scientific glossaryRelative humidity · brief definitionOrigin: Yolia Secure Educational CatalogExact route: /library?path=glossary/relative_humidity COMPLETE THEMERelative humidity · brief definition Name and classification Psychrometric environmental variable, that is, related to the water vapor in the air. It is expressed as a percentage and is not a property of the plant but of the air that surrounds it. In Yolia it is interpreted together with the temperature and not as an isolated data. What is it It is the proportion of water vapor contained in the air compared to the maximum it could contain at that same temperature. That is why the same percentage represents different amounts of water depending on whether it is hot or cold, and hot air can carry much more vapor than cold air. Where and how it was studied It is measured with hygrometers and psychrometers, and today with capacitive electronic sensors. In plant physiology it is studied in controlled environment chambers and greenhouses, where it is related to leaf water loss, growth and health. The accuracy of the sensor and its location greatly influence the data. Why it matters for cultivation Determines how easily the plant loses water to the air. With very dry air the loss can exceed what the root replaces; With very humid air, transpiration is reduced, nutrient transport is limited and the risk of fungal diseases increases. Each extreme affects differently. Physics — how it works The saturation vapor pressure grows exponentially with temperature, and the relative humidity is the relationship between the real vapor pressure and that of saturation. As the air cools with constant vapor, the percentage rises until it reaches the dew point, where the water condenses. Mechanism of action The internal spaces of the leaf are almost saturated with vapor, and the difference with the outside air drives the outflow of water through the stomata. The drier the air, the greater the gradient and loss. That is why it conditions the flow, although the final control is exerted by the stomata. Where it occurs in the plant It acts in the layer of air next to the leaf surface, called the boundary layer, and in the stomata, where vapor is exchanged. It also affects the surface of leaves and flowers, where prolonged humidity favors microorganisms. Its effect is felt throughout the plant through transpiration. How it is regulated The plant does not control the humidity of the air, but it responds to it: it closes stomata with very dry air, adjusts the conductance and over time modifies waxes and trichomes. In closed spaces, management depends on the environment, since ventilation, irrigation and temperature change humidity quickly. What triggers/affects it It depends on the temperature, ventilation, evaporation of the substrate, the water that the plants themselves transpire, the outside climate and the time of day. In general, it is highest at night and early morning, when the air is coldest, and lowest at midday. In closed spaces the density of plants also increases it. What other processes does it interact with? It combines with temperature to give the vapor pressure deficit, which governs transpiration; It is linked to water stress, intense heat, stomata opening and foliar health. It also influences the root zone indirectly, because it determines how much water the aerial part demands. Relevant differences between species in the catalog Species have different preferences: those with thin and large leaves tend to suffer more from dry air, and those with waxy or thick leaves tolerate dry environments better. Some are more prone to mold in high humidity. Appropriate ranges change with species and stage, and should be consulted for each crop. How it is observed in practice It is observed with a hygrometer placed at the height of the plants, away from sources of heat and currents. In the plant, very dry air leaves dry tips and fallen leaves, and very humid air leaves condensation, stains and fungus. Recording variations throughout the day is more useful than a single reading. What if it's extreme? With very low humidity and heat the plant loses water faster than the root replenishes it, closes stomata and may show burnt edges and wilting. With very high and sustained humidity, perspiration decreases and the risk of fungus increases. The effects depend on the associated temperature. How Yolia Uses It Yolia combines relative humidity with temperature to better interpret the real context of the crop, and does not read it alone. Consider that extreme values, very high or very low, generate different types of stress, and that very low humidity leads to losing water faster than the root can replenish it. Common interpretation errors A mistake is to judge relative humidity without temperature: the same percentage in a cold environment and in a hot one implies very different demands. Another is to believe that more humidity is always better, or that the value of the weather station is equivalent to the microclimate next to the plant. Evidence and limits of current knowledge The physical relationship between temperature, vapor and transpiration is well established. It is less precise to define a single ideal range, because it depends on species, stage and management. Inexpensive sensors can have errors of several percentage points and drift over time, so it's a good idea to check them. Related topics and modules This issue is related to vapor pressure deficit, transpiration, water stress and intense heat in the field. Also with the temperature of the environment, foliar health and ventilation of the crop, which modify the humidity that surrounds the plants. Related topicsRelative humidity