|Asymmetric Shadow Hiding ~ There is something odd about this picture of tree shadows and an opposition effect 'glow'. It was taken at Whistler Mountain, BC, Canada from the Peak-2-Peak Gondola by Alan Clark of the University of Calgary. Alan Clark, shown with permission
|When flying over a forested, stony or even grassy landscape an apparently bright spot is often seen directly opposite the sun and travelling along with the aircraft. This is an ‘opposition glow’. Near the antisolar point (ASP) objects hide their own shadows making this area relatively brighter than elsewhere.
The ASP in this picture is exactly marked by the fuzzy dark gondola shadow. We see no tree shadows near it. However, they are also (mostly) absent from the top half of the view especially directly above the gondola shadow. The treescape not only looks brighter around the ASP but also above it.
Why the asymmetry?
It arises (1) because the sun is not high and (2) because the gondola viewing position was relatively close to the trees compared to that of an airplane or mountain top.
At right we see slanting (but parallel) sun rays casting shadows to the side of each tree. From the gondola position marked by the ‘eye’ the tree shadows on the diagram right are visible. In the ASP direction the tree shadow is exactly hidden by the tree itself as expected. But to the diagram left (leftwards in the diagram is upwards in the image) the tree shadows continue to be hidden by the shadow casters. In effect, the ‘opposition glow’ is extended from the ASP.
The asymmetry is always present but is much less so when the sun is high, when the shadow casters are less elongated and when the viewpoint is far away compared to the shadow casters’ heights.
There might be another optical effect present. Some of the silvery foliage trees near the ASP appear almost glowing, especially at their edges. This could be ‘sylvanshine’ – a term coined by atmospheric optics expert Alistair Fraser.
Sylvanshine is related to the heiligenschein whereby dew drops suspended on leaf hairs focus sunlight on the leaf surface beneath which then backtracks through the watery lens to appear as another bright glow at the ASP. Some conifers show a strong ASP glow but they are hairless and could not produce a heiligenschein. Instead, the waxy leaf/needle surface causes water drops to have surface contact angles of less than 90° (i.e. form attached drops) and these become very efficient retro-reflectors.