What is Air Study Guide (page 2)

Updated on Sep 25, 2011

The Air's Gases

The atmosphere is a mixture of many gases. Though the air gets thinner with altitude, for the most part, the percentages of gases relative to each other are fairly constant. A few exceptions exist to this rule; for example, the ozone that we've already seen is mostly in the stratosphere. That's because special chemical reactions take place in the stratosphere that create the ozone. But for the most part, we can study the kinds of gases in the air as percentages and not pay attention to the altitude from which the air came. Table 9.1 is a list of the main gases in Earth's atmosphere.

Table 9.1 Gases of the Atmosphere

Note that the top three gases—nitrogen (N2, 78.08%), oxygen (O2, 20.95%), and argon (Ar, 0.93%)—together make up 99.96% of the atmosphere. All the other gases are only 0.04% of the total. Also note a gas that is missing from the table: water vapor. Water vapor is not included because it varies so much from place to place, from season to season, and across different temperatures. The table shows what is known as the composition of the standard dry atmosphere. In other words, water vapor is not included. Also not included in the table of the standard dry atmosphere gases are the aerosols, which include salt crystals, ice crystals, smoke particles, dusts, sulfates, unburned carbons from fossil fuel combustion, and others. These, too, obviously vary from place and to place, from condition to condition.

Let us consider first the crucial gas water vapor, and then move on to consider the other gases that are greenhouse gases.

Water vapor creates what we call humidity. Two terms for humidity are used by meteorologists: absolute humidity and relative humidity. Absolute humidity is the actual amount of water vapor in the air, say in terms of grams of water per cubic meter. In terms of actual percentages, compared to the percent ages of the other gases in the standard dry atmosphere, water vapor typically varies between 0.3% to 4%. In other words, compared to the other gases, water vapor usually is gas number 3 or 4 in rank.

Much of the time, the concern is about relative humidity, which is given as a percentage relative to how much water vapor a parcel of air at that temperature and pressure could hold. At zero relative humidity (which almost never exists), the air contains no water vapor. At 100% relative humidity, the air has its maximum amount of water vapor. At 100% relative humidity, the air is said to be saturated with water vapor. At that point, the amount of water vapor in the air is said to be at the saturation vapor pressure.

For our purposes, the factor that determines the saturation vapor pressure (in other words, the amount of water vapor at 100% relative humidity) is temperature. The higher the temperature, the more water vapor the air can hold. That's why clouds appear high up in the sky where the air is cooler. As air rises from the ground and cools, its saturation vapor pressure drops. If that pressure drops below the amount of water vapor actually in the air, all the vapor cannot stay in the air. Some of the water vapor condenses.

When the condensation occurs as small invisible droplets, clouds start to form. Within clouds, the droplets can grow larger and larger and become rain. Inside the cloud are air currents, and when these take the droplets up high, the droplets can freeze into ice crystals, starting the process of becoming snowflakes. Clouds are important to climate, not only as the sources of precipitation but as reflectors of sunlight. Globally, clouds reflect about 30% of the sunlight back into space.

Water vapor is also what is known as a greenhouse gas. The greenhouse gas called carbon dioxide is much in the news these days, and it will continue to be so for the coming decades. But most people don't realize that the most powerful greenhouse gas in the atmosphere is water vapor, accounting for about 80% of Earth's overall greenhouse effect.

What is a greenhouse gas? In simplest terms, a greenhouse gas is a gas that lets the sun's energy come in but blocks the earth's energy from leaving to space. In more technical terms, a greenhouse gas is transparent to shortwave radiation and is absorbing to long wave radiation. Most of the sun's energy is in the short wavelengths of electromagnetic energy, the so-called shortwave radiation to which the greenhouse gases are transparent. Visible light is shortwave radiation. The fact that the greenhouse gases are transparent to short wave radiation is why the sky looks basically clear to us.

Longwave radiation, on the other hand, is thermal infrared radiation. Thermal infrared is the wave length of energy given off by the earth. (Earth's temperature is much less than that of the sun, so the radiation energy given off by the earth consists of much longer wavelengths.) Infrared radiation to space is the means by which the earth cools itself (releases energy to space), balancing the energy it receives from the sun. Greenhouse gases absorb infrared radiation. But they don't absorb all of it. You can think of it this way: The more greenhouse gases, the more absorption of Earth's infrared radiation, the warmer the planet.

Greenhouse gases occur in small amounts in the atmosphere. The largest gases in the atmosphere in terms of amounts are not greenhouse gases. Nitrogen (N2, 78.08%), oxygen (O2, 20.95%), and argon (Ar, 0.93%) are not greenhouse gases. What makes a gas a greenhouse gas or not is fairly technical, but we can use a simple rule of thumb: A greenhouse gas has three or more atoms in its molecule.

Thus, we can list the greenhouse gases in Earth's atmosphere, in decreasing percentages: water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Check this list with the chemical formulas in Table 9.1 to verify that they all have three or more atoms in their molecules.

Without some greenhouse gases, the earth's surface would be very much colder, frozen in fact. So the greenhouse gases are important for life as we know it. However, the greenhouse gas called carbon dioxide (as well as others, e.g., methane) is increasing in the atmosphere from human activities (as are some of the other greenhouse gases), and this is cause of concern for the future of Earth's climate and global warming.

Practice problems of this concept can be found at: What is Air Practice Questions

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