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Understanding humidity and temperature

Updated: Mar 13, 2020

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Want to maximize your knowledge on humidity and temperature? Here are six topics that are sure to help.


In order to maintain the A1 environment for plants to grow in a controlled setting with artificial lighting, it is essential for you to understand the nature of the environmental influences and how to measure and evaluate them. This blog describes the physical and chemical resources of the following environmental components and their calculations: humidity, temperature, CO2 concentration, air flow rates, and number of air exchanges per hour. In addition, the basic concepts of energy balance, radiation, and heat conduction and convection are outline in detail.

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Temperature, Energy, and Heat

Temperature is an indicator of the realistic heat energy content of an object or a substance. Many plant physiological processes are affected by plant temperature, which is controlled by the transfer of heat between plant tissues and the surrounding environment. That being so, monitoring and controlling the air temperature is critical for managing plant physiological activity and response. In a indoor environment, air temperature is often controlled at a comparatively constant level, resulting in constant plant temperature and, as a result, consistent physiological activity.

Energy Balance

Any object with a temperature above 0 K (absolute zero) emits thermal radiation, including the plants themselves and their environment. Energy received by plants includes absorbed radiant energy from lights and the absorbed infrared irradiation from the environment. Energy leaving microgreens includes energy lost through emitting infrared radiation, heat convection, heat condition and heat loss thru evaporation. The heat by conduction and convection from leaves is referred to as sensible heat, and that connected with the evaporation or condensation of water as latent heat. Microgreens leaves have high absorption in the photosynthesically activity radiation (400 to 700 nm), but the chemical energy fixed by photosynthesis is inconsequential small compared to the total energy of the plant. Leaves of nearly all species have a low absorption in the close by infrared scale (700 to 1500 nm) because those wavelengths are transferred through or reflected from the leaf. In difference, absorption is high (roughly 95%) in the far infrared waveband (1500 to 30,000 nm), that can contribute notably to the thermal energy load on the plant.


Radiation in the far infrared wavebands is essentially blackbody radiation discharged by environment objects. Objects of higher temperature discharge larger quantities of far infrared radiation than objects at a lower temperature. The main source of radiation energy in indoor environments are lights and reflectors. Conventional lights for indoor grow rooms and greenhouses, such as high pressure sodium lights and metal halide lights, have exterior temperatures of over 212ºF and emit large amounts of far infrared radiation. This radiation is absorbed by plants, causing increased plant temperature regardless of environment air temperature, through hindering control over plant physiological activity. In indoor environment, this challenge is compounded by the small interval between lights and plants that is advantageous for maximizing space use efficiency and plant productivity. So, it is preferable to use light sources that emit much less far infrared radiation, such as LEDs (30ºC/86ºF) and fluorescent lights (40ºC/104º).

Heat Conduction and Convection

Energy is manage between a plant and its environment at the molecular level. Energy is transferred by conduction from the leaf cells to the air molecules in contact with the leaf. Conductive heat moves the interface between leaf and air is restricted without convective motion due to the low thermal conductivity of air. Conductive heat interchange can also happen between plant parts and other solid or liquid media. However, the impact of this conductive heat interchange on the plant's energy blueprint is small, because plants do not have physical contact with solid objects or liquid media. Controlling leaf and air temperatures evenly at every growing level is important in indoor grow rooms. If air circulation in a grow room is inadequate, air temperatures at the higher growing levels will be warmer than lower levels, causing the leaves in the higher canopy to also be warmer. by providing air movement in the whole grow room, the vertical air and leaf temperature inclines can be minimized, as well as differences within each horizontal canopy.


