An atmosphere is a layer of gases that surounds a planet. Earth's atmosphere is relatively thin in vertical extent compared to the size of the solid part of the planet (see Figure). In that figure, the "habitable atmosphere" refers to the portion of the atmosphere where humans can survive without breathing assistence, which extends up to only about 6 kilometers (or 4 miles) above sea level. Thus in order to survive without breathing assistance, we cannot venture more than about 6 km above sea level. Think about how short 6 kilometers seems when moving along a horizontal path. People often use the term "thin air" to describe the air at high alitudes, where "thin" refers to the density of air (the number of gas molecules that is contained in a given volume, like the volume of your lungs for example). It gets more difficult to breath as the air becomes "thinnner" or less dense. In the atmosphere, the gas density is highest at the surface and gets lower and lower as one moves away from the surface until at some point (on the order of a few hundred kilometers above sea level), it becomes difficult to determine whether you are still in the Earth's atmosphere or in outer space.
The atmosphere is retained by Earth's gravity. The mixture of gases that is found on Earth today is commonly called air. Beside gases, the atmosphere also contains very minute quantities of microscopically small suspended particles of solid and liquid (called aerosols), which includes things like dust, pollen, and cloud droplets. Most aerosols are much too small to see individually. However, when concentrations of aerosols are high, such as the microscopic liquid droplets that make up clouds or heavy smoke from a fire, the combined effect of concentrated microscopic particles can be seen. Gases are composed of individual molecules that are not chemically bonded together. The gas molecules and suspended aerosols are individually very tiny and have very little mass. Thus, they have very little weight on Earth compared with the solid and liquid substances, which are composed of countless numbers of chemically bonded molecules, that compose the Earth's ground surface. In essence, the larger and heavier solids and liquids sink to the bottom, and the gases (the atmosphere) float to the top and become the outer layer surrounding the Earth. Not all planets have atmospheres of gas surrounding them, while many other planets have much more massive atmospheres than the Earth. Living on the surface of the Earth (at the bottom of our atmosphere), we have become so adapted to our particular atmosphere that we cannot survive more than a few minutes without it. Just as fish need to be surrounded in an environment of water to survive, we must be surrounded in an environment of air. And anywhere we go, e.g., under water, outer space, etc., we must have air with us for long-term survival.
Meteorology is the study of the atmosphere and the processes (such as cloud formation, lightning, and the movement of the wind) that cause what we refer to as the "weather". Most of the world's weather systems and their related features, including clouds and rain, develop in the lowest layer of the atmosphere, called the troposphere. Such weather systems, or patterns of air movement, develop as a result of the flow of heat from warmer regions of the Earth near the equator to colder regions nearer the poles. The air, and the heat it carries however, does not flow in a straight line, because of the Earth's rotation. As a consequence, the air flow is deflected, forming the swirling patterns of air circulation we observe on Earth.
Despite its relative thinness, our existance is completely dependent upon the atmosphere. Essential functions of the atmosphere include:
The atmosphere also influences our existance in several non-life essential ways: