Introduction to the Atmosphere © 2008 Ann Bykerk-Kauffman, Dept. of Geological and Environmental Sciences, California State University, Chico* *Supported by NSF Grant #9455371. Permission is granted to reproduce this material for classroom use. D–1 Composition of the Atmosphere The atmosphere is a mixture of gases. Take a deep breath. Quick, name the gases you have just inhaled. Most people think of oxygen and carbon dioxide. Now study Figure 16.4 on p. 450 of the textbook. Surprise! The air is over 78% nitrogen (N2). We don't hear much about nitrogen because it has no direct importance to our lives1, but every time we take a breath, more than 3/4 of the air molecules we inhale are nitrogen molecules. Oxygen (O2), essential for all human and animal life, makes up almost 21% of the air--not as much as most people think, but still a reasonably large percentage. The next most common gas in our atmosphere is argon (A). Argon makes up almost 1% of the atmosphere (0.93% to be exact). If you've never heard of argon before, you're not alone. Argon is a “noble” gas, which means that it doesn't react with anything and, therefore, it just floats around in the air doing nothing but taking up space. Carbon dioxide (CO2), by contrast, is very important to life on Earth. Green plants use it to make sugar which they, in turn, use to make fat, protein and other important nutrients. Carbon dioxide also plays a crucial role in regulating the temperature of the atmosphere, as we shall see later in this chapter. Yet carbon dioxide makes up an incredibly tiny proportion of the air--only 0.035%. Nitrogen, oxygen, argon and carbon dioxide are mixed together very thoroughly in the atmosphere. Any outdoor air sample you might take, anywhere in the world at any altitude, would have the same proportions of nitrogen, oxygen, argon and carbon dioxide. By contrast, water vapor--the gaseous state of water--is poorly mixed in the atmosphere. Air at low altitudes has a much higher proportion of water vapor than does air at high altitudes. In addition, the water vapor content of the air varies considerably from place to place, depending on the proximity to a water source, the temperature of the air and other complex factors. The air in a desert contains much less water vapor than does the air in a rain forest, for example. Thus the proportion of 1Nitrogen does, however, have a very important indirect effect on our lives. Plants need nitrogen to make protein. We need to eat protein in order to build and maintain our muscles--we can't live long without it.D–2 Introduction to the Atmosphere water vapor molecules in the air you breathe can vary between 0% and 4%.2 Water vapor is the source of clouds and, of course, all rain and other precipitation. In addition, like carbon dioxide, water vapor plays a crucial role in regulating the temperature of the atmosphere. But, as you might surmise from their relative abundances, water vapor plays a much larger role in regulating air temperature than does carbon dioxide. In fact, water vapor is THE most important “greenhouse gas” (you will learn more about greenhouse gases and the greenhouse effect soon). Temperature What is Temperature? Air temperature is central to any discussion of the weather. We often say things like “This is the hottest summer I can remember.” or “I've had enough of this cold weather.” The predicted daily high and low temperatures help us determine what to wear each day. We heat and cool our homes in order to maintain a temperature ideal for our comfort. We measure the air temperature with countless thermometers placed inside and outside of our homes, at bank buildings, at airports and many other locations. We deal with the concept of air temperature every day, but many of us don't stop to think about what temperature really is. We know that temperature is related to heat. If we add heat to something, its temperature goes up. Is “temperature” just another word for “heat?” We can answer this question by doing the following thought experiment (you may wish to actually do this experiment at home): Place two different-sized pans full of cold water on a stove. Then, turn on the burners underneath the pans, being careful to place both burners on exactly the same setting so that each will put out the same amount of heat. What happens? The temperature of the water in the small pan increases much faster than does the temperature of the water in the large pan. The water in the small pan may be boiling hot when the water in the large pan is still lukewarm. The same amount of heat has been added to each pan, so why is the water in the small pan hotter? Because, in the large pan, the heat is spread out over a larger amount of water. 2Because the water vapor content of the air is so variable, we don't include water vapor when we report the proportions of the various gases in the atmosphere. That is why, in the discussion above, the percentages of nitrogen, oxygen, argon, carbon dioxide and “other” gases add up to 100%. The water vapor is, in a way, “extra.”Introduction to the Atmosphere D–3 So “temperature” is not exactly the same thing as “heat.” It's not the total amount of heat stored in the water that determines the temperature of the water. Rather, the temperature of the water is determined by the amount of heat stored per given amount of water (or any other substance we want to measure the temperature of)3. Scientists use the term internal heat energy to designate the amount of heat that is stored in a given amount of a substance divided by the mass of that substance. Thus temperature is a measure of the internal heat energy of a substance. Let's probe the issue of temperature a bit deeper. How is internal heat energy stored in matter? All matter is made up of extremely tiny molecules. Those molecules are in constant random motion, either vibrating in place (in solids), jostling against each other (in liquids) or freely whizzing along in all directions at tremendous speeds (in gases) (See Figure 17.2 on p. 479). Any object that is in motion possesses kinetic energy (energy of motion). It is the combined kinetic energy of all of the individual molecules of a substance that gives the substance its internal energy. The more kinetic energy these molecules have, the higher is the internal energy of the substance. Let's probe even deeper and look at what causes the kinetic energy of molecules to