The sky is simply the unobstructed view upward from Earth’s surface, including our atmosphere and outer space beyond.

On a clear day, the sky looks blue because air molecules scatter shorter blue wavelengths of sunlight more than longer ones. This effect is called Rayleigh scattering.

If Earth had no atmosphere, the sky would appear black even in daytime, like it does from the Moon.

There is no physical ceiling to the sky. It transitions gradually into outer space.

The air we breathe is mostly nitrogen (about 78 percent), oxygen (about 21 percent), and tiny amounts of other gases.

Most weather happens in the lowest layer of the atmosphere, the troposphere. The word ultimately comes from the Greek “tropos,” meaning turning, combined with “sphere.”

About 99 percent of the atmosphere’s mass lies below 30 kilometers above Earth’s surface.

Sunlight travels through more atmosphere at sunrise and sunset, so blue light is scattered out and we see red and orange hues.

Clouds look white because they scatter all colors of sunlight equally.

Thicker clouds look darker or gray because less sunlight gets through them.

Rainbows form when sunlight is reflected and refracted inside water droplets.

Circumhorizontal arcs, sometimes called fire rainbows, are specific halos made by plate shaped ice crystals and appear as colorful bands in the sky.

Rare light ribbons called STEVE can appear in the night sky and are distinct from auroras.

Even when stars are not visible in daylight, the sky still glows faintly at night due to airglow, a tiny emission of light from the upper atmosphere.

Clouds are made of billions of tiny water droplets and ice crystals. If they were solid, they would weigh around three million pounds on average.

Some high altitude ice crystal clouds, called noctilucent clouds, are visible after sunset and glow against the dark sky.

At any given moment, clouds cover around 67 percent of Earth’s surface.

Mammatus clouds look like bubble wrap hanging from the sky and often indicate stormy conditions.

Lenticular clouds can form lens shapes that people sometimes mistake for UFOs.

When you see stars in the night sky, you are actually looking back in time. Their light traveled years to reach Earth.

Stars twinkle because Earth’s atmosphere bends their light before it reaches your eyes.

Meteor showers happen when tiny space debris enters Earth’s atmosphere and burns up, making shooting stars. Meteors usually become smaller than grains of sand, and most never hit the ground.

The sky on other planets can look dramatically different. Mars often has a reddish orange sky because of dust.