* Scattering of Light: Clouds are made of tiny water droplets or ice crystals. These particles scatter sunlight in all directions. Even when the sun is below the horizon, some sunlight is still scattered towards the clouds, making them appear bright. This is similar to how the sky can be blue during the day, even though the sun is directly overhead.
* Reflection: Clouds also reflect sunlight, particularly when the sun is low in the sky. This reflected light can make the clouds appear even brighter.
* Moonlight: During the night, the moon's light can illuminate clouds, giving them a soft glow.
* Artificial Light: In urban areas, artificial light from buildings and streetlights can also illuminate clouds, making them appear brighter than the surrounding darkness.
Here's an analogy: Imagine a flashlight shining on a white wall in a dark room. The wall will appear bright even though the rest of the room is dark. The light from the flashlight is being scattered and reflected off the wall, making it visible.
The same principle applies to clouds. Even when the sun is below the horizon, there is still light in the atmosphere that can be scattered and reflected by the clouds, making them appear bright.
