When you look up in the sky, you probably don’t think about all of the atmospheric layers. This lesson will explore these layers in the atmosphere. Then, we will examine how data is graphed for the atmosphere in thermodynamic diagrams.
Earth’s Atmospheric Temperature Profile
When you hear the word ‘profile,’ you probably envision a social media site with a profile picture along with a bio about your likes and dislikes. Just like you have a profile, so does the earth.
No, the earth isn’t logging onto social media sites, taking selfies, and writing about its likes and dislikes, but, like your profile and bio, the Earth’s Atmospheric Temperature Profile still tells you a lot about the earth. And, while your profile may look like this, the earth’s atmospheric temperature profile looks like this (see video).Specifically, this profile tells you the temperatures for different layers of the atmosphere. These layers are divided up based on differences in temperature, chemical composition, densities, and movement.
Let’s take a moment to identify the different atmospheric layers based on temperature.
You might think that the layers of the atmosphere get colder the further away from the earth’s surface you go. And, you’d be partially right. But, the truth is that temperature both decreases and increases as you ascend the atmosphere.
Let’s start by looking at the layers based on altitude, or how far away they are from the earth’s surface.Let’s start with the troposphere, which is the layer closest to the earth. Here, the temperature decreases as you gain altitude because the air is heated from the ground. On average, the temperature at sea level is about 15 degrees Celsius and drops by about 6.
5 degrees Celsius for every kilometer you ascend. By the time you reach the top of the troposphere, the temperature plummets to about -57 degrees Celsius in most places. Now, this is all based on some specific criteria, but that’s another lesson!If you kept ascending you’d reach the stratosphere. The stratosphere is located from about 11 to 48 kilometers off of the earth’s surface, and the temperature increases from the cold upper layers of the troposphere to about 0 degrees Celsius.
See, I told you the temperature doesn’t always decline as you ascend. You can see this temperature increases on the atmospheric temperature profile.After the stratosphere is the mesosphere, where the temperature declines again. In fact, in this layer of the atmosphere, the temperature keeps declining as you ascend finally reaching -90 degrees Celsius.
Brrrr….Finally, we have the thermosphere, which is about 55 kilometers above the earth’s surface and goes until 500 to 1,000 kilometers.
Again, the temperature increases in this layer, which you can see on the temperature profile. Depending on the time of day or night, the temperature fluctuates from 500 to 2,000 degrees Celsius! Wow! It’s hot up there!
Now that you have an idea of how the temperature changes as you ascend in the atmosphere, let’s take a look at thermodynamic diagrams, or diagrams that examine temperature, pressure, moisture, and wind in the atmosphere. Thermodynamic diagrams look really confusing with lines running all over, and it takes some practice to understand how to read them.
There are several different types of thermodynamic diagrams, including the Stuve diagram, the emagram, the Skew-T, and the tephigram.The Skew-T is the graph that is most utilized in meteorology, so that will be the focus of this lesson. Data is obtained for the graph by releasing weather balloons in the atmosphere. These balloons transmit data to weather stations and help meteorologists predict weather, like thunderstorms.
First take a moment to note the axes (see video). You have pressure in millibars, temperature in degrees Celsius, and altitude in kilometers. Different Skew-T graphs show a myriad of information, but the graph we will focus on in this lesson shows isotherms, potential temperature, equivalent potential temperature, saturated mixing ratio, and isobars. We will plot temperature and dew point temperature on this graph shortly, after we go over what these new terms mean!In order to fully understand the graph, let me give you some background information on the adiabatic process, which states that as air ascends, it cools due to less pressure.
Obviously, based on what you learned about the atmospheric profile, this only occurs to a certain point. In order to illustrate this idea, think of a balloon that ascends in the atmosphere. At the earth’s surface, the pressure exerted on the balloon keeps the balloon a certain size. As the balloon ascends, less pressure is exerted, so the air inside the balloon starts to expand, expanding the balloon. Eventually, the balloon will explode because the molecules inside the balloon will spread out so much, the balloon’s rubber cannot contain them.
