In this section, we’ll explore basic properties of the atmosphere, focusing on its pressure, composition, and the way in which we define layers with altitude. Before you continue, try the following quiz to review what you’ve already learned about the atmosphere in Chapter 4.

Atmospheric Pressure

We’ve already talked quite a bit about atmospheric pressure, including how it compares between planets. But what exactly is pressure, and how do we measure it for air? Let’s investigate.

Defining Pressure, Air Pressure, and Atmospheric Pressure

The general scientific definition of pressure is “force per unit area.” You can understand what this means with the simple experiment shown in Figure 6.1.1–2, in which alternately you hold two different objects of the same weight in your hand. Notice that even though both objects shown in the figure have the same weight (5 kg) and therefore push downward with the same total force (due to gravity), the object on the right presses down on a smaller area of your hand than the one on the left. You will easily feel the greater pressure from the object on the right, which results from fact that its force concentrated on a smaller area of your hand.

The idea of pressure can also be applied to air, because even though we don’t usually think about it, air also exerts force. The source of this air pressure is the many collisions occurring between molecules in the air with each other and with any surface they come in contact with. The only significant difference between the pressure due to air and the pressure created by the weights in Figure 6.1.1–2 is that while the weights only push downward, air pushes in all directions. That is why, for example, pumping up a tire creates air pressure that inflates the tire by pushing outward in all directions (Figure 6.1.1–3).

The fact that air pushes in all directions means that one of those directions is downward, and this downward push represents the weight of the air. More specifically, atmospheric pressure measures the total weight of the air that sits above any given area. You can understand this idea by thinking of air as sitting above the ground somewhat like a huge stack of pillows (Figure 6.1.1–4). The pillows all push down on each other, but the ones lower down are more compressed — meaning they are subject to greater pressure and have higher density —because there is more total weight above them. In the same way, the fact that air is more compressed near the surface means that the atmospheric pressure and air density is highest at the surface and lower as you go upward.

You can now understand the basic units that we use to measure atmospheric pressure as follows:

On Earth, the atmospheric pressure at sea level is approximately 1 kilogram per square centimeter, which is equivalent to about 15 pounds per square inch. This is a substantial amount of pressure, as you can confirm with the next activity.

Measuring Atmospheric Pressure

The above activity proves that air has weight, but how do we know how much weight it has? A good way to understand the answer is by thinking about drinking with a straw.

It may seem like drinking through a straw means “sucking” the water (or other drink) upward, but if you think about it more deeply, you’ll realize that can’t quite be what’s actually happening. After all, there is nothing directly connecting your mouth to the water, so you couldn’t possibly be pulling the water upward just by moving your mouth. If you’re still not convinced, one simple way to prove this is by “sucking” the water only part way up the straw, then quickly putting a finger over the top of the straw (Figure 6.1.1–5). Try this, then use the questions that follow the figure to think about what you observe.

Do the demonstration shown in Figure 6.1.1–5, then discuss the following questions with a classmate. Then click to open the answers to see if they agree with what you came up with.

1. What happens to the water in the straw when you stop sucking on it and instead put your finger over the top?
   
   Answer

   The water remains in the straw at whatever level you “sucked” it up to.

2. Think about what happens if you pick up a rock and then let go of it. The rock will fall back down, because you are no longer supporting its weight. Water also has weight. If your mouth was exerting a “sucking force” that picked up the water through the straw, what would happen when you stopped sucking?
   
   Answer

   Just like letting go of a rock, letting go of the water would cause the water’s own weight to make it fall downward. But that isn’t what happens! So…

3. Using your answers to the first two questions, what can you conclude about whether you were actually pulling water upward with your mouth?
   
   Answer

   You were not pulling the water upward, because if you had been, it would have fallen back when you removed your mouth from the straw.

4. Keep holding your finger on the straw, and pull the straw out of the cup. Notice that the water stays in the straw as you lift it out. Explain how this proves that it is the push of air pressure that is keeping the water in the straw.
   
   Answer

   The fact that the water stays in the straw as you lift it out of the cup means that something must be preventing the water from falling out of the straw. Because the straw is no longer in the cup and there’s nothing else around, the air is only thing that could possibly be preventing the water from falling. That is, air pressure must be pushing upward on the water to keep it in the straw.

Figure 6.1.1–6 summarizes what really happens when you drink through the straw. Notice that, rather than “sucking” the water upward, the following processes are occurring:

In fact, this same idea also applies to vacuum cleaners, suction cups, and much more: What we usually call “sucking” or “suction” doesn’t really exist as a force. Instead, “suction” is simply the action of mismatched air pressures.

