The slanting sunlight over the river sky is most commonly observed during the golden hours of sunrise and sunset. This phenomenon occurs due to the angle at which the sun's rays strike the Earth's surface. During these times, the sun is low on the horizon, causing its light to travel through a greater thickness of the atmosphere. This results in the light being scattered in all directions, with shorter blue and violet wavelengths scattered out of the direct path, leaving longer red and orange wavelengths to dominate the sky. The river reflects this light, creating a stunning visual of slanting sunlight over the water's surface.

The golden hours typically last for about an hour before sunrise and after sunset. During this period, the sun's rays are at a 45-degree angle to the horizon, which is the optimal angle for creating the most dramatic lighting effects. The slanting sunlight over the river sky is most vivid in areas where the horizon is unobstructed, such as over open water or in expansive plains.

According to a study by the National Oceanic and Atmospheric Administration (NOAA), the angle of the sun's rays at sunrise and sunset varies depending on the latitude and the time of year. For example, at the Tropic of Cancer (23.5 degrees north), the sun is at a 45-degree angle to the horizon at noon during the summer solstice. During the winter solstice, the sun's angle is much lower, around 23.5 degrees, resulting in shorter daylight hours and less intense slanting sunlight.

The intensity of the slanting sunlight over the river sky also depends on atmospheric conditions. Clear skies allow for the most dramatic lighting effects, while clouds can soften the light and create a more diffused glow. Additionally, the presence of moisture in the air, such as from a river or lake, can enhance the reflection and create a more vibrant scene.

In conclusion, the slanting sunlight over the river sky is a captivating natural phenomenon that occurs during the golden hours of sunrise and sunset. This effect is influenced by the angle of the sun, atmospheric conditions, and the presence of water. The most dramatic displays are observed in areas with unobstructed horizons and clear skies, particularly during the summer months when the sun's angle is most favorable.