Adiabatic Processes and Lapse Rates
As long as the air parcel is unsaturated (relative humidity less than 100%), the rate of adiabatic cooling or warming remains constant. This rate of heating and cooling only applies to unsaturated air and is about 10 degrees C for every 1000m of change in elevation. This is known as the Dry Adiabatic Lapse Rate (DALR) (see below). As rising air cools, its relative humidty increases, as the air temperature nears the dew-point temperature. The relative humidty of the rising air becomes 100% if its temperature drops to the dew-point temperature. After further lifting condensation occurs, a cloud forms and latent heat is released inside the rising air parcel. Due to the heat added during condensation offseting some of the cooling due to expansion, the air no longer follows the DALR but instead follows a lesser rate, the Saturated Adiabatic Lapse Rate (SALR) (see below) An average value for of heating/cooling for the SALR is around 6 degrees C per 1000m.
If a saturated air parcel containing water droplets were to sink it would compress and warm at the SALR because evaporation of the liquid droplets would offset the rate of compressional warming. Therefore, the rate at which rising or sinking saturated air changes temperature - the SALR - is less than the DALR.
The SALR is not constant, unlike the DALR, and varies greaty with temperature, and hence, with moisture content - as warm saturated air produces more liquid than cold saturated air. The added condensation in warm, saturated air liberates more latent heat. Consequently, the SALR is much less than the DALR when the rising air is warm. However, when the rising air is very cold the two rates are nearly the same.
Looking at the figure below, if we take the initial state of a material to be represented by point A on the p - V diagram, the line AB is the path the material takes when it undergoes isothermal transformation. Curve AC represents the transformation if the same material underwent a similar change in volume but under adiabatic conditions. Curve AC is called an adiabat.
The adiabat is steeper than the isotherm because under adiabatic compression, the internal energy increases, and therefore the temperature of the system rises. For isothermal compression the temperature remains constant.
A p - V diagram showing an adiabat and isotherm
An adiabatic process is one in which a material undergoes a change in its physical state without any heat being added or taken away from it. For example, an air parcel that expands and cools, or compresses and warms, with no interchange of heat with its surroundings, is an adiabatic process.
