Why Do Clouds Glow at Night?
Clouds can appear to glow at night due to several factors, primarily the reflection and scattering of available light sources. It's not a single phenomenon but a combination of effects, depending on cloud type, altitude, and atmospheric conditions.
In some cases, particularly at high latitudes during certain months, the sun may be just below the horizon. This means that high-altitude clouds are still illuminated by sunlight, even after the sun has set for lower-lying areas. This sunlight illuminates the clouds from below, making them appear to glow. This is especially true for noctilucent clouds, which are extremely high in the atmosphere. [Source: In these months and at the right latitudes, the Sun only just sets below the horizon at nighttime. That means these very high clouds can still be lit by bright sunlight from below, even though the rest of the surface is in darkness – making the clouds appear to glow. ]
Reflection of Light Pollution
Clouds can also appear to glow due to the reflection of artificial light from cities and other human settlements. Low-lying clouds, or even haze, are particularly effective at reflecting this light pollution back towards the ground, making the night sky seem brighter than it would be on a clear night. [Source: On nights when the clouds or even semi-transparent haze are present and unusually low, they can reflect light pollution more often than normal.]
Scattering of Moonlight and Starlight
While less dramatic than the effects of sunlight or light pollution, moonlight and starlight can also be scattered by clouds. This scattering can create a diffuse glow, especially in clouds that aren't particularly dense or thick. [Source: At night, moonlight and starlight can be scattered the same way, ambient light glow from Earth can reflect light from clouds, and thicker clouds ...]
Noctilucent Clouds: A Special Case
Noctilucent clouds are a unique type of cloud that forms in the upper atmosphere. They are only visible during twilight hours and appear to glow with a blue or silvery hue because they are illuminated by sunlight from below the horizon. [Source: Noctilucent clouds are thin, wispy clouds that glow with a blue or silvery hue at night when illuminated by sunlight from below the horizon.]
In Summary:
Several factors contribute to the glowing appearance of clouds at night:
- Sunlight below the horizon: High-altitude clouds can be illuminated by sunlight even after sunset.
- Reflection of light pollution: Clouds reflect artificial light from the ground.
- Scattering of moonlight and starlight: Clouds scatter these light sources, creating a diffuse glow.
- Noctilucent clouds: A specific type of high-altitude cloud only visible during twilight.
