Vapour-pressure deficit the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa. In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation. To compute the VPD, we need the ambient (greenhouse) air
Air well (condenser) It is distinct from fog, in that fog is made of droplets of water that condense around particles in the air. Condensation releases latent heat which must be dissipated in order for water collection to continue. An air well requires moisture from the air. Everywhere on Earth, even in deserts, the surrounding atmosphere contains at least some water. According to Beysens and Milimouk: "The atmosphere contains of fresh water, composed of 98 percent water vapour and 2 percent condensed water (clouds): a figure comparable to the renewable liquid water resources of inhabited lands ." The quantity of water vapour contained