The Earth’s atmosphere, the layer of air that surrounds the planet, is a protective blanket that shields us from harmful radiation and traps heat energy, storing warmth from the sun and keeping temperatures in a range that makes life possible. Animals need air to breathe, plants need it for photosynthesis, and living things also use air to fly, carry scents and sounds, and keep warm. With its many functions in our lives, it’s worthwhile exploring the nature of air.
Held in place by Earth’s gravity, the atmosphere is an envelope of air that extends outward from the surface for some 500 miles, becoming thinner and sparser with distance. Dry air is composed primarily of three gases, with nitrogen making up 78%, oxygen 21%, and argon just under 1%. The remainder is made up of other gases, such as carbon dioxide, ozone, helium, and hydrogen. In addition, air contains water vapor in varying amounts from almost none to 4% of the total, which we feel as humidity. A comfortable roomful of air might contain two quarts of water. The percentage of carbon dioxide in air has been increasing over the last fifty years due to the burning of fossil fuels, reaching a record level of 400 parts per million (.04%) in 2014, a cause for great concern because of its effects on climate. In addition to gases, air contains tiny particles like dust, dirt, pollen, bacteria, and salt. These act as condensation nuclei for cloud droplets, raindrops, and snowflakes, producing the precipitation that returns water to the ground, water that is essential to life.
The Earth’s atmosphere began to form when ancient volcanoes released gases from inside the planet. Later cyanobacteria (commonly known as bluegreen algae), which evolved several billion years ago, carried on photosynthesis, a process that produces oxygen. Over time this changed the composition of the atmosphere to one rich in oxygen in which plants and animals could flourish.
Plants use carbon dioxide from the air and water from the soil to make sugars and starches by photosynthesis. As a byproduct of this process, they release water vapor and oxygen into the atmosphere. Thus plants, both those on land as well as algae in the sea, produce the oxygen that animals need to breathe. At night, without sunlight, plants can’t photosynthesize, but they carry on respiration, using oxygen and releasing carbon dioxide.
In respiration, oxygen is used to generate energy for bodily functions. All animals need oxygen, whether they live in the soil, in water, or on land. Fish extract oxygen from the water with their gills, as do tadpoles and some aquatic insects. Worms, and to some extent frogs and toads, exchange gases through their damp skin. Insects have a series of holes along the sides of their bodies called spiracles. These are connected to a network of tiny tubes that deliver air throughout their bodies.
Many animals breathe air with lungs as we do, including mammals, birds, reptiles, and most adult amphibians. The millions of tiny air sacs in our lungs are lined with moist membranes that contain many tiny blood vessels. This allows for an exchange of gases between the blood in our capillaries and the air we breathe. With each breath, carbon dioxide, the waste product of our cellular respiration, passes into the lungs from the blood and is exhaled. And oxygen passes through the membranes and into the blood, attaching to red blood cells, which carry it to the rest of the body. Although we humans can do without food or water for a number of days, we cannot survive for more than a few minutes without oxygen.
In addition to breathing, air has other functions in our lives as well. When trapped in tiny spaces, air makes excellent insulation, a property used by many warm-blooded animals like mammals and birds to stay warm. Within their furred or feathered coats, air trapped in tiny spaces creates an insulating layer of warm air close to their bodies. Air has no odor of its own, but it can carry odors that can be detected by animals. Many animals use scents to communicate with others of their kind, and a plant’s odors give out information to animals and other plants. Female cecropia moths release a scent that can call males from several miles away. Corn plants attract wasps to attack caterpillars feeding on their leaves. Sound waves move easily through air. In spring and fall, frogs, birds, and insects like crickets and katydids fill the air with the sounds of their courtship songs. Breezes carry the pollen of flowers and trees, the spores of fungi, mosses, and ferns, and the seeds of maples, dandelions, and many other plants. And those animals with the right equipment – wings, feathers, fins – make use of the air for flight. Insects, birds, bats, flying squirrels, and even flying fish use the air to get around.
Even though air is invisible, we know it is something tangible because we can feel it and hear it when we take a breath or swing our arms around. Like all matter, air is made of molecules, tiny particles too small to see even with a microscope. In solids the molecules are bonded to each other; in liquids the bonds are weaker, allowing them to slide by one another. In gases the bonds are even weaker, and molecules can spread out to fill any available space. When air is heated the molecules move faster and it expands; cooling the air slows molecules down and it contracts.
How is a gas similar to or different from a solid or liquid? The molecules of air are so spread out compared to the densely packed molecules in a solid that the weight is much less. Air is lighter than water, too, and air bubbles in a cup of water will float to the surface. Still, a one-inch column of air extending from sea level to the farthest edges of the atmosphere would exert a pressure of 14.7 pounds, or about a ton of pressure on every square foot of ground. We don’t feel the air pushing down on us because we have evolved to live in these conditions.
Solids have distinct shapes and are very difficult to compress. Liquids take the shape of the container they’re in but are also difficult to compress. Gases like air take the shape of their container, but because there is a lot of space between molecules they can be squeezed into a smaller space, as when pumping up a tire. As air is squeezed, it pushes outwards with greater and greater force. This is because air seeks to reach an equal pressure with its surroundings, its molecules spreading or pushing outward to achieve a uniform density. The force of air under pressure is used in tires, pneumatic machinery, and many other applications. In a parachute, the air trapped under the cloth is compressed so it pushes against it, slowing the fall of the person or object strapped beneath it. When we breathe, our diaphragm moves downward and our ribs outward, expanding the chest cavity. This lowers the pressure in the lungs so air flows in. When we exhale, the diaphragm and ribs relax, making the chest smaller. This increases the pressure in our lungs so air flows out.
Although air forms only a small fraction of the total mass of our planet, it plays a weighty role in our climate, keeping temperatures in a range in which living things can exist. Not just a protective blanket, it also provides us with nitrogen, oxygen, carbon dioxide, and water – elements essential to life. This lightweight, odorless, colorless gas supports, propels, buoys us up, and supplies force to do work for us. Clearly, air is not something to be taken lightly.
Hamblin, W. Kenneth and Eric H. Christiansen. Earth’s Dynamic Systems. 10th ed. Upper Saddle
Houghton, John. The Physics of Atmospheres, Cambridge, England: Cambridge University Press, 2001.
Lehr, Paul E., R. Will Burnett, and Herbert S. Zim. Weather. Golden Guide. New York, NY: St.
Martin’s Press, 2001.
River, NJ: Prentice-Hall, 2004.
http://health.howstuffworks.com/human-body/systems/respiratory/lung.htm (detailed discussion of how lungs function)
http://www.chem.purdue.edu/gchelp/liquids/character.html (simple illustrated comparison of solids, liquids, gases)
http://www.npr.org/blogs/krulwich/2014/04/29/307981803/plants-talk-plants-listen-here-s-how (a simple illustrated discussion of chemical communication by plants)
http://www.theweatherprediction.com/habyhints/40/ (discussion of the connection between air temperature and water vapor)
http://www.ucmp.berkeley.edu/bacteria/cyanointro.html (a discussion about the importance of cyanobacteria in the formation of our atmosphere and the evolution of plants)