Which Of The Following Will Increase In A Rising Parcel Of Air?

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. If you give the rock in the picture at right a push it will move away from where it started (and not come back).

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.

Unsaturated air (relative humidity less than 100%) always cools at a rate of 10^o C/km. This is known as the dry adiabatic lapse rate. The term lapse rate just means rate of decrease with increasing altitude, adiabatic means that heat is not being exchanged between the air inside and outside the parcel.

Saturated air cools a little more slowly, we will use an average rate of 6^o C/km (the moist adiabatic lapse rate). As saturated air rises, expands, and cools, condensation releases latent heat inside the parcel. The latent heat energy offsets and reduces the cooling due to expansion. There isn't enough latent heat energy to cause the rising parcel to warm.

We also need to know the temperature of the atmosphere at different altitudes above the ground.

The atmosphere can do just about anything. The middle figure shows temperature decreasing at a rate of 8^o C/km (the environmental lapse rate). The left and right examples show the air cooling more slowly and more rapidly, respectively, with increasing altitude. Usually atmospheric temperature doesn't decrease at a uniform rate as shown above. A more realistic example is shown below.

A plot of temperature versus altitude is called a sounding.

We now have all the tools we need.

In this first example we assume the environmental lapse rate is 4^o C/km. This is the left column of figures in the figure above. The next two columns show the temperature inside rising parcels of unsaturated (green) and saturated (orange) air (they cool at 10^o C/km and 6^o C/km, respectively). 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 and below the purple, environment, curve. Rising parcels of unsaturated or saturated air will both end up colder and denser than the surrounding environmental air. If they are lifted and released, they will sink back to the ground. The atmosphere is absolutely stable in this situation.

We'll change the environmental lapse rate to 11^o C/km in Example #2.

Now, because the atmosphere is cooling so quickly with increasing altitude, lifted parcels of both unsaturated and saturated air end up warmer and less dense than the surrounding air. Both the orange and green curve lie above and to the right of the purple curve on the graph. When released these parcels will continue to rise on their own. The atmosphere is absolutely unstable in this case.

We'll pick an intermediate value, 8

^o C/km, for the environmental lapse rate in the next example.

The atmosphere is conditionally unstable. A rising parcel of unsaturated air ends up cooler and denser than the surroundings. A parcel of saturated air, which cools at a slower rate, ends up warmer than the air around it. The condition for instability is that the air must be saturated. It was a little harder coming up with a rock/hill analogy in this case. The rock is in either a stable or unstable position depending on which direction you push it.

We'll won't change the environmental lapse rate the same for the last and most instructive example.

In this case the parcel of air starts out unsaturated. It becomes saturated when lifted to 1.5 km altitude, once it has cooled to a temperature of 0^o C. This is called the Lifted Condensation Level (LCL) and you would see cloud begin to form at this point. From that point on upward the rising parcel will cool at the moist adiabatic rate.

Initially the rising parcel is colder and denser than the surrounding air. If the parcel is lifted to 3 km it has the same temperature as the air around it. If lifted above 3 km the parcel air finds itself warmer and less than the air outside. If lifted just a little bit beyond 3 km altitude the parcel will be able to continue to rise on its own. This dividing line between stable and unstable is called the Level of Free Convection (LFC).

The atmosphere is conditionally unstable in this case. A rising parcel must first of all become saturated. Then it must be lifted to and just above the level of free convection. The LFC can be higher or lower than 3 km. It depends on how quickly the atmosphere is cooling with increasing altitude and at what altitude the rising parcel becomes saturated.

The value of the environmental lapse rate is one of the main factors that determines whether the atmosphere will be stable or unstable.

environmental lapse rate	atmospheric stability
less than the MALR (eg. 4^o C/km)	absolutely stable
between the DALR & the MALR (eg. 8^o C/km)	conditionally unstable
greater than the DALR (eg. 11^o C/km)	absolutely unstable

Warming the air above the ground and/or cooling the air next to the ground will make the atmosphere more stable. The ground and the air above it cool during the night. The atmosphere is usually most stable early in the morning.

A temperature inversion represents an extremely stable situation. Rising parcels always cool with increasing altitude (at either the dry or moist rate). In an inversion the surrounding air gets warmer and warmer with altitude. The difference between the cold parcel air and the warmer surroudings gets larger and larger with increasing altitude.

Sunlight warms the ground and the air next to it during the day. This steepens the environmental lapse rate and makes the atmosphere more unstable. Cooling air above the ground has the same effect.

One last figure before we leave this topic. The figure shows the different types of clouds that form in stable and conditionally unstable conditions.

The violet curve shows the environmental temperatures as a function of altitude. The other curve shows the temperature of a rising parcel of moist air. The rising parcel starts out unsaturated and follows the green portion of the curve. Once it becomes saturated it begins cooling at the moist rate and follows the orange line.

The top figure shows a conditionally unstable situation. Because the parcel is lifted above the Level of Free Convection (LFC) it is able to continue rising on its own and develops into cumuliform cloud (perhaps a thunderstorm).

The air in the lifted parcel does become saturated but never becomes warmer than the surrounding air. The parcel won't go any higher than it is lifted. There isn't any Level of Free Convection in this case; stratiform clouds tend to form under these conditions.