Overview of Thermodynamic Diagrams

Of all of the varieties of meteorological tools, thermodynamic diagrams are the most confusing, complicated, and full of information! These charts are sometimes also known as "adiabatic" charts, since they show how parcels of air change as a function of altitude. These charts are used to explore the behavior of the atmosphere, typically from the surface to a specific pressure level, such as 100 mb. Actual heights of the pressure levels are calculated and plotted on the charts. From these charts, we can determine factors such as the lifting condensation level (LCL), the height of the tropopause, and the stability of the atmosphere. The charts are particularly useful in that they give us a visual representation of the behavior of the atmosphere from the surface to the top of the troposphere (and sometimes far beyond!). Combined with a text-based output, these charts provide the meteorologist and the air quality modeler with critical information.

There are several types of thermodynamic diagrams (click on the name of the diagram below to see an example in a separate window). These charts are generated twice a day (00Z and 12Z) based on data from using radiosondes (weather balloons) released at 00Z and 12Z.

1. Stuve diagrams: a standard thermodynamic diagram that display lines of constant pressure. Lines of constant temperature are displayed vertically.
2. Skew-T diagrams: a thermodynamic diagram that displays temperature in a "skewed" manner, that is, not as vertical lines but at a skewed angle of 45 degrees. Sometimes these diagrams are referred to as "Skew-T Log-P" diagrams, since lines of constant pressure (in units of millibars) are plotted on a logarithmic scale.
3. Hodographs: A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).
4. Emagram: An emagram is very close to a Skew-T except the temperature lines are vertical, not skewed to the right

It is beyond the scope of this reading to detail all three of these types of diagrams. We have chosen to describe in detail the Skew-T diagram, one of the more common types of charts found on typical Web-based meteorological pages. Many sites, such as the two below, provide you with several options when choosing a thermodynamic diagram.

• University of Wyoming: http://www-das.uwyo.edu/upperair/sounding.html (good site, but often limited stations available)
• NOAA: http://orbit-net.nesdis.noaa.gov/goes/soundings/skewt/html/skewtus.html (best site, but often a slow loading time)

Each of the successive readings in this unit will focus on one aspect of the thermodynamic diagram:

1. Pressure and heights
2. Temperatures (dry-bulb and dewpoint)
3. Dry adiabatic lapse rates
4. Mixing ratios

By way of introduction, however, we present you with a full Skew-T diagram, this one for Greensboro, North Carolina. If you click on the graphic at right, a current Skew-T diagram will appear in a new browser window. This particular Skew-T diagram comes from the University of Wyoming site mentioned above. This diagram was taken for Greensboro, North Carolina on October 19, 2000, not a particular eventful weather day in central North Carolina. In addition to the diagram itself, notice a list of meteorological information on the far right of the diagram.

Let's give some basics, then provide you with an opportunity to explore this meteorological tool:

1. Notice that the pressure in millibars is indicated on the left, beginning at the standard pressure of 1013.5 mb, the first marked pressure being 1000 mb. Most charts go up to 100 mb, some sites provide the option of going up to 10 mb.
2. Two temperatures are plotted: the first, on the right-hand side (sometimes represented by a red line), is the "dry-bulb" temperature, meaning the temperature of the atmosphere at the various levels. If you have a Skew-T chart that is not in color, the dry-bulb temperature is always the right-most temperature line. The second is the dewpoint temperature (usually noted at "T_d", and is indicated on most charts as a blue line.
3. Temperatures, in degrees Celsius, are indicated on the bottom of the chart, in this case in red. Notice how the temperature lines are "skewed" at a 45-degree angle from left to right.

We'll get into more detail regarding some of the other lines in later pages in this unit.

Before we move on, however, let's introduce you to an interactive version of a Skew-T thermodynamic diagram! This tool, created by the Forecasting Systems Laboratory (FSL), a branch of NOAA, allows you to interact with the diagram to find lots of different types of information. If you go to the interactive Skew-T diagram for Greensboro, NC, you will get a plot of the weather in Greensboro for the day that you are reading this page! (NOTE! If you do not get a sounding, click on the "load sounding" button at the bottom left of the sounding window, use the menu box to go back several hours earlier in time).

• On the red line: the dewpoint in Celsius (and also Fahrenheit below 750 mb).
• On the blue line: the temperature in Celsius (and also Fahrenheit below 750 mb).
• on the right-hand side of the plot: the wind direction and speed in knots.
• on the left-hand side of the plot: the pressure in millibars, the height in meters, and the height in feet
• at the cursor point: in non-bolded letters, the pressure, height in feet, and temperature in both Celsius and Fahrenheit
• most diagrams provide a text-based description of important parameters. For example, in the interactive sounding, the lift index (L.I.) is reported. On some diagrams, the lift index -- a measure of the buoyancy of an air parcel, calculated by subtracting the temperature of a lifted parcel of air from the surface to 500 mb from the ambient temperature of the air -- is indicated by shading, using a color such as light blue. Lift indexes give a good indication of the stability of the atmosphere -- a negative number representing a negative buoyancy, an indicator of a very unstable atmosphere with likelihood of severe weather. You can experiment with various temperatures of the atmosphere and the parcel of air using the buoyancy calculator.

Quick Quiz: For the sounding below, what can you say about the stability of the atmosphere? (Hint: the dark red line is the profile of the ambient temperature, the solid green line is the temperature of the parcel of air)

(Click on image to see full-sized)
Stable
Unstable
Neutral
Difficult to determine