An air-conditioning device and an air conditioning method make it possible to reduce the problem that, on entry in a tunnel with a warm internal temperature, in the case of otherwise cold outside temperatures as a result of which, inter alia, the vehicle windowpanes and exterior mirror surfaces are cooled, this cold windowpane and exterior mirror surfaces, the surface temperature of which is lower than the dew-point temperature of the warm air in the tunnel interior, immediately fog up, since the absolute humidity on the surface then corresponds to more than 100% relative humidity. The air-conditioning device according to the invention and the air-conditioning method according to the invention use data supplied by already existing sensors and relating to the ambient weather, for example temperature data from an outside temperature sensor and moisture data from a dew-point sensor, in order to detect weather which is critical to this fogging. When critical weather is detected and a tunnel entry is detected, the vehicle windowpanes and exterior mirror surfaces are then fully automatically heated and/or the windshield wipers are actuated for a predetermined period of time.