1f. Potential InstabilityAir with a large temperature lapse rate is
said to be statically unstable. Another type of instability that
influences vertical air motions, called potential instability, arises
when a layer of dry air rests above moist air below. Lifting the two
layers can cause the temperature lapse rate to increase, thus making the
air statically unstable.The Development of Potential
InstabilityConsider the situation shown in Figure 6-2-1. In parcel 1,
located in the moist layer, the temperature (Ta) equals 30°C and the dew
point (Td) is 26°C. In parcel 2, located in the dry layer, the
temperature and dew point are 25°C and 18°C, respectively. Now consider
what happens if some process lifts the two layers containing the
parcels. Both parcels are initially unsaturated, so they cool at the dry
adiabatic lapse rate (DALR) of 1°C per 100 m, and their dew points
decrease at 2°C per 100 m. After 500 m of ascent, the temperature of
both parcels has fallen by the same amount, so the temperature
difference between them is unchanged. However, parcel 1 is now
saturated, so further lifting will cause its temperature to decline at
the saturated adiabatic lapse rate (SALR). Meanwhile, parcel 2 is still
unsaturated, so further lifting will cause it to cool at the DALR as
before. Now let’s lift the two parcels another 500 m. Assuming an SALR
of 0.6°C/100 m, the lower parcel cools by 3°C to 22°C, while the upper
parcel cools at the DALR to 15°C. We can now see how uplift of the
column of air containing the two parcels has affected its stability.
Initially, the temperature difference between the parcels was 5°C,
corresponding to a laps rate of 1°C per 100 m (neutral stability). After
ascent, both parcels are colder, but the dry parcel cooled more and
became 7°C colder than the parcel below. This yields a temperature lapse
rate of 1.4°C per 100 m, making the air statically unstable. Thus, the
air that is statically stable has the potential to become statically
unstable, given sufficient uplift—hence the term potential
instability.Potential Instability and Severe ThunderstormsBoth theory
and experience show that potential instability is an important factor in
the development of severe thunderstorms. During spring and summer, the
southern Great Plains region often has warm, humid air near the surface
advected from the Gulf of Mexico. In the middle troposphere above the
region, westerly winds bring dry air from the southern Rockies. This air
in the middle troposphere sinks somewhat after passing the Rockies to
form a subsidence inversion (described later in this chapter), which
inhibits the development of localized thunderstorms. But given
sufficient uplift, the surface layer of air can become statically
unstable and severe thunderstorms can develop....FIGURE 6-2-1 Potential
Instability. Potential instability occurs when warm, dry air overlies
moist air. Here two air parcels rise 1000 m. Before uplift the lower
parcel is colder, and the atmosphere is stable. Initially both cool by
1°C per 100 m. After 500 m of uplift the moist parcel reaches
condensation and cools more slowly, 0.4°C per 100 m. After the full
ascent the moist parcel is warmer than the dry parcel above and thus the
layer has destabilized.What kind of temperature and moisture pattern
causes potential instability? Get solution
2f. Potential InstabilityAir with a large temperature lapse rate is said to be statically unstable. Another type of instability that influences vertical air motions, called potential instability, arises when a layer of dry air rests above moist air below. Lifting the two layers can cause the temperature lapse rate to increase, thus making the air statically unstable.The Development of Potential InstabilityConsider the situation shown in Figure 6-2-1. In parcel 1, located in the moist layer, the temperature (Ta) equals 30°C and the dew point (Td) is 26°C. In parcel 2, located in the dry layer, the temperature and dew point are 25°C and 18°C, respectively. Now consider what happens if some process lifts the two layers containing the parcels. Both parcels are initially unsaturated, so they cool at the dry adiabatic lapse rate (DALR) of 1°C per 100 m, and their dew points decrease at 2°C per 100 m. After 500 m of ascent, the temperature of both parcels has fallen by the same amount, so the temperature difference between them is unchanged. However, parcel 1 is now saturated, so further lifting will cause its temperature to decline at the saturated adiabatic lapse rate (SALR). Meanwhile, parcel 2 is still unsaturated, so further lifting will cause it to cool at the DALR as before. Now let’s lift the two parcels another 500 m. Assuming an SALR of 0.6°C/100 m, the lower parcel cools by 3°C to 22°C, while the upper parcel cools at the DALR to 15°C. We can now see how uplift of the column of air containing the two parcels has affected its stability. Initially, the temperature difference between the parcels was 5°C, corresponding to a laps rate of 1°C per 