A computing device determines that a first time for a first sound generated by a vehicle to reach a first sound maximum amplitude and a second time for a second sound generated by the vehicle to reach a second sound maximum amplitude are within a range. An object that reflects the second sound is identified based on an interval between receipt of the first and second sounds. A speed of sound is determined based at least in part on the time interval and a distance to the object. A humidity is determined based at least in part on the speed of sound. A vehicle subsystem is adjusted based on the humidity.