We can use this idea to measure the atmospheric pressure. Imagine making really long straws, so that you could try to drink through them while standing on platforms at different heights. Further imagine that you had superhuman lung strength, so that you could “suck” through the straws with super power (by expanding your lungs so much that air continues to flow into them). If you did this while standing on platforms at heights of, say, 3 meters or 5 meters, you’d have no problem drinking through the straw. But if you tried to do it from above about 10 meters (33 feet), you would find it impossible to drink through the straw, even with your superhuman lung strength (Figure 6.1.1–7).

The fact that you cannot “suck” water upward more than about 10 meters was actually discovered in ancient times, though obviously not with long straws and superhuman lungs. Instead, people trying to pump water from wells or into buildings found that no matter how good of a pump they made, they could never get water to rise more than about 10 meters . This fact also tells us Earth’s atmospheric pressure! Slide Show 6.1.1–8 shows how.

• 
  Slide 1 - If you place an open tube part-way into a large vat of water, the atmospheric pressure pushing on the water will be the same both inside and outside the tube, so the water level will also be the same. Note: You can see this idea in action by placing an open straw into a glass of water. The water level in the straw will be the same as in the rest of the glass.
• 
  Slide 2 - Imagine a tight-fitting piston that gives an added downward push to the water around the tube. Because the downward pressure is now greater around the tube than inside it, the water in the tube will rise upward.
• 
  Slide 3 - Now imagine removing the piston, but sealing the top of the tube and removing some of the air from it. The reduced air pressure in the tube will have exactly the same effect as the increased pressure from the piston did: The downward pressure will be greater around the tube than inside it, so the water in the tube rises upward.
• 
  Slide 4 - If you remove more air from the top of the tube, the pressure difference becomes larger, so the water rises higher.
• 
  Slide 5 - If you remove all the air from the top of the tube, making a perfect vacuum, you would find that the water in the tube would rise to a height of about 10 meters (if you are at sea level).
• 
  Slide 6 - The fact that the water won’t rise any higher means that the pressure created by the weight of the water itself must now be balancing the atmospheric pressure. In other words, the atmospheric pressure at sea level must be equal to the pressure created by an approximately 10-meter tall column of water.

Slide Show 6.1.1-8 – Study these slides and captions carefully to make sure you understand how the water level in giant straw can tell us the atmospheric pressure.Credit: Zofostro Science.

Be sure you’ve studied the slide show carefully, so that you understand its concluding point:

At sea level, Earth’s atmospheric pressure is the same as the pressure created by the weight of the water in an approximately 10-meter tall column.

How much pressure is that? As shown in the Do the Math below, it is about 1 kilogram per square centimeter, just as we stated earlier. But now you know how this fact is measured.

Barometers

These ideas suggest a way of making a simple device, called a barometer , for measuring atmospheric pressure at any time and in any place. In principle, all you’d need to do is make an instrument with a long tube in a large vat of water, just like the ones shown in Slide show 6.1.1-8. By creating a vacuum at the top of the tube, the air pressure would push the water up as high as possible. Of course, since this height is about 10 meters at sea level, such an instrument would be very difficult to carry around!

That is why liquid barometers use mercury, which is a liquid metal, instead of water (Figure 6.1.1–9). Mercury is almost 14 times as dense as water, and therefore a mercury barometer needs to be only about 1/14 as tall as a water barometer. More specifically, at sea level on Earth, a mercury barometer will make a column of mercury about 0.76 meters tall, which is the same as 760 millimeters. For this reason, atmospheric pressure is often described as “760 millimeters of mercury” or, equivalently, “29.9 inches of mercury.”

Today, most barometers actually measure relative changes in air pressure , which allows them to be much smaller than mercury barometers, as well as safer (since mercury is toxic). Modern digital barometers are quite small (Figure 6.1.1-10). Indeed, most smart phones now contain one of these tiny sensors, allowing the phone to be used as a barometer .

The value of 760 millimeters that we’ve given for the height of the mercury column is the average atmospheric pressure at sea level. The actual pressure at any moment and any location may be different from this by up to a few percent, and it changes with the weather. That is why you’ll sometimes hear meteorologists talk about a rising or falling barometer, meaning an increase or decrease in the local atmospheric pressure. We’ll return to this idea when we discuss winds in Section 6.2 and weather in Section 6.3.

