Managing Vapor Pressure Deficit in Greenhouses
Relative Humidty
There are two ways to measure humidity: absolute humidity and relative humidity. The actual amount of water vapor in a given volume of air is the absolute humidity; it is expressed as weight per volume. Absolute humidity is rarely measured in greenhouses; instead, relative humidity is used. The most common descriptor of humidity, relative humidity (RH), is also the most practical measurement in greenhouses. Relative humidity can be defined as the ratio of the amount of moisture in a volume of air to the total amount of moisture that can be held at saturation (i.e., saturation point) at a given temperature and atmospheric pressure and is expressed as a percentage. Anything in excess of the saturation point will condense out as liquid water.
Dew Point Temperature
Saturation occurs when the air is cooled to the point that the saturation vapor pressure exceeds the ambient vapor pressure and condensation occurs—the temperature at which this occurs is called the dew point. The practical significance of the dew point is that it is directly related to the amount of water vapor in a volume of moist air. The dew point temperature has direct application to container greenhouse management because of its relationship to condensation, which occurs when humid air comes in contact with a cooler surface, such as the inside of the greenhouse covering or a leaf. Growers manage humidity in their greenhouses to minimize condensation, which can cause disease problems.
Role of Humidity in Plant Growth and Development
Humidity can be the most difficult environmental factor to control in greenhouses. Even the most sophisticated environmental control equipment cannot perfectly control the humidity level in greenhouses. Humidity levels fluctuate with changes in air temperature, and plants are constantly transpiring, which adds water vapor to the air. When humidity levels get too high, the plant’s ability to cool itself or transport nutrients and minerals can be limited.
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