Sky

The sky (also called the atmosphere) is the visible appearance of the upper air as seen from the ground. The term encompasses the layers of air surrounding the Earth, as well as the upper space above the air where stars are visible at night.

The color and light of the sky depend on the position of the sun, atmospheric conditions, and clouds. During the day the sky typically appears blue; at sunrise and sunset it often appears red and orange; at night it appears dark with stars visible.

Why the sky is blue

Sunlight contains various colors with different wavelengths. When this light passes through the air, molecules in the atmosphere scatter it. Shorter wavelengths—blue and violet—are scattered more than longer ones such as red. This causes the sky to appear blue in all directions. When the sun is near the horizon, light travels through a longer path of air, and the blue is scattered away before reaching the eye, which is why the sky appears red and orange instead.

Layers of the atmosphere

The air surrounding the Earth is divided into layers. The lowest layer, the troposphere, is where most clouds and weather occur. Above it lies the stratosphere, which contains the ozone layer that blocks some ultraviolet radiation. Further up are the mesosphere and thermosphere. Air density decreases as altitude increases, gradually transitioning into space.

The sky at night

At night, when sunlight is absent, the moon, planets, and stars become visible. The appearance of the night sky changes according to Earth's rotation and the season of the year. Light from cities reduces the ability of people to see stars, a phenomenon known as light pollution.

Cultural and weather uses

For a long time, people used the sky to indicate time and season. Sailors and nomads navigated by the stars to determine direction and the months of the rainy season. Study of the sky's appearance underlies astronomy and weather forecasting.