the atmosphere of planet Earth
The Earth's atmosphere is a layer of gas that surrounds the entire planet. This layer is approximately 350 miles thick. The force of gravity keeps the atmosphere in place around the planet. The atmosphere itself is in four main layers. All of human activity takes place in the first layer, the Troposphere. Most of our weather is found in this layer.
The other layers of the atmosphere (in ascending sequence from the Earth's surface) are the Stratosphere, the Mesophere, and the Thermosphere.
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The atmosphere comprises of a mixture of nitrogen, oxygen and various other gases. Nitrogen is the main ingredient with around 78% of the atmosphere consisting of this gas. Only 21% of the atmosphere consists of oxygen.
The greatest density of atmospheric gases is at the surface of the planet. The atmosphere gets thinner and thinner as one goes away from the planet surface. We see this when people need additional oxygen to get to the top of several of the mountains on planet Earth.
The atmosphere protects us by blocking some dangerous rays coming from the Sun. About one third of the energy that reaches planet Earth from the Sun is radiated back into space, thus maintaining a balance of incoming and outgoing energy.
Without the energy from the sun reaching the Earth's surface, the temperature on planet Earths would be around -18c.
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