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In a nutshell, VPD is a calculation of how much moisture the air can hold at different temps. If your moisture goes higher, condensation occurs(dew) and if you go under the plants pull more moisture from the roots, aka require more water. Running a little dry is better than too moist outside of germination/cloning IMO. I personally only worry when I get too wet....if you feed heavy running dry can lead to nutrient issues though.