The Full Circle of Rainbows: How and When to See Them
The Full Circle of Rainbows: How and When to See Them
Rainbows, beautiful optical phenomena, are often seen as half-arcs across the sky. However, a complete circle of a rainbow, akin to a full halo, is indeed possible, but not always visible from the ground. This intriguing natural spectacle can be witnessed if the observer is at a sufficient elevation, such as on a mountain or, more commonly, from an airplane. Understanding the science behind rainbows and how they form a complete circle can deepen our appreciation of these fascinating displays.
How Rainbows Form
When sunlight passes through raindrops, it undergoes a process called refraction, where the light bends as it enters the water. This is followed by internal reflection and, in the case of a secondary rainbow, a second internal reflection. The refraction causes the light to bend at an angle of approximately 42 degrees for a primary rainbow. This angle, known as the displacement angle, is the reason why rainbows appear as arcs in the sky.
The optical properties of water mean that light is scattered through a specific angle. For a primary rainbow, the angle is roughly 42 degrees, meaning the light exits the raindrop at this angle. In the case of a secondary rainbow, the light is reflected twice inside the raindrop, resulting in a larger angle of about 138 degrees. This secondary rainbow appears as an arc below the primary one and is fainter and wider.
Seeing a Full Circle Rainbow
To see a full circle rainbow, an observer must be at a high vantage point where the sun is below their horizon. This is because the secondary reflection inside the raindrops produces a more precise angle, often allowing the arc to be seen as a complete circle. However, even from such a high point, only the lower half of the full circle is often visible due to the shadow of the observer's head blocking the upper half.
“From a suitably elevated vantage point, like on a tall mountain or in an airplane, you may see a full circle if the rain is below you.”
When the sun is situated at an angle of around 42 degrees or less, the refracted light can pass through raindrops below the observer. This creates a full circle rainbow, with the non-visible half blocked by the observer's own shadow. The rainbow's curvature is centered on the direction directly opposite the sun, and its radius is approximately 42 degrees, with the colors spreading out into a stunning spectrum.
The Impact of Sunlight and Viewing Angle
The visibility of a full circle rainbow depends heavily on the sun's position and the observer's location. If the sun is higher above the horizon, the rainbow will appear as an arc, due to the shadow effect mentioned earlier. The closer the sun is to the horizon, the more visible the full circle will be, as the observer's head is no longer in the way.
A full circle rainbow visible from an airplane.The effect of sunlight on the rainbow's appearance cannot be overstated. Rainbows are usually best viewed during the early morning or late afternoon when the sun is lower in the sky. During these times, the sun's angle relative to the horizon is more favorable, reducing the shadow effect and allowing the full circle to be observed more easily.
Astronomical and Geographical Factors
Several astronomical and geographical factors can influence the visibility of full circle rainbows. Atmospheric conditions, such as humidity and the presence of fog, can impact the rainbow's appearance. Additionally, the observer's height above sea level and the location's latitude can affect how often and how clearly a full circle rainbow is visible.
For instance, the likelihood of observing a full circle rainbow is greater in areas with higher altitudes and more persistent rain. Mountains and mountain ranges often provide ideal viewing spots, as they offer elevated positions that minimize shadow effects. In some cases, rain in valleys below an observer can create the ideal conditions for a full circle to be seen.
Scientific and Practical Applications
The study of rainbows and their formation has practical applications in various fields, including meteorology and optics. Understanding the behavior of light and water droplets is crucial for improving weather forecasting and interpreting optical phenomena. In aviation, the ability to identify and interpret rainbows can be useful for pilots in navigating through weather patterns and ensuring safety during flights.
The phenomenon of a full circle rainbow also holds cultural and symbolic importance in many societies. Throughout history, rainbows have been associated with hope, renewal, and divine promises. In contemporary contexts, rainbows are used as symbols in LGBTQ communities, representing diversity and acceptance.
Conclusion
While the concept of a full circle rainbow might seem like a myth, it is a real and scientifically explainable phenomenon. Observers with the right conditions—high vantage points, specific sun angles, and the right atmospheric conditions—can indeed witness a complete circle of color. The study of rainbows continues to inspire both scientific inquiry and wonder, challenging our understanding of nature and the beauty it can create.