Lesson Video: The Atmosphere Science • Second Year of Preparatory School

Video Transcript

In this video, we will learn how to describe how the density and pressure of Earth’s atmosphere change with height above the ground.

The atmosphere is a layer of gas that surrounds Earth. All the air around us is part of the atmosphere, including the air we breathe. The atmosphere is made up of a mixture of different gases, so it contains different kinds of gas particles. Most of the air is nitrogen, with some oxygen and a few other gases too. In this video, we’ll be looking at how the particles in the atmosphere lead to atmospheric pressure.

But before we can understand atmospheric pressure, we need to understand the concept of air columns. Much like the name suggests, an air column is simply the column of air above a region of the atmosphere. For example, let’s look at this region on Earth’s surface, which we’ve shaded orange. To draw the air column above this orange patch, we simply draw a cylinder that starts from the region we’re looking at and reaches all the way to the top of the atmosphere, like this. It’s important that the column goes all the way up to the top of the atmosphere, because the column is supposed to include all of the air particles above the orange-shaded region. To help us picture this, we can draw some of the particles in the column, like so.

In reality, there are many, many particles in the column, far more than we could ever draw. And these gas particles are always moving around, which, again, we can’t really draw in a helpful way. So when we draw particles in an air column like this, we’re not really capturing the whole picture. This is more of a reminder, to help us think about what’s going on.

To understand atmospheric pressure, we need to remember that each of these particles has a weight, a downwards force that pulls the particle towards the ground. But because there are so many particles in an air column, a particle cannot move towards the ground without coming into contact with some other particle below it. This means that particles high up in the atmosphere end up pushing down and exerting a force on particles lower in the atmosphere.

All of these forces add up to exert a force on Earth’s surface. So, the force exerted on our orange-shaded region is equal to the combined weight of every air particle in our air column. But this force doesn’t just act at a single point. It’s distributed over the whole orange area. When a force is applied to an area, it creates a pressure. Since the force is due to the weight of the atmosphere, we call this pressure atmospheric pressure. We experience atmospheric pressure all the time. We’re so used to it that we don’t really notice, but it’s always there.

Atmospheric pressure can be measured in units of bar, using a device called a barometer. Near Earth’s surface, at sea level, atmospheric pressure is equal to 1.01 bar. However, atmospheric pressure doesn’t always have the same value. The value of atmospheric pressure changes with a quantity called altitude. Altitude simply means height above the surface of Earth. At greater altitudes, so greater heights, atmospheric pressure is lower.

To understand why this is the case, we can go back to our idea of air columns. Before, we drew the air column above a region on Earth’s surface, like this. This time, let’s draw an air column above a region at a higher altitude, like this. Again, to draw our air column, we simply draw a cylinder connecting the orange region to the top of the atmosphere. We can see that our second air column is much shorter than the first. Let’s draw some air particles in each one. At high altitudes, the air column above a region is shorter and so contains fewer particles. Since there are fewer particles, the combined weight of all the particles in the column is less. This means that the value of atmospheric pressure is lower at higher altitudes.

The height of the air column above a region is not the only thing that affects atmospheric pressure. The density of air in the column is also a factor. Atmospheric density is a measure of how many air particles there are in a particular volume. If a volume contains lots of air particles, then it is a region of higher atmospheric density than a volume that contains very few particles. Atmospheric density is greater near the surface of Earth than it is near the top of the atmosphere. This difference in density is another reason why atmospheric pressure is lower at higher altitudes.

Let’s again compare our two air columns from before. Our first air column, which extends from Earth’s surface all the way to the top of the atmosphere, contains some air with a high density near the surface and some air with low density near the top of the atmosphere. In our second column, however, we only have the low-density air. So not only is this column shorter, but the air it contains is also less dense overall. As a result, there are fewer particles in this column, and so the combined weight of the air in the column is less. This leads to a lower value of atmospheric pressure.

The most important thing to remember is that higher altitudes correspond to lower atmospheric densities and lower atmospheric pressure. We can measure changes in altitude using a device called an altimeter. An altimeter measures changes in altitude by detecting changes in atmospheric pressure. Since atmospheric pressure can be different in different places, scientists often draw special kinds of maps, called isobar diagrams, to keep track of the different values of atmospheric pressure. An isobar diagram is just a map of an area that has isobars drawn on it.

An isobar is simply a line that joins up points where atmospheric pressure has the same value. For example, here we have a map of some imaginary island. Let’s see if we can draw a few isobars. Let’s say that near the edges of the island, all of the ground is at sea level. That means if we were to measure atmospheric pressure at all of these points, we would record a value of 1.01 bar at each and every one. Since all of these points correspond to the same value of atmospheric pressure, we can join them up using an isobar. Then, instead of labeling all of the points as 1.01 bar, we can just label the whole line.

Now let’s say that nearer the middle of the island, the altitude is higher than sea level, and so atmospheric pressure decreases. Let’s also imagine that at all of these orange points, the atmospheric pressure is equal to 0.98 bar. Since all of these points correspond to the same value of atmospheric pressure, we can join them with an isobar, just like before, and label it as the 0.98-bar isobar. Isobar diagrams can look complicated. But they’re not so difficult to interpret when we remember that each isobar is simply joining up points where atmospheric pressure is the same.

