Oslo and Paris are marked on
the chart and you can work out the average temperature by adding the up and
dividing by the number you have added:
(-25ºC)
+ (-24ºC) + (-23ºC) + (-21ºC) + (-21ºC) + (-20ºC) = -134º ÷
6 = -22.3ºC
This is the temperature at FL180 and at a typical lapse rate of 2ºC per 1,000ft the temperature at FL160 will be about 4ºC warmer at approximately -18ºC.