The phenomenon you are referring to is often called "over-the-horizon" viewing or "mirage." It occurs when light undergoes refraction as it passes through layers of air with different densities, leading to an apparent bending of the light rays. This bending can sometimes allow objects that are beyond the curvature of the Earth to become visible.
The Earth's atmosphere is not uniform, and its density can vary due to factors such as temperature, humidity, and air pressure. These variations can cause the speed of light to change as it passes through different layers of the atmosphere. When light encounters a change in density, it bends or refracts, similar to how a straw appears bent when partially submerged in water.
Under certain atmospheric conditions, such as when there are significant temperature inversions or layers of cooler air near the ground and warmer air above, light rays can be refracted in a way that causes them to curve along the Earth's surface instead of following a straight line into space. This bending of light allows the observer to see objects that would otherwise be hidden behind the Earth's curvature.
Furthermore, the curvature of the Earth is not always a significant barrier to visibility, especially over relatively short distances. The Earth's curvature is gradual, and the line of sight from an observer can extend beyond the expected horizon due to the curvature being more gradual than anticipated.
It's important to note that these atmospheric phenomena can cause distortions, mirages, or unusual appearances of objects. The observed images may be inverted, displaced, or appear distorted due to the complex interplay of light and the atmosphere.
Overall, the bending of light due to atmospheric refraction can explain how objects beyond the curvature of the Earth can become visible under certain atmospheric conditions.