The International Standard Atmosphere assumes a lapse rate of 6.5 C / km. (planet3.org)
The equations and parameters used are documented in a book entitled U.S. Standard Atmosphere, 1976 published by the U.S. Government Printing Office, Washington, D.C.. (pdas.com)
This plot shows the full Standard Atmosphere model for 1976 with those layers. (mc-computing.com)