John thanks for the reply. If you have a technical publication on precisely how the environmental lapse rate (-6.5 K/km) is derived I'd be very grateful. I don't have a copy of ISO 2533:1975 or ICAO Doc 7488 but the freely available US Standard Atmosphere 1976 appear not to explain the figure (obviously all the usual hydrostatic and hypsometric equations are there but I can't see how the 6.5 K/km comes into existence). Thanks.