Temperature and Humidity

Temperature and humidity share a single article 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. In fact, this is the principle that freeze drying relies on. Temperature and humidity can also be used to calculate the Vapour pressure deficit (VPD) which provides an indication to the cultivar's capacity for gas exchange.

Ventilation

In Indoor grows, ventilation is a very important part of having control over the temp and humidity. As the plant expels water vapour via transpiration the air becomes more humid and this eccess water vapour must be removed from the grow environment. In other words, a fan to remove humid air is crucial. Blower (centrifugal fans) should be used for this type of ventilation. To control the smell emitted from the plant, carbon filters are often implemented as part of the exhaust of the ventilation.

Vapour pressure deficit (VPD)

VPD is a metric to measure the difference (deficit) 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 of ideal VPD for each life stage can be found at dimluxlighting.com

More on VPD

Ideal conditions

The ideal temperature is range is 24°- 28°C while the ideal humidity is 40-60%. Of course, the exact ideal values differ depending on the current life stages or the plant's genetics.