Now to answer your question, -LSB-[«But there is a normal atmospheric lapse rate, by which temperature tends to decrease as a function of altitude. (hubpages.com)
I don't really know the atmosphere so I'm not sure where that is in hPa, but it's interesting to note that the sonds clearly show that the atmospheric lapse rate in the lowest part of the troposphere, from surface to 500 hPa, is becoming larger in magnitude. (realclimate.org)
θ = potential temperature, which is conserved for dry adiabatic processes and is a useful vertical coordinate for examining various fluid mechanical processes (like Rossby waves) when the atmospheric lapse rate is stable (for dry convection)(which is generally true on a large scale away from the boundary layer). (realclimate.org)