>> With increasing altitude each 100 meters, air temperature decreases 0.5 degrees celcius << Ahh, that's what you were thinking of... sounds reasonable. Just checked... Th = To �?nbsp; 0.0065° / m X h according to the Wikipedia article at http://en.wikipedia.org/wiki/Density_of_air (To = 298.15 K or 25°C). That would be a 0.65 degree decrease per 100 m of altitude. The article didn't say how accurate the formula is!  Steve |