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Look up in the Sky
What is the sky? What is air? What is the atmosphere? The atmosphere is just a thin layer of gases that surrounds the Earth. It can act many different ways and you need it to survive. Just as there are layers inside the Earth, there are also layers in the atmosphere. The layers interact, heat up, and interact with the top layer of the Earth's crust. Sometimes you feel the atmosphere as a cool breeze. Sometimes it's a really hot and humid day that seems to push on you from all sides.
Like an Envelope
If compared to the diameter of the Earth, the atmosphere is very thin. It is a coating of gases that protects the Earth and life on Earth from the vacuum and radiation of space. The thickness of the atmosphere is a balance between the gravity of the Earth and energetic molecules that want to rise and move towards space. The molecules become excited as energy from the Sun hits the Earth. If the Earth were much larger, the atmosphere would be thicker. The gravity of a larger planet would pull those gas molecules closer to the surface and pressure would increase.
The atmosphere is far more than just a layer of gases surrounding the Earth. It is a moving source of life for every creature of the planet. While the atmosphere is mainly composed of nitrogen (N2), it also contains gases such as oxygen (O2) and carbon dioxide (CO2) that plants and animals need to survive. It has specialized molecules like ozone (O3) that filter out harmful radiation from space. The atmosphere also protects us from the vacuum of space. Without the atmosphere, our world would be as barren and dead as the Moon or Mercury.
One result of the circulation of gases and particles is the climate of the planet. There is not one climate for the whole planet. Specialized climates are found in areas all over the planet. There is one type of climate over the equatorial Pacific Ocean and another type at the North Pole. The common trait of all of these Earth climates is the atmosphere.
The hot air from the equator eventually moves north or south to other climates. That warmer air combines with cooler air and mixing begins (and storms form). That constant mixing of the atmosphere helps to keep a stable system for the organisms of this planet to survive. Oxygen will never run out in one area of the planet, and the temperature will not skyrocket in another.
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