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Please explain VPD

It can be confusing, as some explanations are not always so easy. But relative humidity measures capacity percentages. Which is the difference in the amount of water in the air (absolute humidity) compared to the how much water that can be held in the air at that specific temperature. If the temperature changes, so does the percentage, as temperature effects the maximum amount of water that can be held. For example if air was a cup, the cup size would change relative to the temperature. Low temperature would reduce the cup size, high temperature would increase the cup size. But the absolute water in the cup would never change. Though the percentage would, as the cup size changes, effecting the percentage of water in the cup relative to the cup size.

VPD is not about this capacity percentage, its about how the water in the cup effects water potential. Its a measurement of pressure difference not a capacity difference. So it measures the difference in pressure of the absolute humidity compared to the maximum water holding capacity at that temperature. Instead of measuring capacity differences.

The calculations for VPD are formed by relative humidity and temperature. Where as relative humidity is calculated by absolute humidity and temperature.

Capacity vs pressure.
 
Another way to call vpd is transpiration rate. Low vpd=low transpiration and high vpd=high transpiration. It dictates how much water is loss trough the pores in the leaves to keep the right temp inside the leaves. Not a very scientific way to say it but easier to understand.
 
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