Air is the invisible mixture of odorless and tasteless gases that surrounds the Earth. The gases that make air are about 78 percent nitrogen, about 21 percent oxygen, and about 1 percent other gases such as carbon dioxide, neon, and hydrogen.
The gases in the air are used by living things in different ways. Humans and other animals breathe oxygen in the air. Cells in the body use oxygen to get energy from food. This process is called cellular respiration. Plants use carbon dioxide in the air to help make carbohydrates which plants use for growth. This process is known as photosynthesis. Humans and animals, in turn, take carbohydrates from plants to grow as well.
While the air around the Earth is mostly gas, air also holds tiny particles such as dust and pollen. Tiny particles in the air are called aerosols. Air also holds water vapor. Water vapor is the gaseous form of water. (Water can also be a liquid and a solid.) For the most part aerosols and water vapor are transparent but not wholly invisible.
The layers of air surrounding the Earth are called the atmosphere. The air that surrounds the Earth acts as insulation. It keeps the Earth from getting too hot or too cold. Rays from the Sun are very hot, but the air in the atmosphere diffuses the Sun’s heat and allows in just enough to keep the Earth warm but not unbearably hot. Outer space is very cold, but the air in the atmosphere keeps that cold out and warmth that reaches the Earth in place.
The air (atmosphere) that surrounds the Earth was first formed more than 4.5 billion years ago when gas and dust swirling through the solar system clumped together to form planet Earth. Some of the elements that made the Earth were solids and some were gas–air came from some of those gases.
Gravity–the result of the Earth’s rotation–holds air in layers around the Earth; there are five layers of air surrounding the Earth. The layer closest to the Earth is most dense or heavy; the layer furthest from the Earth–where the atmosphere meets space is the least dense.
The heaviness and lightness of air are called air pressure. Very close to the Earth’s surface–sea level–air pressure is high because the weight of the whole atmosphere is pushing down on it. The gases in the air are made of molecules and those molecules are the most squeezed, dense, and, in turn, heavy at sea level. The air pressure on a mountain top is low because there is less atmosphere or levels of air pushing down from above. Air and air pressure are important components of weather.
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