If you bring moving air to rest (or accelerate stationary air to the speed of the aircraft), then you are imparting energy. That energy will appear as a temperature rise.
For a given mass of air the temperature rise will be the kinetic energy change divided by the specific heat (constant pressure).
i.e. 0.5*V^2 / Cp
Your thermometer stuck out of a car window will be stagnating a small proportion of the air flowing onto it, so will show a slightly higher temperature than if you measured it while stationary (at 70 mph it will be about 0.5 degrees rise).