This is called a stable airmass. If the rising air parcel contains enough water vapor, however, then condensation of the water vapor in it occurs, and a cloud is formed.
This condensation releases heat, which warms the air parcel, which can cause the parcel to rise higher still. This is the basis for thunderstorm formation. The more unstable the air mass, the more energy there is for thunderstorms to feed off of. The thunderstorm transports the extra heat into the upper troposphere , and at the same time cools the lower troposphere. We'll start with a relatively easy to understand conceptual test that we can perform to determine whether the atmosphere is stable or unstable.
You simply take some air and isolate it in a parcel. At this point the air inside the parcel is exactly the same as the air outside same temperature, density, and pressure. We'll assume that once the air is sealed in the parcel it can't mix or exchange heat with the surrounding air. Next you imagine lifting the air. The air parcel will expand and the air in the parcel will cool somewhat.
After being lifted, the air inside the parcel may have a different temperature than the surrounding air outside the parcel. This is because the inside the parcel is "insulated" from the surrounding air.
In the figure above the air in the parcel has ended up colder and denser than the surrounding air. In this case the parcel would sink back to the ground. In the analogy shown above at right you can imagine giving the rock in the picture a shove then watch to see what happens. In the situation shown above the rock would roll part way up the slope but then stop, turn around, and come back down to where it started. Now the lifted air parcel has found itself warmer and less dense than the surrounding air.
It will continue to float upward on its own. This indicates an unstable situation. We need a little more information to be able to perform the test described above.
First we need to know how quickly a rising parcel of air will cool. A plot of temperature versus altitude is called a sounding. We now have all the tools we need. This is shown in the left column of figures in the figure above. This lapse rate is approximately 3. The actual stability of an air parcel is determined by the orientation of the environmental lapse rate in comparison with either the dry or moist adiabatic lapse rates. The environmental lapse rate is simply what it says- the rate of change of the temperature of the environment atmosphere with changing altitude.
It is important to relaize that because the atmosphere environment , on average, is not rising or sinking, the environmental lapse rate can look much different than the dry or moist adiabatic lapse rates.
In fact, it is those differences that allow us to determine whether a particular part of the atmosphere is stable or unstable. The dry and moist adiabatic lapse rates can be seen in Figure 1 to the right.
Figure 2: This image illustrates the concept of unstable equilibrium. In this case, the environmental lapse rate is greater than both the dry and moist adiabatic lapse rates. The atmosphere is considered to be unstable if a rising parcel cools more slowly than the environmental lapse rate.
This causes the air parcel to remain warmer and less dense than its surroundings and, therefore, continue to accelerate upward. The orientation of an unstable environmental lapse rate can be seen to the left in Figure 2.
Figure 3: This image illustrates the concept of stable equilibrium. In this case, the environmental lapse rate is less than both the dry and moist adiabatic lapse rates. The atmosphere is considered to be stable if a rising parcel cools faster than the environmental lapse rate. This causes the air parcel to be cooler and more dense than its surroundings and, therefore, lose its buoyancy. Vertical motions tend to be restricted when the atmosphere is in stable equilibrium. The orientation of a stable environmental lapse rate can be seen to the right in Figure 3.
Figure 4: The figure above shows a typical temperature inversion. An inversion occurs when temperature increases with height, a situation "inverted" from the general temperature decrease with altitude in the troposphere. A temperature inversion occurs when the temperature increases with height. The environmental profile associated with a temperature inversion is the most stable type of environment.
A temperature profile featuring an inversion can be found to the left in Figure 4. The inversion is at the top, where temperature increases with height. Figure 5: This figure illustrates the vertical mixing process that occurs in an unstable atmosphere.
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