Atmospheric
Optics
By eye or camera both parallel shadow tubes appear to converge to the same anti-solar point.
Alfred Lee is just inside the shadow at left and looks along its length.
That shadow looks dark.
He is above the shadow of the lower mount at right. He looks across it through a shorter path of dark air. It looks lighter. It is further lightened by scattering from the intervening sunlit air.
A mountain shadow is a column of dark air tens or hundreds of miles long. Seen from a summit, it is viewed along its topmost edge and appears triangular almost regardless of the mountain shape. Whatever the cross section of the shadow tube profile, perspective makes it appear to taper to a point far away. A straight road or railway track does the same.
Why a darker and lighter shadow? As seen by the sun on that morning, the profile of Kinabalu is decidedly asymmetric. To the south (left) it climbs gradually to a relatively flat summit. Northwards and just beyond where Alfred Lee was standing, it falls away steeply. The lower mountain to the north (right) is responsible for the lighter shadow.
Imagine a more extreme Kinabalu made of two blocks.
At sunrise each casts a long parallel shadow. Alfred Lee is positioned at the right-hand corner of the larger block and looks along the main shadow tube.
