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.
When the environment is unstable, air mixes readily in the vertical. This vertical mixing can have a profound effect on various atmospheric phenomena as diverse as air quality, wind speed and cloud type. Vertical mixing in an unstable environment helps bring cleaner air from above down to the surface, while transporting polluted air aloft. Also, stronger winds from above where there is little friction can be transported mixed toward the ground when the atmosphere is unstable.
This is why it is often breezy on a sunny afternoon, and often quite calm in the morning, when vertical mixing is restricted. An example of this vertical mixing can be seen in Figure 5 to the right. If there is sufficient moisture in the atmosphere the water vapor in the rising bubbles of air will condense into clouds if the parcel rises high enough to cool to the dewpoint.
In an unstable environment these convective motions work to produce cumuliform clouds. Figure 6: This image depicts a typical stable, or stratified, environment where no mixing occurs. The atmospheric motions that occur in a stable environment are fundamentally different than those found in an unstable environment.
The most profound difference between these two types of environments is the inhibition of vertical mixing in the stable environment. The lack of vertical mixing leads to a "stratified" atmosphere, where many atmospheric variables are separated into layers instead of being well-mixed. The stratification of the atmosphere when stable leads to, for instance, pollution episodes and drastic changes in wind speed and direction over short vertical distance.
An example of a stratified and stable environment can be seen in Figure 6 to the left. Another atmospheric consequence of a stable and stratified atmosphere involves the process of cloud formation. Assuming that there is sufficient moisture present in the atmosphere, stratiform clouds can form in a stable environment. The following explanations relate to the Handout on Instability Absolutely Stable. In the first situation the air at sea level is 75 degrees.
As ones goes up in elevation note, you are "going up" through a horizontally moving atmosphere , one experiences a average decrease in temperature of 3 degrees per ft. This is the environmental lapse rate. The temperature at ft in the atmospheric environment is 71 degrees. When a single air parcel in this environment is forced to rise from the ground for example from sealevel up a mountain , it cools at 5. This is the dry adiabatic lapse rate.
In this case, the air parcel starts with a temperature 75 degrees, but by the time it rises to the top of the mountain at feet elevation, it will cool to Note that the air parcel now at feet has a temperature of So the air parcel is 1.
Since in this case the air parcel is colder than its surroundings, it is relatively dense. After being forced to the top of the mountain, the air parcel will sink back down to its initial elevation.
In the second situation the temperature of the atmospheric environment at sea level is still 75 degrees. However, for some reason, the temperature of the horizontally moving air at feet is much colder 65 degrees instead of 71 than that for the first example above. This can happen if cold air aloft moves horizontally from the north southward over a region.
In this case, note that the temperature in the atmopsheric environment at ft. Now, if an air parcel at the ground is forced to rise over the moutain, cooling at the dry adiabatic lapse rate, it will still cool to a temperature of But at ft. Thus air parcel will be less dense than the air surrounding at at feet and will continue to rise.
In the third situation, the temperature in the atmospheric environment is still 75 degrees at sea level but is 70 degrees at ft. Note that the only constraint we have changed in discussing these three "states" is the temperature at feet. Thus, the temperature difference between the ground and feet observed by a thermometer as it is carried upwards in a weather balloon would be different on each of these days. If one air parcel is forced to rise to the top of the mountain, note that its temperature would still cool to In the atmosphere, this occurs if the dew point temperature in the air parcel is so low, that it would not be cooled to the dewpoint when lifted to feet elevation.
This is still an absolutely stable atmosphere. However, suppose that surface dew point temperature was not low, and was also 75 degrees. 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. The environmental temperatures and the parcel temperatures are also plotted on a graph on the right side of the figure.
The parcel curves green and red lie to the left of the purple, environment, curve.
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