River in the Sky - OPOD

Last updated on September 16, 2023

River in the Sky: A Phenomenon of Atmospheric Optics

Have you ever witnessed a river in the sky? This captivating atmospheric optics phenomenon occurs when a low sun projects the image of a nearby river onto the underside of clouds. It creates a mesmerizing sight that can leave you in awe of nature's wonders. In this article, we will explore the science behind this phenomenon and delve into its intriguing details.

The Creation of the River in the Sky

When the sun is low on the horizon, its rays interact with calm water on a river's surface, reflecting parallel beams of sunlight back upwards. These rays then impinge upon the underside of clouds, forming an image of the river. The resulting effect is a wavy orange line that appears to flow through the sky.

The Sun's Position and Cloud Height

To witness this phenomenon, the sun needs to be at a specific angle in the sky. Typically, it should be around 5.5° high, casting its rays at just the right angle for the image projection to occur. Additionally, the illuminated cloud patches should be positioned between 6-10° above the horizon. These conditions allow for optimal interaction between the sun's rays, the river's reflection, and the cloud cover.

Estimating the height of the cloud underside can be challenging, but it is believed to range from approximately 4,000 to 9,000 feet. This range is not unrealistic for low-level clouds and contributes to the overall visual effect of the river in the sky.

A Historical Parallel: The "Ice Blink"

Interestingly, this phenomenon shares similarities with an effect observed by Arctic dwellers and explorers known as the "Ice Blink." In this case, distant leads—clear water separating ice floes—were imaged onto the undersides of clouds near the horizon. These images provided valuable information to those navigating icy waters, as they could indicate the presence of open water.

Exploring Atmospheric Optics Further

Atmospheric optics is a fascinating field of study that explores the interactions between light and the Earth's atmosphere. It encompasses various phenomena, such as rainbows, halos, and mirages, each offering unique insights into the behavior of light in our atmosphere. The river in the sky is just one captivating example of the wonders that atmospheric optics can reveal.

Conclusion

Witnessing a river in the sky is a remarkable experience that allows us to appreciate the beauty and complexity of our atmosphere. The interplay between sunlight, water, and clouds creates a visual spectacle that reminds us of the intricate processes occurring above us. Whether you stumble upon this phenomenon by chance or actively seek it out, take a moment to marvel at the river flowing through the sky—a reminder of the astonishing phenomena that surround us every day.

River in the Sky

Captured by Matthew Shepherd at Troy, New York. The wavy orange line is an image of the nearby Mohawk River projected by a low sun onto the underside of the clouds.

All images ©Matthew Shepherd, shown with permission

Calm water on the river reflects parallel sun rays back upwards. They impinge on the cloud underside to form an image of the river.

The upward going sun rays are visible in the lower image.

The sun was 5.5° high and the illuminated cloud patches 6-10° above the horizon. The distances of the reflecting parts of the river are less easy to judge but 7-15km likely brackets them. That gives a cloud underside height of very approximately 4-9000 ft. Not unrealistic for low level clouds.

Arctic dwellers and explorers used to use a similar effect, an "Ice Blink". Distant leads, clear water separating ice floes, were imaged onto the undersides of clouds near the horizon.

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Reference Atmospheric Optics

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