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
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