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.
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 (unit kPa) used 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). It is a more accurate way of measuring the "dryness" of air that takes into account air temperature, leaf temperature and air humidity. VPD directly correlates to rate of transpiration. Cannabis should be exposed to between 0.5 and 1.6kPa. Fluctuations in VPD over 0.5kPa can decrease yields by up to 20%. 
An excellent VPD calculator and chart of ideal VPD for each life stage can be found at dimluxlighting.com
VPD can be calculated by using the Tentens Equation to find the vapor pressure in the leaf and vapor pressure in air, and then subtracting the vapor pressure of air from the vapor pressure of the leaf. e.g.
vapor pressure in the leaf = 0.61078 * e17.27 * T / (T + 237.3) Where T is the leaf temperature in Celsius. If leaf temp is unknown air temp can be used.
vapor pressure of air = RH * 0.61078 * e17.27 * T / (T + 237.3) Where T is air temperature in celsius and RH is relative humidity.
VPD = vapor pressure in the leaf - vapor pressure of air
The ideal temperature is range is 24°- 28°C while the ideal humidity is 40-60%. Ideal VPD is around 1 kPa .
Of course, the exact ideal values differ depending on the current life stage of the culivar or it's genetics.
- ↑ https://www.royalqueenseeds.com/blog-indoor-cannabis-growing-relative-humidity-and-temperatures-n243
- ↑ Centrifugal Fan vs Axial Fan | Industrial Accessories Company - https://www.youtube.com/watch?v=gYxLbBP3evM
- ↑ Minimizing VPD Fluctuations Maintains Higher Stomatal Conductance and Photosynthesis, Resulting in Improvement of Plant Growth in Lettuce, 2021 Inoue - https://doi.org/10.3389/fpls.2021.646144
- ↑ L Breit, M Leavitt, A Boyd, Cannabis Science and Technology 2(2), 52-61 (2019). https://www.cannabissciencetech.com/view/circular-cannabis-systems-tracking-and-minimizing-waste-and-emissions-impacts
- ↑ Chandra et al. (2008) - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550641/pdf/12298_2008_Article_27.pdf
- ↑ Nadia Sabeh, PhD - Vapor Pressure Deficit (VPD) and Cannabis Cultivation - Future Cannabis Project - https://www.youtube.com/watch?v=a6GUPfepLIc&ab_channel=FutureCannabisProject