OPOD - Opposition Streak

Last updated on September 16, 2023

OPOD - Opposition Streak: A Fascinating Atmospheric Phenomenon

Have you ever noticed a bright line rather than a circular spot during the final approach of a flight? Fabiano Diniz, an avid observer of atmospheric optics and astronomy, experienced just that during his flight from Curitiba to Porto Alegre in southern Brazil. This unique optical phenomenon is known as the Opposition Effect Streak, and it occurs when the shadow of an object appears as a bright linear streak instead of a circular spot. Let's delve deeper into this intriguing atmospheric phenomenon and understand the science behind it.

The Science Behind the Opposition Effect Streak

The Opposition Effect Streak is a result of shadow hiding and backscattering of light. When an object casts a shadow, the area opposite the sun (known as the antisolar point, ASP) experiences apparent glows. The glow is usually circular, but the presence of a bright streak suggests an asymmetry in the shadow casting objects.

To visualize this, imagine a large array of parallel horizontal rods set slightly above the ground. When you look downwards along the sun's rays towards the ASP, the shadows of the rods closest to the ASP are neatly tucked in and hidden beneath them. However, as you move further away from the ASP, the shadow of each rod appears shifted towards the antisolar point, becoming increasingly visible. This phenomenon creates a bright linear streak instead of a circular bright area.

Factors Affecting the Appearance of the Opposition Effect Streak

While a perfectly aligned array of crops would be rare, even a slight degree of alignment due to wind or planting can produce a noticeable opposition streak. The alignment of objects plays a crucial role in determining the appearance of this optical phenomenon. Objects that are more aligned with the sun's rays create a more prominent streak, while less alignment may result in a fainter or less defined streak.

Observing the Opposition Effect Streak

The Opposition Effect Streak is best observed when you are in motion. Whether you are walking through fields or flying in an airplane, the movement allows you to perceive the subtle changes in brightness and alignment. These effects can be mesmerizing to witness, offering a unique perspective on the interaction between light and shadows in our atmosphere.

Real-Life Examples and Variations

While the opposition streak observed by Fabiano Diniz during his flight was a linear one, it's important to note that this phenomenon typically appears as a circular glow. The linear streak observed by Diniz indicates an exceptional level of alignment, resulting in a deviation from the usual circular appearance.

In real-life scenarios, the opposition streak is often seen as a circular bright area around the antisolar point. Factors such as soil and rock minerals backscattering light, as well as the presence of dew on the ground, can contribute to the brightness of the glow.

Exploring Atmospheric Optics

The Opposition Effect Streak is just one of the many fascinating phenomena within the realm of atmospheric optics. From rainbows and halos to mirages and crepuscular rays, our atmosphere constantly presents us with stunning visual displays. Understanding these phenomena not only deepens our appreciation for the beauty of nature but also allows us to gain insights into the physics of light and its interaction with our atmosphere.

Conclusion

The Opposition Effect Streak is a captivating atmospheric optics phenomenon that can be observed under specific conditions. This unique optical effect occurs when shadows cast by objects create a bright linear streak instead of a circular spot. Factors such as alignment and backscattering of light play a crucial role in shaping the appearance of the opposition streak. Whether you observe it from an airplane or while walking through fields, witnessing this optical phenomenon in motion offers a mesmerizing experience. As we continue to explore atmospheric optics, we uncover the hidden intricacies of our atmosphere and gain a deeper understanding of the world around us.

Opposition Effect Streak, Brazil

Linear opposition glow observed by Fabiano Diniz (atoptics & astronomy).

" During the final approach of a flight from Curitiba to Porto Alegre (both cities are state capitals in southern Brazil) I saw an opposition effect that caught my attention. It appeared as a bright line rather than a circular spot. As I was sitting in the last row, the plane shadow appears a little bit ahead of the line center." �Fabiano Diniz, shown with permission.

Shadow hiding gives apparent glows opposite the sun (at the antisolar point, ASP). Soil and rock minerals also backscattering light. When the ground is bedewed a heiligenschein adds its own brightness.

The glow is usually roughly circular. This bright streak implies an asymmetry in the shadow casting objects.

Imagine an extreme, a large array of parallel horizontal rods set a short distance above the ground. The rods form an equally spaced array of dark shadows.

Now look downwards on the rods in a direction along the sun�s rays (i.e. towards the ASP). Rods very near the ASP have their shadows neatly tucked in and hidden beneath them. Further away the shadow of each rod appears shifted towards the antisolar point and thus increasingly visible.

Areas away from the ASP have more shadows and so on average appear darker. But, with our extreme array, the effect only occurs along one angular direction. We see a bright linear streak instead of a circular bright area.

Crops so carefully arrayed would be unusual indeed. However, some degree of alignment due to wind or planting is sufficient to produce a bright opposition streak. See another example here.

These subtle effects are best seen when you are moving. They can be seen when walking through fields as well as from airplanes.

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Reference Atmospheric Optics

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