Now we’ve learned all about the atmosphere and atmospheric pressure, let’s look at some example questions.

Fill in the blank. Atmospheric pressure is the pressure at a point in the atmosphere due to the weight of the air blank that point. (A) At, (B) below, or (C) above.

To work out the answer to this question, we need to know how air particles in the atmosphere create atmospheric pressure. To do this, let’s think about a small area within Earth’s atmosphere, which we’ve drawn here shaded in orange. When we’re thinking about atmospheric pressure, it’s often useful to draw in an air column above the region we’re looking at. An air column is simply a column that contains all the air above a particular area in the atmosphere.

To draw an air column above our orange area, we just draw a cylinder that surrounds the area and extends all the way up to the top of the atmosphere, like this. Next, we can draw a small number of air particles into our column to help us think about how they behave. Each of these particles has a weight. This means these particles exert a downwards force on the particles below them. Because of this, particles in our orange region will experience a force, due to the total weight of all the particles above it. Since this force is distributed over the entire orange area, it becomes a pressure. So, we have seen that the pressure at a point in the orange region is due to the weight of the air above that point. This corresponds to answer option (C).

To check our answer, let’s think about the other options. There are particles below the orange region. But they also only exert downwards forces. Since they’re already below the orange region, they don’t affect the pressure actually at the orange region. So option (B) cannot be the right answer. Similarly, the particles that are at the orange region don’t affect atmospheric pressure at the orange region. They only contribute to atmospheric pressure at altitudes below the orange region. Option (A) can’t be the right answer either.

So, we have seen that the pressure at a point in the atmosphere is determined by the weight of the air particles above that point. So the correct answer to this question is (C), above. When we fill in the blank, our final statement reads, “Atmospheric pressure is the pressure at a point in the atmosphere due to the weight of the air above that point.”

Now let’s look at another example question.

A mountain climber uses a barometer to measure how the atmospheric pressure changes as they climb a mountain. At which point does the barometer measure the lowest pressure? (A) At the bottom of the mountain, (B) halfway up the mountain, or (C) at the top of the mountain.

Let’s start by drawing a quick sketch of the mountain to help us picture what’s going on. To answer this question, we need to decide whether atmospheric pressure is lowest at the bottom of the mountain, halfway up the mountain, or at the top of the mountain. In other words, we need to remember how atmospheric pressure varies with altitude. The easiest way to approach this question is to think about air columns. We can recall that atmospheric pressure at a certain region is the pressure due to the weight of the air in the air column above that region. The greater the weight of the air above a region, the greater the atmospheric pressure at that region.

So, to answer this question, we can start by drawing air columns at our three different altitudes. First, we need to mark the top of the atmosphere, so we know where our air columns should end. Then, we can draw an air column for the area at the bottom of the mountain. We can see that the column extends from the ground all the way to the top of the atmosphere. Then, we can add in some air particles. Next, let’s draw a column for the region halfway up the mountain and add in some air particles like so. Finally, let’s draw the column for the region at the top of the mountain. Since atmospheric pressure is due to the weight of the air in each column, the region where atmospheric pressure is lowest corresponds to the column with the least air in it.

If we compare all three columns, we can see straight away that the column on the left is the tallest and the column on the right is the shortest. So this tells us that the column on the right probably has the least air in it. But we should also be thinking about the density of the air at different altitudes. Atmospheric density is higher nearer to the surface of Earth and lower at higher altitudes. So the column on the left contains a region of high-density air at the bottom, as well as a region of low-density air at the top. The column on the right only contains low-density air.

Since the column on the left is the tallest and contains air with the highest overall density, it contains the most air particles of the three columns. Since the column on the right is the shortest and contains air with the lowest overall density, it contains the least air particles of the three columns. So this is the column that corresponds to the lowest atmospheric pressure. So looking through our answer options, we can see that the answer to this question must be (C). The barometer measures the lowest pressure at the top of the mountain.

Let’s look at one more example question.

Consider this isobar diagram. At point I, the atmospheric pressure is 1,000 millibars. What can be deduced about the atmospheric pressure at point II? (A) At point II, the atmospheric pressure is 1,000 millibars. (B) At point II, the atmospheric pressure is less than 1,000 millibars. Or (C) at point II, the atmospheric pressure is greater than 1,000 millibars.

To answer this question, we need to recall how isobar diagrams are used to map atmospheric pressure. On an isobar diagram, each isobar joins up points where the value of atmospheric pressure is the same. On the diagram we’ve been given, we can see that point I and point II both lie on the same isobar. This means that atmospheric pressure must have exactly the same value at both points. We’ve been told that atmospheric pressure at point I is equal to 1,000 millibars. So, atmospheric pressure at point II must also equal 1,000 millibars. The correct answer to this question is therefore option (A). At point II, the atmospheric pressure is 1,000 millibars.

Now, let’s finish up by summarizing some key points. The atmosphere is a layer of gas that surrounds Earth. Because the particles in the atmosphere have weight, the atmosphere exerts a pressure, which we call atmospheric pressure. Atmospheric pressure decreases as altitude increases. Atmospheric density also decreases as altitude increases. Finally, we can use isobar diagrams to map atmospheric pressure. Isobar lines join points where atmospheric pressure has the same value.