Weather - Noctilucent clouds: the glow of the upper atmosphere

Noctilucent clouds, or night shining clouds, are tenuous ice crystal formations located in the upper atmosphere near the polar mesopause. From the ground, they are only visible during astronomical twilight, when an observer remains in Earth's shadow while the high-altitude clouds catch sunlight. Usually seen in summer between fifty and seventy degrees latitude, they appear from space as polar mesospheric clouds. Recent research indicates rising atmospheric methane emissions may generate extra water vapor, creating or intensifying these luminous displays.

Source: Wikipedia