sky

The sky refers to the arc or dome of atmosphere and space that is visible from the surface of Earth. By day it appears as a blue vault overhead and by night as a dark canopy speckled with stars. The sky is not a solid object; it is the appearance resulting from light and matter in the atmosphere as seen from the ground.

Composition: It is the lowest part of Earth's atmosphere, extending from the ground through the troposphere and up into the stratosphere, mesosphere, and beyond to the exosphere. Most weather occurs in the troposphere. When sunlight enters the atmosphere, molecules and small particles scatter light. Shorter wavelengths (blue and violet) scatter more, making the sky look blue to observers because violet is absorbed by the upper atmosphere and the Sun.

Color variations: The color of the sky shifts with solar angle and atmospheric conditions. During clear days it is blue; at sunrise and sunset, the path through the atmosphere lengthens, scattering short wavelengths and leaving reds and oranges. Overcast skies appear gray; pollution and aerosols can alter color as well. At night the sky darkens as sunlight vanishes, revealing celestial objects, though light pollution can wash out stars.

Phenomena and significance: The sky hosts weather phenomena such as clouds, rainbows, halos and sun dogs, and auroras at high latitudes. Culturally and scientifically, the sky has served navigation, timekeeping, and astronomy, guiding explorers and shaping calendars and myths. Modern science studies the sky through observational astronomy and space-based instruments.