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Temperature and humidity areshare a pairedsingle togetherarticle as they are so closely linked. Generally, humidity in a growing context refers to [https://en.wikipedia.org/wiki/Humidity Relative Humidity] (RH). As temperature decreases, the amount of water vapour itthe air can hold before itthe water condenses back into a liquid, decreases. In other words, hotter air can hold more water vapour per volume than cold air can<ref>https://www.royalqueenseeds.com/blog-indoor-cannabis-growing-relative-humidity-and-temperatures-n243</ref>. TheTo Temperaturereiterate andin humidityanother canway, alsoit beis usedpossible to calculatelower thehumidity Vapourby pressurewarming deficitup (VPD)the whichair. providesAll adue measure of how opento the cultivar'srelationship [[Anatomy_of_Cannabis#Stomata|stomata]]between aretemperature and it's capacity for [[Plant_processes#gas_exchange|gas exchange]]humidity.
 
This article by [https://www.royalqueenseeds.com/blog-indoor-cannabis-growing-relative-humidity-and-temperatures-n243 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.
 
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 [[Plant processes#Respiration|Respiration]] and [[Plant processes#Transpiration|transpiration]].
 
==Ventilation ==
VentilationIn isIndoor agrows, veryventilation importantis partthe ofmost havingeffective controlway overof influencing the temp and humidity of the environment. As the plant expels water vapour via [[Plant processes#Transpiration|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<ref>Centrifugal Fan vs Axial Fan | Industrial Accessories Company - https://www.youtube.com/watch?v=gYxLbBP3evM </ref>. To control the smell emitted from the plant, [[Carbon filter|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 [[Plant processes#Transpiration|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%. <ref>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</ref>
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 of ideal VPD for each [[Life Stages|life stage]] can be found at [https://www.dimluxlighting.com/knowledge/vapor-pressure-deficit-vpd-calculator/ dimluxlighting.com]
 
[https://www.cannabissciencetech.com/view/circular-cannabis-systems-tracking-and-minimizing-waste-and-emissions-impacts More on VPD]<ref>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</ref>
 
==== Calculating VPD ====
VPD can be calculated by using the [[wikipedia:Tetens_equation|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 * <big>''e''</big><sup>17.27 * T / (T + 237.3)</sup> 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 * <big>''e''</big><sup>17.27 * T / (T + 237.3)</sup> Where T is air temperature in celsius and RH is relative humidity.
 
'''VPD''' = '''vapor pressure in the leaf - vapor pressure of air'''
 
==Ideal conditions==
The ideal temperature is range is 24°- 28°C<ref>Chandra et al. (2008) - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550641/pdf/12298_2008_Article_27.pdf</ref> while the ideal humidity is 40-60%. Ideal VPD is around 1 kPa <ref>Nadia Sabeh​​, PhD - Vapor Pressure Deficit (VPD) and Cannabis Cultivation -
 
Future Cannabis Project - https://www.youtube.com/watch?v=a6GUPfepLIc&ab_channel=FutureCannabisProject</ref>. Of course, the exact ideal values differ depending on the current [[Life Stages|life stage]] of the culivar or it's [[genetics]].
 
== References ==
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