Water vapor is the gases state of water and humidity is a measure of its content in the air. The amount of climatic water vapor can range from nearly zero up to 4% of the total mass of air. Absolute humidity, or humidity ratio, is a measure of the real water vapor content in the air and is communicated as the ratio of mass of water vapor to the mass of dry air for a defined volume of air. The air can hold on to more water vapor at higher temperatures than at lower temperatures. Relative humidity is temperature dependent and used to communicate the water vapor content of air found on the maximum amount of water the air can hold for a given temperature and pressure. It is almost all expressed as a percentage or ratio of the given water vapor content to the maximum at a given temperature. As a blueprint, if the air temperature become less with no change in water vapor content, the maximum water holding volume of the air drops, resulting in a higher relative humidity. Water vapor is produced by evaporation from open water surfaces and evaporation from wet surfaces such as soil and plants. In a indoor environment, plants are constantly adding water vapor to the air through transpiration, which is the evaporation of water from plant surfaces to the environment. Well, actively growing plants can transpire a large amount of water, resulting in a rapid increase in the water vapor content and humidity in a semi closed indoor environment. When the air conditioning system is operating, humidity is kept under control because water vapor condenses on the cooling coils, dropping the moisture content, and thus humidity, of the air. For that reason, one approach to controlling humidity in a indoor environment is to alternate the functioning of the lights to generate heat and cause the air conditioner to run, resulting in concurrent cooling and dehumidification of the grow room. Dehumidifiers can be installed in the indoor environment that do not rely on the operation of air conditioners. These units may be used in indoor environment applications that require day/night cycles, when turning on the lights for dehumidification would be undesirable. They can also be used to avoid operating lights and air conditioners during peak hours use periods, lower energy cost.

Vapor Pressure Deficit (VPD)

Relative humidity is commonly used as a measure of air humidity, it supplies no direct information about the driving force of transpiration and evaporation. Instead, the vapor pressure deficit (VPD) is a measure of the driving force, meaning that transpiration and evaporation rates are proportional to VPD. VPD is the difference (deficit) between the amount of moisture in the air and how much moisture it can hold when it is saturated at the same air temperature and is expressed in units of pressure. While water vapor content increases, water molecules apply more force on each other, resulting in a higher vapor pressure. Because air can hold more water vapor at higher temperatures, the maximum water vapor pressure is higher at higher temperatures. When the VPD is too low, transpiration will be reserved and can lead condensation on leaves and surfaces inside the indoor environment. Also, when the VPD is high, the plant will draw more water from its roosting an effort to avoid wilting. If the VPD gets too high, plants close stomata and shut down the transpiration altogether in an effort to prevent excessive water loss. In indoor environment, the idea range for VPD is from 0.8 kPa to 0.95 kPa, with an optimal setting of around 0.85 kPa.

CO2 Concentration

CO2 is a naturing occurring chemical compound. It is a linear covalent molecule and is an acidic oxide, and reacts with water to give carbonic acid. CO2 is a nonflammable, colorless, odorless gas at standard temperature and pressure and exists in earth's atmosphere at this state as a trace gas. Atmospheric CO2 concentration varies with time of day and location depending on adsorption and respiration of plants and animals, and human activity. CO2 is produced from the combustion of coal or hydrocarbons, the fermentation of liquids, and the respiration of humans, animals, and fungi.

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Air Current Speed

Air current speed is defined as distance air travels over a specified period of time, such as one meter per second. Air velocity is the term used when the direction of air current speed is specified. Inadequate air current speed around plants suppresses gas diffusion in the leaf boundary layer, which later on reduces rates photosynthesis and transpiration and hence plant growth. Maintaining suitable air speeds indoor environment creates small turbulent eddies around the leaf surface that facilitates gas exchange between the plants and the surrounding environment, promoting plant growth. Low air speeds can cause variations in air temperature, CO2 concentration, and humidity inside the plant canopy, resulting in inconsistent growth on leaves and other surfaces in the grow room, helping to prevent unwanted growth of bacteria and molds. Fans can be used to circulate air movement and control air speed within the plant canopy in the grow room. To achieve exact air speed control, special calculation and design master plans regarding the location, number, and capacity of fans are required when a indoor grow room is built.

Number of Air Exchange Per Hour

Number of air exchange per hour is a measure of how many times the air within a defined space is replaced by new air, which is defined as the ratio of hourly ventilation rate divided by volume of room air. If possible the number should be small for the purpose of controlling the environment and preventing entry of pathogens and pest. But, a minimum air exchange rate should be maintained to prevent the accumulation of ethylene in a indoor grow room, which can cause damage to the plants.

What am I missing here? Let me know in the comments and I'll add it in.

Next week I'll post about lighting.....

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