What is Air Study Guide (page 2)

Introduction

From ancient times, people knew that air was crucial to life. But they did not know that air, which seems to be a single substance, is actually a mixture of many different gases. We will examine the contents of air later in this lesson. But first, we start with the atmosphere's general properties of temperature and pressure and examine how these properties change with altitude.

Pressure, Temperature, and Altitude

We all know that air pressure is less at the top of a mountain than at sea level. But what exactly is air pressure? Quite simply, air pressure is the total weight of the air above any spot. For example, one of the common numbers for air pressure is 14.7 pounds per square inch, at sea level. Imagine yourself at a sunny beach looking at a square inch area of sand (approximately at sea level). Extend that column of one square inch all the way up, miles and miles to the upper reaches where the atmosphere fades into outer space. All the air in that thin column weighs about 14.7 pounds.

Meteorologists—scientists who study the weather—have found it convenient to define another unit for air pressure called the bar. One bar is the downward force exerted by 1 kilogram of mass upon 1 square centimeter of area at sea level. It turns out that the mass in a column of air of one square centimeter in cross-section area, extending from sea level all the way to the outer reaches of the atmosphere, is just about one kilogram. In fact, the mass of air in that column is 1.013 kilograms. So this standard air pressure is 1.013 bar.

Meteorologists measure small differences in air pressure that occur when various air masses move across the land. That's why the air pressure at sea level changes slightly with the weather. To speak in terms that are simple to understand, meteorologists therefore like to use the unit of the millibar, or one-thousandth of a bar. Another way of expressing the standard air pressure of 1,013 bar is 1,013 millibar, or 1,013 mbar.

Yet other ways of expressing air pressure exist, for example by putting the mass of air in our column into equivalent masses of other substances. Air pressure at sea level, for example, is equal to a column of mercury about 760 mm or 29.9 inches high. Water is less dense than mercury, so the equivalent height of a column of water that is equal to air pressure at sea level is higher, about 33 feet. Still another way to measure air pressure is to use the metric unit of pressure, noted in pascals (Pa). The standard sea level air pressure is 101,325 Pa. Note that 1 mbar equals 100 Pa.

Measurements of air pressure are made using a barometer. In the earliest days when it was understood that air had measurable pressure, barometers were literally made in columns of mercury. And when early scientists, such as the Frenchman Blaise Pascal (for whom the metric unit of pressure was named), took columns of mercury up the slopes of mountains, they discovered that air pressure decreased with altitude.

The reason for the decrease in air pressure with increase in altitude is simple. You climb a mountain and less air is above you. As you climb a mountain, all the air you see below you is no longer above you pressing down on you. When you get to space, no air at all is pressing down on you and the air pressure is zero.

The drop in pressure with increase in altitude is, however, not linear. That means that as you go up, the drop in air pressure is not equal to equal increments of altitude. Consider the following facts about our column of air (either 1 square inch or 1 square centimeter in cross-section, it doesn't matter). About 50% of the total mass of the atmosphere in that column is within the lowermost 5 to 6 kilometers. You have to include the next 25 kilometers to get nearly the equal amount of mass. So it's not just the air pressure that decreases with increase in altitude; the density of the air decreases with altitude as well. In mathematical terms, we can say that air pressure drops exponentially with increasing altitude.

The ocean's water presents a contrasting situation. Ocean pressure increases with depth linearly. This is because the water is virtually incompressible. As the water down deep is squeezed by the weight of the water above, it has more pressure on it but almost no change in density. So water pressure is linear with depth. The water pressure at two miles down is twice that found at one mile down.

As you might now suspect, the reason for the exponential behavior of air pressure is because air is compressible. Its density changes with pressure. So not only is the air pressure greater at sea level than, say, at the top of Mount Everest, but so is its density. The air at sea level is squeezed and made more dense by its local, higher air pressure.

Another important property for us to consider about the atmosphere is temperature. Like air pressure, temperature, for much of the mass of the atmosphere, decreases with altitude. The reason, however, is completely unrelated. And, as we shall see, important regions of the atmosphere exist in which the pattern reverses, that is, in which temperature increases with altitude.

What causes the earth's surface to be so much hotter than the biting cold of outer space? Recall that outer space is about 3° above absolute zero—the temperature at which all molecular motion ceases. Though some energy comes up from the interior of the hot, inner Earth, that energy is very small. Over long times, the earth's inner heat does drive plate tectonics, but on a day-to-day basis, the inner energy that comes to Earth's surface is small. By far, the main source of energy that warms the earth to the temperature we enjoy is the sun.

Consider the fact that we see the sun clearly through the atmosphere (but please don't look at it directly). That means the air is mostly transparent to the rays of energy from the sun that our eyes respond to (which happen to be the same kinds of rays that contain most of the energy from the sun). The atmosphere scatters some of the sun's energy, which is why the sky is blue. And clouds reflect sunlight back to space. But where the sky is clear, a lot of the sun's energy goes through the atmosphere and is absorbed at the surface of the earth—by the ground or the surface of the ocean.

