When a parcel of air ascends or descends in the atmosphere, the changes that occur in its temperature are given by the adiabatic relation (3.4.5). Using this relation, we may define a temperature, 9, which a parcel of air at temperature T and pressure p will assume if it is compressed (or expanded) adiabatically and brought to a standard pressure, po, which is usually taken as 1000 mb. It is called the potential temperature and defined by the relation
The potential temperature 9 is a conservative property of air in any adiabatic process and, as such, widely used for identification in air mass analysis. Lines of constant potential temperatures are called adiabats. So, when a parcel of dry air rises adiabatically in the atmosphere, it follows a dry adiabat. The concept of potential temperature is useful in studies of vertical stability of the atmosphere.
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