How to calculate outside air temperature at a certain altitude?
The temperature will drop around 2° Celsius (-1.98 to be more precise) for each 1000 feet you climb.

So if the OAT at 6000ft is -13°C, at 7000ft it will be -15°C, or minus 16 from the International Standard Atmosphere (ISA) which should be +1°C (15 - 2*7 = 1).