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 the air can hold before the water condenses back into a liquid, decreases. In other words, hotter air can hold more water vapour per volume than cold air can. To reiterate in another way, it is possible to lower humidity by warming up the air. All due to the relationship between temperature and humidity.

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 of the cultivar's capacity for Respiration and transpiration.

Ventilation

In Indoor grows, ventilation is the most effective way of influencing the temp and humidity of the environment. As the plant expels water vapour via transpiration the air becomes more humid and this excess water vapour must be removed from the growing 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 stage of the culivar or it's genetics.