In an International Standard Atmosphere the temperature is +15ºC at sea level and has a lapse rate of 1.98ºC per 1,000 up to 36,090 feet.

          At FL330 (33,000 feet) the temperature is       +15º - (33 x 1.98º)

                                                                                              =  +15º - 65.34º

                                                                                              = - 50.34ºC