The SALR depends on the moisture content, so the more moisture air contains the slower it will cool.
We use 1.8 degrees/1000 ft in the mid latitudes. Close to the Equator where the air is warm and can contain a lot of moisture, the SALR can be as low as 1 degree/1000 ft. At the poles where it is very cold, the air cannot hold much moisture, so the SALR can be 2.8 degrees/1000 ft as it is virtually dry.
As we go up in the atmosphere the air gets colder and cannot hold the same moisture, so the SALR varies with height.