Adiabatic processes are those where no heat is added to or subtracted from an ascending or descending air parcel. The changes in temperature that are brought about in such air parcels are referred to as adiabatic temperature changes.
The rate of change in temperature through this process is called the adiabatic lapse rate. Thus, adiabatic process signifies a thermodynamic change of state of a gaseous system in which there is no transfer of heat into or out of the system.
An ascending volume of dry air moves to regions of lower atmospheric pressure, expands, and cools at the rate of 10 degrees Celsius for every kilometer of ascent. Similarly, descending air is compressed and warmed at the same rate. This rate of temperature changes in dry air is called the dry adiabatic lapse rate.
The exact mechanism of the adiabatic temperature changes is simple to understand. When an air mass is heated at the surface of the earth, it expands and becomes lighter so that the surrounding cold and denser air forces it to rise.
The rising air enters into a region of progressively decreasing air pressure. Thus, the rising air enters into a region of lower atmospheric pressure from that of higher pressure. According to gas laws there is an increase in the volume of an ascending parcel of air.
As we are aware, when an object expands it needs certain amount of energy. If the required amount of energy is not available from outside, then the object undergoing expansion spends its own internal energy which results in cooling.
In this way an ascending air mass expands and cools in a continuous process. It may be pointed out that in adiabatic process, despite changes occurring in the temperature of a rising air parcel, its heat content remains unchanged. Thus, it is clear that in adiabatic processes there is no exchange of heat between an ascending air and its environment.
A descending air parcel, on the other hand, finds itself into a region of higher pressure, so that it is compressed and gets heated. The descending air parcel is also warmed at the adiabatic lapse rate of 1° Celsius for every 100 meters of descent.
It is noteworthy that adiabatic temperature changes are brought about solely due to changes in the air pressure. Moreover, in adiabatic processes no transfer of heat between the ascending or descending air and its surroundings by conduction or radiation is involved.
The following example makes the adiabatic process of temperature change more clear. Suppose a dry air parcel ascends up to a height of 5250 meters above the mean sea level (air pressure 1016 millibars).
At this height the atmospheric pressure to which the rising air is subjected is just half of the sea-level pressure so that its volume is doubled. In other words, if the volume of the ascending air parcel is one cubic centimeter at the sea level, then at therefore mentioned height it will become two cubic centimeters.
Similarly, the volume of a descending air parcel is decreased. Therefore the changes brought about in the temperature of an ascending or descending air parcel only because of its expansion or contraction are called adiabatic temperature changes.
The noteworthy aspect of these temperature changes is the fact that irrespective of the temperature of air, the rate of adiabatic heating and cooling of the air that is unsaturated through vertical movement is constant.
It is always 10° Celsius per 1000 meters. It may be pointed that the rate of cooling of an ascending air mass is much more rapid than the normal lapse rate, i.e. 6.5″ Celsius per kilometer.