The atmosphere is not a static system, although it is in balance. It is a mixture of gases, with tiny grains of dust suspended in it. It holds in heat and shelters us from the cold of space, as it protects us from radiation and falling space debris.
Air is mostly nitrogen, about 78%. This element is inert in air, though bacteria associated with plants convert it to organic forms without which life is impossible. Oxygen is second most common in air, at 21%, and of course we need to breathe it. Coming next are a group of noble gases, led by Argon at nearly 1%, which may be left over from the primordial atmosphere.
Carbon dioxide insulates and warms the earth, and plants need it as animals need oxygen. Methane and nitrous oxide from volcanoes, swamps, and animals also warm the atmosphere. Water vapor hangs in the atmosphere as part of the water cycle, until it rains down again.
Water vapor in air is invisible. Clouds and fog, formless as they seem, are vapor that has coalesced around some tiny core. The amount of water in air varies considerably from place to place. Its weight may be as much as 1/25th of the weight of the air it is mixed with in a rain forest, but it may make up only 1/10,000th of a mass of polar air. When it condenses back to liquid form, the vapor gives up heat that supplies energy to storms, which is why hurricanes get stronger over water.
The dust particles in air are mostly invisible too: there might be 250,000 minute particles to an inch. Most of it comes from the surface of the earth. It is the smoke of prairie and forest fires, as well as particles carved off the dry land by wind erosion. Volcanoes in eruption spew particles whose spread draws maps of air circulation around the world. Dried salt crystals drawn up by winds blowing over wave crests form nuclei for new droplets of rain. From space, dust comes from meteors vaporized as they meet the air.
The inputs to air change in each area as the world turns. Plants and algae only make oxygen in sunlight. Humans also have their times of sleep and work, mostly adding to the atmosphere with industries and cars when their part of the world is facing the sun.
Another cycle is formed by the cells of air circulation that produce the horse latitudes, the trade winds, and the westerlies, while they transfer heat from the equator to the poles. These are the predictable bands of wind that the sailing ships used to chart their voyages.
The atmosphere can be described as formed in shells, like the layers of an onion. Division of these zones can be made according to temperature, air density, or function. The troposphere rises to between 7 and 15 km above the earth. This is the layer where clouds form, and precipitation begins. Above it is the stratosphere, rising to about 50 km, where most of the earth’s ozone is found. Ozone is the special form of oxygen that absorbs ultraviolet radiation that could otherwise harm life on earth.
The mesosphere is above that, to about 80 or 85 km above the surface of earth. This is where meteors burn up. The thermosphere is the next rarefied portion of the atmosphere. It is a shell rising to 640 km above the surface of the earth. Here is where the radiant Aurora Borealis and Aurora Australis form, where the ionosphere is found, and where the International Space Station orbits. The last, highest, shell is the exosphere, rising to 10,000 km above earth. Molecules here can actually escape into space.
The biosphere, of course, is the area where life is possible, and air is part of it. The biosphere is sometimes narrowly defined as including only living material, but if it is defined as the sum of all ecologies, then we must include the air.
Life is possible in the earth’s extremes. Live bacteria have been found 41 kilometers above the ground, though they may have been thrown there by volcanism. Endoliths live within stones, in the dry valleys of Antarctica. At hydrothermal vents beneath the sea, piezophiles withstand unbelievable pressure, and make their food of raw elements and heat pouring from vents, the way plants make food from sunlight, water, and soil. Life could well go on without air, but not life as humans now know it.
