At face value, VPD (sucking energy) appears to be the same as relative humidity - because relative humidity is the ratio of the particular vapor pressure within the air to the SVP. First is the saturation vapor pressure (SVP): consider this as the maximum amount of water vapor a given block of air can hold. Remember the mechanics of vapor https://www.vaporsuper.com/ultroner-take-it-go-80w-box-mod pressure deficit, VPD. Think of VPD because the water sucking power of the air, https://www.vapefeatured.com/cookies-x-g-pen-connect-vaporizer as a result of it is actually the VPD that pursuits your plants, not the relative humidity.