Pressure and Altitude

Figure 6.1.1–11 shows a graph of how pressure depends on altitude on Earth. To read this graph, be sure to note that:

• The x-axis shows pressure “relative to sea level.” Therefore, at sea level, the pressure is “1.00,” because it is exactly the sea level pressure.
  As you’ve already learned, this sea level pressure is approximately equivalent to 1 kilogram per square centimeter or 760 millimeters of mercury. This pressure is also often given in several other units, so you may see it shown on similar graphs as approximately “1 atmosphere,” as about “1 bar” (1,000 millibars), or as about “100,000 kilopascals (kpa).”
• The y-axis shows altitude. Because pressure is lower at higher altitudes, it is easiest to read the graph if you start from where the curve shows you the sea level pressure (1.00) and follow it to the left, so that you see how pressure drops with altitude.
• If you want to know the pressure at any altitude, simply find that altitude along the y-axis, then look where that altitude occurs long the curve, and look down to the x-axis to read the pressure. For example, the light green dashed lines show you that at an altitude of 10 kilometers, the pressure is about 0.26 times that at sea level, or about 26% of the sea level pressure.
  You can similarly find the altitude that goes with any pressure by starting with a pressure value on the x-axis, then looking to see the corresponding altitude along the curve.
• You can also think of the pressure numbers as telling you approximately how much of the total air in the atmosphere lies above that altitude. For example, the pressure of 0.26 at altitude 10 kilometers means that only about 26% of the total air in the atmosphere lies above that altitude. This also means that 100% – 26% = 74% of the atmosphere lies below an altitude of 10 kilometers.

The “Edge of Space”

Note, however, that small amounts of gas are still present for another few hundred kilometers above this (Figure 6.1.1–12). The International Space Station and many satellites orbit Earth within these outer reaches of the atmosphere, and the low-density gas exerts a little bit of drag on these orbiting spacecraft. That is why satellites in low-Earth orbit slowly spiral downward, eventually burning up as they reenter the denser layers of the atmosphere.

Figure 6.1.1-12 – Like the International Space Station, the Space Shuttle orbited low enough to still have a
little bit of atmospheric gas around it. This photo shows the visible glow of this low-density gas around a
Space Shuttle’s tail. The colorful spikes of light in the distance are the aurora as seen from space.
Credit: NASA.

Atmospheric Composition

Composition and Altitude

However, there are two small but important changes that occur in the chemical makeup at high altitudes. Both result from the way air interacts with the different forms of light coming from the Sun (see the Brief Review below if you need to review the different forms of light). Here is how you can understand these two important changes:

Figure 6.1.1–14 shows where the ozone layer and ionosphere are located on the graph that you saw earlier (in Figure 6.1.1–11).

Clouds and Humidity

The atmosphere has one other major component besides those shown in Figure 6.1.1–13: water. The water can be in any of the three basic phases , meaning it can be solid crystals of ice, liquid droplets, or the gas we call water vapor . The amount of water in the atmosphere can vary quite a bit:

This variability explains why we don’t show water in the pie charts of Figure 6.1.1–13.

The fact that we can see (and feel) water droplets or ice crystals makes it easy to get a general sense of how much liquid or solid water is in the air. But how do we describe the amount of invisible water vapor in the air? The answer is that we use the term humidity : a higher humidity means more water vapor in the air. We’ll talk more about humidity later in the chapter, for now keep in mind this simple idea: Warmer air can hold more water vapor than cooler air. Therefore, when air cools, it is more likely that water vapor in it will condense. That is why clouds usually form at high altitudes where the air is colder. It is also why we often see condensation on cold surfaces, such as dew that forms on plants or the ground on many mornings, or the condensation we often see on car windows (which tend to stay cooler than the surrounding air).

Dust and Aerosols

Atmospheric Temperature: Four Major Altitude Layers

If you hike up a mountain, you’ll generally notice that in addition to the air becoming thinner with altitude, it also becomes colder. This fact explains why snow is more likely on tall mountains than at lower altitudes in the same geographical regions (Figure 6.1.1–16).

This trend that we see with mountains continues upward even above the altitudes of the highest mountains. For example, most commercial airplanes fly at altitudes close to 10 kilometers, and if you’ve ever listened to a pilot tell you about outside conditions on such a flight, you’ll know that the outside temperatures at that altitude are typically below −50°C (about −60°F).

