Potential temperature

Summary

The potential temperature of a parcel of air is the temperature it would have at a reference pressure, usually $1000$ hPa. The parcel is imagined to be brought there without exchange of heat. It removes the cooling by expansion and the warming by compression that a parcel feels when it rises or sinks. It is conserved in dry adiabatic motion, so it labels air parcels and measures the static stability of the atmosphere.