Air and the Atmosphere

The atmosphere is an envelope of gases, water vapor (humidity), and suspended solids surrounding the earth to a distance of between 600 to 1500 kilometers.   About 1/2 of this matter lies below 18,000 feet (5500 meters).   At the lowest part of the atmosphere, the gases, known as "air", are 76.8% nitrogen (N₂) and 23.2% oxygen (O₂) by weight.   By volume, the respective ratios in air are 79.1% nitrogen and 20.9% oxygen.   There are other minute, although essential, gases in the air, such as ozone (O₃) (protection against ultraviolet radiation from the sun) and carbon dioxide (CO₂) (keeps the earth warm by trapping heat). ^{Considine 255}

The atmosphere is held in placed around the earth by the force of the earth's gravity.   It exerts a pressure proportional to its height, being zero at the outer limits and about 14.7 lbs. / sq. inch (psi) at sea level.   Each person has about 20 metric tons of force exerted against his/her body at all times.   The pressure of 14.7 lbs/sq. inch is called 1 atmosphere (= 760 mm Hg = 760 torricelli = 1 g / sq. cm.), which is a convenient pressure unit of measure for meteorological work.   A barometer is an instrument for measuring atmospheric pressure.

The oxygen and carbon dioxide in air are essential to photosynthesis in plants and respiration in animals; therefore, air is essential to life.   In addition, air has many industrial applications.   Oxygen is an essential element in all burning (combustion, oxidation) of fuels.   Further, oxygen and nitrogen are extracted from the air and liquified for many industrial applications.   Also, oxygen and nitrogen readily combine with other elements to form many useful compounds, so air is a major chemical raw material.   Moving air, "wind", exerts force, so it can be used as source of power.   If sufficiently strong and steady, it can sail ships and move grist (grain-grinding) windmills.   Because air is a non-pollutant, wind power has an environmental advantage over power generated by fossil fuels, such as coal, oil, and gas.   The total or partial absence of air, a vacuum, in an enclosed space is extremely useful because the pressure exerted by the air on the outside of the evacuated enclosure has potential energy to do work in air pumps.   Pumps produce air under pressure ("compressed air") to drive pneumatic machinery and tools, e.g., pneumatic drills that remove rock to make holes to hold explosives to remove large amounts of rock and soil in mines and excavations.