OPOD - Asymmetric Shadow Hiding

Last updated on September 16, 2023

OPOD - Asymmetric Shadow Hiding: Exploring the Phenomenon in Detail

Have you ever noticed a peculiar bright spot directly opposite the sun while flying over a forested, stony, or grassy landscape? This intriguing phenomenon is known as an "opposition glow." It creates a sense of mystery and wonder as it accompanies your aircraft. But what causes this asymmetrical shadow hiding effect, and why does it appear more pronounced in certain conditions?

The concept of asymmetric shadow hiding becomes evident when we examine a photograph taken at Whistler Mountain, BC, Canada by Alan Clark of the University of Calgary. In the image, we observe tree shadows and an opposition effect glow. Interestingly, the trees near the antisolar point (ASP) – the point directly opposite the sun – seem to hide their own shadows, resulting in a relatively brighter area compared to its surroundings.

The asymmetry in shadow hiding arises from two key factors: the sun's angle and the observer's position relative to the objects casting shadows. In this particular photograph, the sun is relatively low, and the gondola viewing position is closer to the trees compared to an airplane or mountain top. These conditions accentuate the asymmetrical effect.

To understand the mechanics behind this phenomenon, let's examine the diagram provided. The slanting sun rays cast shadows to the side of each tree. From the gondola's perspective, we can see the tree shadows on the right side of the diagram. As expected, when we look towards the ASP direction, the tree shadow is hidden by the tree itself. However, when we shift our gaze to the left side of the diagram (which corresponds to upwards in the image), the tree shadows continue to be concealed by the shadow casters. This extension of the opposition glow from the ASP contributes to the overall asymmetry observed in the photograph.

It's important to note that while the asymmetry is always present, its prominence diminishes when the sun is higher in the sky. This occurs because the shadow casters become less elongated, and the viewpoint is farther away from the shadow casters' heights. Consequently, the effect becomes less pronounced.

Apart from the shadow hiding phenomenon, another optical effect called "sylvanshine" may also be present in the photograph. Some of the silvery foliage trees near the ASP appear to glow, particularly at their edges. This glow is attributed to the interaction between sunlight and dew drops suspended on leaf hairs, a phenomenon known as the heiligenschein. The dew drops focus sunlight onto the leaf surface, which then refracts back through the water droplets, creating an additional bright glow at the ASP. While some conifers exhibit a strong ASP glow, they lack leaf hairs necessary for heiligenschein. Instead, their waxy leaf or needle surfaces cause water drops to become efficient retro-reflectors, producing a similar sylvanshine effect.

In conclusion, the concept of asymmetric shadow hiding adds an intriguing dimension to atmospheric optics. This phenomenon, observed in the photograph taken from Whistler Mountain, BC, Canada, showcases how objects near the antisolar point can hide their own shadows, resulting in a distinct brightness around this area. The asymmetry arises from the sun's angle and the observer's proximity to the shadow-casting objects. While the effect is always present, it becomes more pronounced when the sun is low and the observer is relatively close to the objects compared to their height. Additionally, the presence of sylvanshine, caused by interactions between sunlight and water droplets on leaf surfaces, further enhances the visual spectacle. As we delve deeper into atmospheric optics, phenomena like asymmetric shadow hiding continue to captivate our curiosity and offer glimpses into the fascinating interplay between light and our natural surroundings.

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.

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