Temperature and Humidity

Temperature and humidity are paired together as they are so closely linked. Generally, humidity in a growing context refers to Relative Humidity (RH). As temperature decreases the amount of water vapour it can hold before it condenses back into a liquid decreases. In other words, hotter air can hold more water vapour per volume than cold air can. The Temperature and humidity can also be used to calculate the Vapour pressure deficit (VPD) which provides a measure of how open the cultivar's stomata are and it's capacity for gas exchange

This article by Royal Queen Seeds covers the physical relationship between temperature and humidity and the values that are best for the plant at each stage of growth.

Ventilation

Ventilation is a very important part of having control over the temp and humidity. In other words, a fan is crucial. Blower (centrifugal fans) should be used for this type of ventilation

Vapour pressure deficit (VPD)

The VPD is the difference between how much water there is in the air (AVP) and the maximum amount of water the air can currently hold (SVP). An excellent VPD calculator and chart can be found at dimluxlighting.com

Ideal conditions

The ideal temperature is range is 24°- 28°C while the ideal humidity is 40-60%