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Isentrops are lines of constant entropy. Hence, isentropic surfaces are surfaces of constant entropy. Entropy is directly proportional to potential temperature. In large scale stratospheric flow conditions, air parcels move adiabatically (i.e. without loosing or gaining heat) or along isentrops and thus conserve potential temperature. The Potential Temperature (theta) of an air parcel is defined as the temperature which the parcel of air would have if it were expanded or compressed adiabatically from its existing pressure and temperature to that at 1000 hPa.

Poisson's equation (above) is used to convert from the the temperature at a given pressure to the potential temperature at that pressure. Geostrophic wind in isentropic coordinates blow parallel to isolines of Montgomery Stream Function. Potential Vorticity (PV) is conserved on isentropic surfaces. Regions of strong PV gradients on isentropic surfaces can act as semi-permeable transport "barriers" to chemical tracers such as ozone. During the winter the air inside these stong PV gradient areas are isolated from the air outside of the PV gradient area. If the temperatures inside this area are cold enough (T < -78C) Polar Stratospheric Clouds can form. Then in the presence of sunlight heterogeneous chemistry can quickly void the area of ozone. This is why atmospheric scientists monitor isentropic surface between 450-650 K during the winter/spring period. It is within this range of isentropic surfaces that Antarctic and Arctic ozone depletion occurs the greatest.

Below are examples of the monthly mean temperature profile with latitude for a given February and the resultant potential temperature profile.

lat_pres_temp_feb lat_pres_theta_feb

Below are examples of the monthly mean temperature profile with latitude for a given July and the resultant potential temperature profile.

lat_pres_temp_jul lat_pres_theta_jul

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