So the atmosphere at Earth's surface is heated by the sun. The following two facts mean that the average pattern will be cooling of the atmosphere with altitude: (1) that most of the heating from the sun takes place at the surface, and (2) that outer space is cold. Indeed, we have all felt this. When you climb a mountain on a hot day, you might start off in a T-shirt but need a heavy jacket at the top of the mountain. And look where snow occurs on mountains: at the tops. It's cold up there.

The exact relationship between temperature and altitude is called the lapse rate. There are two kinds of lapse rates. To develop the theory to explain the lapse rate, meteorologists use a specialized term: adiabatic. This term simply refers to the fact that the meteorologists assume that no additional heating from the sides come in to the parcel of air that is being considered (this will be clear in a moment). Two kinds of lapse rates have been denned: dry and moist. We will consider the dry adiabatic lapse rate first.

As a parcel of warm air from the ground rises, what happens? Because the pressure is lower as it rises, the parcel expands. This happens all the time—when air currents rise to form clouds, for example. Calculations show that for the part of the atmosphere we live in (the lower troposphere), the dry adiabatic lapse rate is about 1° C per 100 meters (or 1° F per 183 feet). That means that for every hundred meters you climb, the temperature, on average, will drop about 1° C.

Water vapor in the atmosphere, however, creates dynamics that make this number lower (that is, the drop in temperature is less). This is a more realistic situation for places where substantial water vapor is in the atmosphere, for instance, above tropical rain forests or above the ocean. (The dry adiabatic lapse rate works in other places, such as in deserts.) When water vapor plays a role in creating the lapse rate, the lapse rate is called the moist adiabatic lapse rate.

The basic idea behind the moist adiabatic lapse rate is the fact that as the air cools when it rises, water vapor condenses out of it. When the condensed vapor accumulates into droplets, we see clouds. Furthermore, when the water vapor condenses, it releases heat. This is simply the opposite process of water evaporating, which requires heat. So evaporating water into water vapor takes energy, and condensing water vapor into water releases heat. That heat goes into the air that is rising, warming it slightly. This is why the moist adiabatic lapse rate is less than the dry adiabatic lapse rate. The heat released by the condensed water vapor some what counters the cooling created by the air expanding as it rises.

What is the number for the moist adiabatic lapse rate? It depends on latitude and air temperature. But the average is about 0.6° C per 100 meters of altitude.

This discussion of the reason for why temperature cools with altitude and the two different kinds of lapse rates holds true for the most important zone of the atmosphere, the lowermost layer called the troposphere. We live in the troposphere, and most of the mass of the atmosphere is in the troposphere. All our most familiar weather takes place in the troposphere, because the troposphere is where those rising and falling currents of air make clouds, where water vapor condenses into rain and snow, and where the variable winds we feel every day blow. On average, the troposphere goes from the earth's surface to about 15 kilometers in altitude. This altitude at which the troposphere ends is called the tropopause?, and it varies with latitude and seasons.

The next layer of the atmosphere is called the stratosphere. The stratosphere begins at the tropopause and extends approximately 50 kilometers up beyond that. Commercial jets fly in the lower stratosphere (or upper troposphere, but anyway above the clouds), to escape the variable weather conditions of the lower troposphere, as well as to reduce friction and conserve fuel. Temperature in the stratosphere does not follow the same rules that apply to the troposphere. In the stratosphere, temperature increases (not decreases) with altitude. How can that be?

For temperature to increase with altitude in the stratosphere, there has to be a source of energy. And there is. The stratosphere contains a certain type of molecule called ozone (O₃, three atoms of oxygen in a molecule of ozone). Ozone has the property of absorbing ultraviolet light from the sun. Though as we said before, much of the sun's energy is contained in light rays that go through the atmosphere to the ground or ocean's surface, a small amount of energy is contained in the ultraviolet rays. It is this energy that is absorbed by the ozone in the stratosphere. This absorption warms the stratosphere and causes tem perature to increase with altitude. The altitude at which the stratosphere ends is called the stratopause.

Additional layers of the atmosphere can be found above the stratosphere. But remember that the air here is very thin. In fact, even though levels of the stratosphere are technically warm, you would freeze if you tried to live there unprotected, because your body would radiate much more heat away from your skin than you would gain from the small amounts of warm air molecules hitting against your skin.

Above the stratosphere comes the mesosphere. In the mesosphere, which extends from the stratospause up to about 80 kilometers, temperatures again drop with increasing altitude. The mesosphere ends with the mesopause, and the thermosphere begins.

Some of the gases in the uppermost, very thin region called the thermosphere absorb some of the sun's energy, and, in addition, protons and electrons given off by the sun are absorbed in the thermosphere and serve as a source of energy. Thus, temperature again rises in the themosphere with altitude. Within the thermosphere is a sublayer called the ionosphere, which has electrically charged particles that some times light up the skies in the high latitudes with colorful lights called the auroras. At an altitude of about 500 kilometers, the atmosphere fades gradually into outer space (see Figure 9.1).