5.2: Charting Air Parcels

Using Stüve Diagrams

As you may have noticed, calculating air parcel temperatures at various altitudes can be a computationally intensive process. The good news is that there is another way! Stüve Diagrams include a helpful set of lines that represent the lapse rates for rising parcels. These lines are called 'Adiabats' and are shown in Figure \(\PageIndex{1}\). Dry air parcels follow the dry adiabat, represented by the solid green lines, while saturated parcels follow the moist adiabat, represented by the dashed blue lines. The dotted orange lines represent constant water vapor mixing ratio, but we'll ignore those for now as we focus on the adiabats. By drawing a line on the adiabat, you can track the temperature of any parcel at any given altitude.

Figure \(\PageIndex{1}\): A blank Stüve Diagram with dry adiabats shown using solid green lines and moist adiabats shown using dotted blue lines. (CC BY-SA 3.0; Daelomin via Wikimedia Commons) Alternative description of image.

Here’s how to do it: You need two pieces of information to start, the altitude (or pressure level) at which you need to find the parcel's temperature, and the temperature of the parcel before rising. Start at the bottom of the diagram, i.e., at 0 km altitude or the altitude of the parcel before it begins to rise, and place a dot at the temperature of the parcel before it starts to rise in the atmosphere. Then, draw a line as you travel along (or parallel to) the nearest dry or moist adiabat, depending on the condition of your air parcel, until you reach your desired pressure level. The temperature on the X-axis at the point where you stopped is the parcel's temperature at your desired altitude. For example, consider a dry parcel of air at the surface with a temperature of 17°C.

5. Because the parcel is dry, it will travel along the nearest:
   1. Solid-green dry adiabat
   2. Dashed-blue moist adiabat 

Figure \(\PageIndex{2}\) is a Stüve Diagram for a parcel of air starting at the surface with a temperature of 17°C. Drawn on the figure are two lines: one for a dry parcel and one for a moist parcel rising in the atmosphere.

Figure \(\PageIndex{2}\): Stüve Diagram with plotted lines for a dry (green) and moist (blue) parcel of air that begins at the surface with a temperature of 17°C. (CC BY-SA 4.0; Modified from Daelomin via Wikimedia Commons) Alternative description of image.

6. Using the lines in Figure \(\PageIndex{2}\), a moist parcel will have the temperature of approximately ___ at 400 mb.  
   1. -50°C
   2. -27°C
   3. -35°C
   4. -15°C

7. On the other hand, a dry parcel will have a temperature of approximately ____ at 400 mb.  
   1. -50°C
   2. -27°C
   3. -35°C
   4. -15°C

8. As would be expected, given its smaller lapse rate, the _____ parcel is the warmer of the two at 400 mb. 
   1. dry
   2. moist

Let's practice tracking rising and sinking air parcels using a Stüve Diagram. Print out a blank Stüve Diagram for the following few problems. Consider a dry parcel of air with a surface temperature of 28°C. Draw a dot for that parcel on your blank diagram. Now follow the parcel up to 800 mb. Hint: If your dot doesn't exactly fall on a dry or a moist adiabat, draw a line parallel to the nearest dry or moist adiabat.

9. At 800 mb, the parcel will have a temperature of:  
   1. -8°C
   2. 18°C
   3. 8°C

10. Suppose that at 800 mb, the parcel has cooled to its dew point and is not saturated. Continuing up, the parcel will now travel along the:  
    1. Solid-green dry adiabat
    2. Dashed-blue moist adiabat  

11. At 400 mb, the temperature of the parcel is:  
    1. -43°C
    2. -28°C
    3. -35°C
    4. -50°C

Now let’s allow the parcel to sink. There is one exception to the Las Vegas rule that hasn’t been covered yet: Once a parcel becomes saturated, the moisture within the parcel condenses into liquid water and is no longer considered a part of the air parcel. If the saturated parcel is still rising, it can continue to condense more moisture and thus must follow the moist adiabat. However, as soon as any parcel begins to sink, it becomes unsaturated (dry).

12. Based on the previous information, the parcel of air from questions 9-11 will now follow the _____ as it sinks.
    1. Solid-green dry adiabat
    2. Dashed-blue moist adiabat

Draw a line following the previous parcel from 400 mb, back down to the surface, and note its temperature at the surface.

13. Once it sinks back to the surface, the parcel is _____ than it was when it initially rose from the surface.  
    1. warmer
    2. cooler

14. Because it lost moisture to condensation as it rose, it will also be _____ than it was when it initially rose from the surface.  
    1. drier
    2. wetter

15. Submit the Stüve Diagram to your instructor.

Congratulations! You created a Stüve diagram plot tracking the temperature of an air parcel as it rose and sank in the atmosphere.