100 m (neutral stability). After ascent, both parcels are colder, but the dry parcel cooled more and became 7°C colder than the parcel below. This yields a temperature lapse rate of 1.4°C per 100 m, making the air statically unstable. Thus, the air that is statically stable has the potential to become statically unstable, given sufficient uplift—hence the term potential instability.Potential Instability and Severe ThunderstormsBoth theory and experience show that potential instability is an important factor in the development of severe thunderstorms. During spring and summer, the southern Great Plains region often has warm, humid air near the surface advected from the Gulf of Mexico. In the middle troposphere above the region, westerly winds bring dry air from the southern Rockies. This air in the middle troposphere sinks somewhat after passing the Rockies to form a subsidence inversion (described later in this chapter), which inhibits the development of localized thunderstorms. But given sufficient uplift, the surface layer of air can become statically unstable and severe thunderstorms can develop....FIGURE 6-2-1 Potential Instability. Potential instability occurs when warm, dry air overlies moist air. Here two air parcels rise 1000 m. Before uplift the lower parcel is colder, and the atmosphere is stable. Initially both cool by 1°C per 100 m. After 500 m of uplift the moist parcel reaches condensation and cools more slowly, 0.4°C per 100 m. After the full ascent the moist parcel is warmer than the dry parcel above and thus the layer has destabilized.How does potential instability affect the weather in the southern Great Plains? Get solution
2f. Potential InstabilityAir with a large temperature lapse rate is said to be statically unstable. Another type of instability that influences vertical air motions, called potential instability, arises when a layer of dry air rests above moist air below. Lifting the two layers can cause the temperature lapse rate to increase, thus making the air statically unstable.The Development of Potential InstabilityConsider the situation shown in Figure 6-2-1. In parcel 1, located in the moist layer, the temperature (Ta) equals 30°C and the dew point (Td) is 26°C. In parcel 2, located in the dry layer, the temperature and dew point are 25°C and 18°C, respectively. Now consider what happens if some process lifts the two layers containing the parcels. Both parcels are initially unsaturated, so they cool at the dry adiabatic lapse rate (DALR) of 1°C per 100 m, and their dew points decrease at 2°C per 100 m. After 500 m of ascent, the temperature of both parcels has fallen by the same amount, so the temperature difference between them is unchanged. However, parcel 1 is now saturated, so further lifting will cause its temperature to decline at the saturated adiabatic lapse rate (SALR). Meanwhile, parcel 2 is still unsaturated, so further lifting will cause it to cool at the DALR as before. Now let’s lift the two parcels another 500 m. Assuming an SALR of 0.6°C/100 m, the lower parcel cools by 3°C to 22°C, while the upper parcel cools at the DALR to 15°C. We can now see how uplift of the column of air containing the two parcels has affected its stability. Initially, the temperature difference between the parcels was 5°C, corresponding to a laps rate of 1°C per 100 m (neutral stability). After ascent, both parcels are colder, but the dry parcel cooled more and became 7°C colder than the parcel below. This yields a temperature lapse rate of 1.4°C per 100 m, making the air statically unstable. Thus, the air that is statically stable has the potential to become statically unstable, given sufficient uplift—hence the term potential instability.Potential Instability and Severe ThunderstormsBoth theory and experience show that potential instability is an important factor in the development of severe thunderstorms. During spring and summer, the southern Great Plains region often has warm, humid air near the surface advected from the Gulf of Mexico. In the middle troposphere above the region, westerly winds bring dry air from the southern Rockies. This air in the middle troposphere sinks somewhat after passing the Rockies to form a subsidence inversion (described later in this chapter), which inhibits the development of localized thunderstorms. But given sufficient uplift, the surface layer of air can become statically unstable and severe thunderstorms can develop....FIGURE 6-2-1 Potential Instability. Potential instability occurs when warm, dry air overlies moist air. Here two air parcels rise 1000 m. Before uplift the lower parcel is colder, and the atmosphere is stable. Initially both cool by 1°C per 100 m. After 500 m of uplift the moist parcel reaches condensation and cools more slowly, 0.4°C per 100 m. After the full ascent the moist parcel is warmer than the dry parcel above and thus the layer has destabilized.How does potential instability affect the weather in the southern Great Plains? Get solution
