What are the four most common gases in dry air?

Dry air is composed primarily of nitrogen, oxygen, argon and carbon dioxide. Nitrogen and oxygen comprise approximately 99 percent of the atmosphere. The other gases are present in minute amounts, each of them equaling less than 1 percent.

The dry air of the Earth's atmosphere contains 71 percent nitrogen and 28 percent oxygen. Other gases comprise the remaining 1 percent. The atmosphere of the Earth is responsible for:

• protecting people, plants and animals from the sun's ultraviolet rays
• using the greenhouse effect to keep the Earth warm
• preventing extreme temperature changes between day and night

The atmosphere of the Earth is divided into five different layers:

• Troposphere - people, animals and plants live in this layer. It is the lowest layer and the one in which most of the weather occurs.
• Stratosphere - this is the layer above the troposphere and contains the ozone layer and the jet stream.
• Mesosphere - located above the stratosphere, the coldest temperatures in the Earth's atmosphere are found in this layer. Not much is known about this layer, as it is too high for weather balloons and too low for satellites.
• Thermosphere - this is the layer that is commonly referred to as outer space. The International Space Station orbits in this level, as did the Space Shuttle.
• Exosphere - air in this level is extremely thin and, near the top, blends into the vacuum of outer space.