Water vapour lidars need calibration to accurately measure trends in the atmosphere. A new method tracks air parcels measured by radiosondes to improve calibration accuracy. This method shows better results when the water vapour field is not uniform. The uncertainty in calibration due to radiosonde measurements is around 4%, while the uncertainty from the lidar's counting system is about 0.3%. This new trajectory technique allows for more precise calibration of lidars, even when radiosondes are not in the same location, and can use more nights for calibration.