produced when the sun's light shines between objects. This creates the long tunnels of lit or unlit air that we perceive as beams or shadows.

Select images of mountain shadows can be viewed on-line at Atmospheric Optics. One in particular is of special interest. Michael Connelley's photograph of the shadow of Mauna Kea in the Hawaiian Islands shows the mountain's shadow stretching out above a heavy layer of clouds. The photo caption explains that this photo demonstrates that mountain shadows extend through the atmosphere; they are not cast upon the clouds. The caption also explains that we can see a full moon rising in the heart of the shadow because mountain shadows, although up to 300 miles long, are not long enough to reach the moon and dim it. The Earth's shadow, on the other hand, is more than 1 million miles long and can reach the moon and darken it.