Okay, back to our parcel of air. Heat energy is exchanged when molecules collide into one another. When the molecules are closer together, as seen at the earth’s surface, they collide more often. When they get higher in the atmosphere, they spread out so they collide less. With fewer collisions, there is less heat energy, therefore, the parcel cools.So, with that information, let’s examine the graph more closely (see video).
Let’s start with isotherms or areas of equal temperature. On this particular graph they are represented by red lines. You’ll notice these temperature lines are angled or ‘skewed’ (hence the name a Skew-T graph).There are solid green lines on the graph, which represent the potential temperature. This is the temperature the air would be at if it underwent the adiabatic process to a given pressure. For example, what would happen to the air parcel if it ascended or descended to a certain pressure.Next check out the dashed green lines, which represent the equivalent potential temperature, which is the temperature of the parcel of air assuming two stipulations.
The first is the temperature if all of the water vapor has condensed out. The second is that the parcel of air is at a certain pressure, specifically the pressures at sea level.The dashed purple lines represent the saturated mixing ratio.
That just means how much water vapor the air can hold at a certain temperature. Typically, the warmer the air, the more water vapor it can hold.The blue lines are the isobars.
You notice they are running from left to right, and these are equal areas of pressure.I know that was a fast explanation on the Skew-T, but hopefully it gave you enough background so we can briefly talk about how you plot data. We are going to plot temperature and dew point, which is the point where water vapor condenses and evaporates at the same rate.
So far you’ve seen a blank Skew-T, but let’s add some data. You can get data off of the internet at various sites. Here is some data obtained from the University of Wyoming Department of Atmospheric Science for Barrow, Alaska, on June 11, 2015.
|Pressure (mb)||Temperature (degrees C)|
On your Skew-T, find 1005 millibars.
Next, the corresponding temperature is -1.7 degrees Celsius, so find that point. You’ll notice that the temperature is skewed, meaning the red lines veer off to the right. So you’ll need to follow that red line to get the right point.Now place a dot where those two lines meet. You’ll notice we are using red. Temperature is typically plotted in red.
Let’s practice plotting a couple more points for Barrow, just so you get the idea.
|Pressure (mb)||Temperature (degrees C)|
So, just like for the first set of data, find 1000 millibars and -2.1 degrees Celsius, and place a dot where those two meet.
Next, do the same for 968.8 millibars and -4.2 degrees Celsius.If you continued this for all of the data points, you would have the temperature graphed on your Skew-T for Barrow. The temperature is usually plotted in red. For the sake of time, we won’t graph all of it.The other thing often plotted on the Skew-T is dew point, which is usually plotted in blue.
Just like temperature, there are numerous websites that provide dew points for different weather stations. Let’s stick with Barrow. Here’s the data:
|Pressure (mb)||Dew Point C|
Just like before, find 1000.5 millibars and -3.3 degrees Celsius, and place a dot at these points.
Next, find 1000 millibars and -3.5 degrees Celsius, and place a dot.Finally, locate 968.8 millibars and -4.8 degrees Celsius, and place a dot.
You connect these dots with a blue line. Of course, there are many, many more points, but you get the idea.
That was a whole lot of information, wasn’t it! It takes time to understand Skew-T graphs, but hopefully this gets your foot in the door.
In this lesson, we explored the Earth’s Atmospheric Temperature Profile, which means the temperatures for different layers of the atmosphere. The earth’s atmosphere is made up of different layers, each with unique properties. Most people assume the temperature slowly decreases as you ascend in the atmosphere, but that isn’t always the case.The troposphere is the layer closest to the earth, and the temperature decreases as you ascend in this layer. Next is the stratosphere, and the temperature actually increases in this layer. Next is the mesosphere, where the temperature declines, and finally the thermosphere, where the temperature increases again.Thermodynamic diagrams allow meteorologists to look at many properties of the atmosphere, including temperature, pressure, moisture, and wind.
The most common thermodynamic diagram used is the Skew-T, which looks like a bunch of confusing crisscrossing lines. But, each line has a specific meaning. For example, a Skew-T graph shows isotherms, potential temperature, equivalent potential temperature, saturated mixing ratio, and isobars. You can use websites to get raw data that you can plot on a Skew-T graph. Types of data you can plot include temperature and dew point.