What is the dry adiabatic lapse rate?

Air parcels are not really all that complex. They are masses of air that have the same characteristics and that generally travel in the same direction as a cluster. You can call them air masses or air clusters or air blobs. Air does not move within an air parcel, but when it does this, it is called adiabatic process. Adiabatic processes are reversible and involve no heat exchange. In fact, the term "adiabatic" means that the temperature remains the same.

Consider any parcel of air with a certain pressure and a certain temperature. If you could physically lift this air parcel, it would be in a situation where all of its surrounding environment was all a lower pressure, mainly because of the higher altitude. Your imaginary air parcel has more pressure in it than the air around it so it will naturally expand because of course it has no real walls. In order to do this, energy is taken from the air molecules and it will naturally cool down the air parcel.

Suppose you can take this same air parcel and now move it downward toward the ground. Here it is surrounded by warmer air under higher pressures. Under these pressures, it must contract. As it contracts, the molecules are closer together and because of this, the air parcel will be warmer. It would be the same as if you took a number of people on a cold day and smash them together into a small huddle of people. Everyone would warm up. This process is also adiabatic because no heat is exchanged between that air parcel and the surrounding environment.

Think about it. This is different than what you would see if you mixed two air masses of different temperatures, averaging their temperatures afterward. In this case, there is no mixing. The temperatures change only because of a change in the kinetic energy of the air parcel itself. Even though an air parcel has no walls, you can think of it as truly being a bubble of air.

WHAT IS THE DRY ADIABATIC LAPSE RATE?

Parcels of air have varying degrees of relative humidity. If the relative humidity of any air parcel is less than 100 percent, the temperature change you see with rising and falling air masses will be a constant temperature. This constant temperature downward with height is called the lapse rate. Meteorologists also call it the dry adiabatic lapse rate

if there is no humidity in the air parcel. This dry adiabatic lapse rate is approximately 10 degrees Celsius per kilometer; in actuality is precisely 9.8 degrees Celsius per kilometer. Again, this is predicted by decrease in the internal energy of the expanded air mass.

If the environment in an area is considered neutral from a meteorology perspective, air masses neither rise nor fall. In any unstable equilibrium situation, however, air parcels will either rise or fall, although rising is more common than falling. How do you know if the air mass or air parcel is stable? Let's look at this.

Consider an air mass that has just moved. You will know if it is stable by checking the temperature and pressure of the air around it. If you lift this air parcel it will cool and, if it cools to a degree greater than its environment, the air will be denser and will fall back to where it was. This is a stable air mass because it resists being moved vertically. On unstable air mass will not resist this and will rise to whatever level its own temperature matches the environment. Again, it is mainly the temperature of an air parcel that predicts where it reaches equilibrium. You will know if the air parcel is stable or not by checking the temperature of the rising air and of the environment at different altitudes around which the air parcel resides. As you can imagine, this process involves weather balloons and radiosondes in order to measure the vertical temperature at any given area. This process is called sounding.

We call any air parcel that is not completely saturated a dry air parcel. It is easy to measure its stability. If the environment involves a lapse rate that is equivalent to the dry adiabatic lapse rate, we know what happens to the temperature with elevation and we call this air parcel neutrally stable.

If the environmental lapse rate is less than the dry adiabatic lapse rate, the air parcel will either be lifted and colder than its environment or pushed down and warmer than its environment. In this situation, the air mass is still stable; it will have an average tropospheric lapse rate of approximately 6.5 degrees Celsius per kilometer. On the other hand, if the environmental lapse rate is greater than the dry adiabatic lapse rate, this air parcel will be warm on elevation compared to its environment and colder if it is pushed down compared to its environment.