VPD — Vapour Pressure Deficit. It sounds like something a physicist invented to torture growers. It's not. It's just how thirsty the air is.

VPD is the difference between how much water the air could hold at your temperature and how much it actually holds. When the gap is small (humid air), your plant's transpiration stalls — she can't move water, nutrient uptake slows, and mould starts eyeing up your densest bud. When the gap is wide (dry air), she loses water faster than the roots can replace it, closes her stomata, and growth stalls the other way.

Get it in the band for your stage and she transpires properly: water and nutrients move, leaves work, nothing rots. The leaf-temp offset is there because leaves run a degree or two cooler than the air — evaporation cools them, same as it cools you.

One number, checked daily, prevents the mistake that takes the fattest cola two weeks before harvest. Ask anyone who's found the grey fuzz.