Get a $20 IR thermometer, take readings from multiple spots on the canopy, and then get the average. VPD uses the concept of the "offset" between the ambient temp and the LST ("leaf surface temperature"). If the LST is higher than ambient, the offset is a positive value, if lower, it's a negative value.@Marshydro I see you have leaf temperatures in your chart, I haven't seen anyone else reference this, what are you using to measure this?