5.4: Atmospheric Stability
- Page ID
- 42070
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In addition to topographic lifting, it is important to understand how air parcels behave as they rise, as this determines atmospheric stability. This is because rising air parcels are often completely isolated from the rest of the atmosphere, meaning that they have a different temperature than the surrounding atmosphere, which can be heated/cooled by other factors. The stability of the atmosphere is determined by comparing the parcel's temperature (which has risen from the surface) with the temperature of the surrounding atmosphere.
- Suppose the parcel of air is warmer than the surrounding atmosphere. In that case, its adiabatic temperature line lies to the right of the weather-balloon-measured temperature on a Stüve diagram. The parcel will continue to rise because it is less dense than the surrounding atmosphere. A layer in the atmosphere in which an air parcel is warmer than its surroundings is called the Convective Available Potential Energy (CAPE).
- Suppose the air parcel is cooler than the surrounding atmosphere. In that case, its adiabatic temperature line lies to the left of the weather-balloon-measured temperature on a Stüve diagram. The parcel will sink because it is denser than the surrounding atmosphere. This phenomenon is known as Convective Inhibition (CIN).
- If the parcel temperature matches the surrounding air temperature, the parcel will neither rise nor sink. This is called the Equilibrium Zone (EZ).
Figure \(\PageIndex{1}\) is a Stüve Diagram of air temperature, represented by a black line on the right, dew point temperature, represented by a black line on the left, and parcel temperature, represented by a red line taken from a weather balloon launch over Dallas-Fort Worth International Airport at 0000 UTC (6 pm local time) on May 20, 2019. In this figure, a layer of CIN, where the air parcel is cooler than the surrounding atmosphere, exists between 750 mb and 600 mb, and a layer of CAPE, where the air parcel is warmer than the surrounding atmosphere, exists between 600 mb and approximately 160 mb. In this case, if a parcel of air were “pushed” to the 600 mb pressure level, it would quickly rise on its own. Situations like this are ideal for the formation of severe weather, including Tornadoes, Hail, High Winds, severe lightning, and other interesting weather phenomena. Indeed, the Storm Prediction Center issued a “High Risk” alert for the Dallas area on this particular day, and the region experienced several tornadoes.
Let us determine atmospheric stability and tornado potential for another location using a similar upper-air plot. Figure \(\PageIndex{2}\) shows a Stüve Diagram plot of weather balloon data and calculated parcel temperatures (red line) from Topeka, Kansas, at 0000 UTC on May 29, 2019, which corresponds to 6 pm local time on May 28, 2019. Using this information, let's answer the following questions.
- According to Figure \(\PageIndex{2}\), a layer of CIN _____ present between 950 mb and approximately 700 mb over Topeka, Kansas, at 0000 UTC on May 28.
- is
- is not
- Above the 700 mb level, a layer of CAPE ______ present.
- is
- is not
- Conditions for Topeka at this time ______ similar to conditions experienced at Dallas-Fort Worth on May 20 (Figure \(\PageIndex{1}\)), when a tornado outbreak occurred near Dallas.
- are somewhat
- are not at all
Let's verify our atmospheric stability conclusions by checking the NOAA Severe Weather reports for the United States on May 28, 2019, shown in Figure \(\PageIndex{3}\).
- According to Figure \(\PageIndex{3}\), there _____ several reports of tornadoes in the Kansas region on May 28.
- were not
- were
- This _______ that the conditions shown in Figure \(\PageIndex{2}\) indicated severe weather for Topeka, Kansas on May 28, with the possibility of tornadoes.
- confirms
- contradicts