However, if could keep going upward (or use a weather balloon to make temperature measurements at higher altitudes), you’d find that that the simple pattern of declining temperatures comes to an end at an altitude that averages around 12 kilometers . Above that, temperature follows a more complex pattern with altitude. Figure 6.1.1–17 shows the pattern. As you study the figure, notice that the temperature pattern defines the following four layers in our atmosphere, starting from the surface:

• The troposphere is the lowest layer, extending to an average altitude of about 12 kilometers. Here, the temperature drops with altitude, just as we’ve discussed for climbing mountains or flying on commercial airplanes.
  Note that unless you become an astronaut, you will probably spend your entire life in the troposphere, since it extends to an average altitude above any mountain top and higher than most commercial airplanes fly.
• The stratosphere begins where troposphere ends. Here, the temperature rises with altitude, rather than falling. This trend of rising temperatures continues to an altitude of about 50 kilometers.
• The mesosphere lies above the stratosphere. Here, the temperature once again falls with altitude, with this trend continuing up to about 80 kilometers in altitude.
• The thermosphere is the uppermost layer of the atmosphere. Here, the trend changes one last time, with temperature again rising with altitude.

This temperature pattern with altitude may at first seem a bit strange, but you can understand it using ideas you’ve learned previously. The following questions will help you do that.

Discuss each question with a classmate, then click to open the answer to see if it agrees with what you came up with.
Hint: Remember that the Sun emits many types of light into space, including visible light, ultraviolet light, and x-rays. Think about what happens to these different forms of light in Earth’s atmosphere.

1. Let’s begin at the top of Figure 6.1.1–17. The fact that temperatures rise with altitude in the thermosphere means that some form of light from the Sun must be providing the energy to heat the air at these high altitudes. What form of light is it, and how does it also explain why the ionosphere (as discussed earlier) overlaps with the thermosphere?
   
   Answer

   It is x-rays from the Sun, which are absorbed by atoms high in the atmosphere, causing the atmosphere to heat up at these altitudes. The x-rays can also strip electrons from atoms to create the ions that make up the ionosphere.

2. The temperature “bump” near the top of the stratosphere (and bottom of mesosphere) similarly tells us that some form of light from the Sun must be providing energy to heat this region of the atmosphere. What type of light is it, and what is absorbing this light? Hint: Think about what you’ve already learned about ozone .
   
   Answer

   It is ultraviolet light from the Sun, and ozone is absorbing the energy of this light, heating this region of the atmosphere. You’ll know this if you recall that ozone is located at altitudes around temperature “bump” in the graph (in what we call the ozone layer ) and that ozone absorbs ultraviolet light.

3. Visible light from the Sun reaches the surface, where it can be absorbed by the ground and oceans, heating them up. How does this fact explain why temperature declines with altitude in the troposphere?
   
   Answer

   The absorption is occurring at or near the surface, which effectively makes the surface a source of warmth. Therefore, as you go higher in altitude, you are getting farther from this source of warmth, which is why temperatures drop. (The trend reverses in the stratosphere, because you are now moving closer to the heating caused where ozone absorbs ultraviolet light.)

4. How does the fact that temperature declines with altitude in the troposphere explain why air in the troposphere can undergo convection ? Hint: Be sure to review the discussion of convection in Section 5.3.2.
   
   Answer

   Convection can occur whenever there is heating from below, and in this case the “below” is the surface (ground, oceans) that heats air above it. Just as in the convecting pot of water shown in Figure 5.3.2–4 except with air, warmer air is less dense and rises upward, while cooler air high in the troposphere is more dense and sinks downward.
   

   View Figure 5.3.2–4

5. There is no convection in the stratosphere, which is what makes its air stratified (layered). How does the fact that temperature declines with altitude in the stratosphere explain its lack of convection?
   
   Answer

   Building on the previous question, convection requires heating from below. But the heating in the stratosphere is occurring near its top (in the temperature “bump”), and it is coldest at its bottom. Therefore convection cannot occur.

6. Notice that temperatures in the thermosphere rise quite high. Yet if you could somehow go outside at such a high altitude, you’d find that it would feel extremely cold, and you’d need a heated and pressurized flight suit (or spacesuit) to survive. Why? Hint: Think about the density of the atmosphere at very high altitudes and review the difference between temperature and heat in Section 4.2.3..
   
   Answer

   The heat that you feel depends on both temperature and density (see Figure 4.44), and the air density in the thermosphere is extremely low. Therefore, although the high temperature tells us that the atoms and molecules are moving very fast in the thermosphere, there are so few of them that they cannot transfer much heat to your body. That is why you would feel cold despite the high temperature.
   

   View Figure 4.44

If you’ve answered the above questions successfully, you now know that the temperature pattern with altitude is the result of heating that occurs in three regions of the atmosphere:

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