A measure of the difference between the moisture content of the air and the maximum moisture it can hold at a given temperature; a key metric for optimizing transpiration and clone health.
Dakota uses an AC Infinity VPD gun to check conditions inside propagation domes. He found VPD inside some domes reached 1.5 due to overheating from new heat pads and the uninsulated roof, which killed a batch of clones. This prompted installation of the AC Infinity Terraform 7 and CloudForge T7 humidifier.
Discussed in: Ep 004