![]() ![]() ![]() Where T is temperature in ☌ and saturation vapor pressure is in kilopascals. Saturation vapor pressure = 0.61078 * exp The Tetens equation for saturation vapor pressure uses the exponent function: The Tetens equation is accurate to within 0.1% over the range of 0 to 50 ☌. Our VPD calculator uses the equation set out by Tetens in 1930 to calculate vapor pressure of liquid water, which is equal to the vapor pressure of air at saturation. There are various equations that we can use to calculate vapor pressure. Leaf temperature is normally a few degrees cooler than air temperature due to evaporative cooling. You can get an estimate of leaf temperature by measuring canopy temperature with an infrared sensor, or an air temperature sensor (aspirated is best) located near the plant canopy. Leaf temperature (or canopy temperature).For the standard calculation, you will need: Where we commonly represent vapor pressure using kilopascals, millibars, or pounds per square inch. Vapor pressure deficit = vapor pressure of leaf - vapor pressure of air Vapor pressure deficit can be calculated as: This means that vapor pressure deficit can be reliably used as a gauge for plant transpiration at different temperatures. In contrast, vapor pressure is an absolute measure of the water vapor in the air and does not change with temperature. Air with 60% relative humidity at 35 ☌ would feel much drier than air with the same relative humidity but at a colder temperature (such as 18 ☌). ![]() As a consequence, relative humidity is a poor indicator of the "drying power" of air. Warm air can "hold" more water vapor than cold air, so the absolute difference of vapor pressure between the relative humidities of 60% and 100% will change depending on temperature. Relative humidity tells you the amount of moisture in the air, expressed as a percentage of how much moisture the air can hold. The answer is that relative humidity is what its name indicates: relative. This is why growers use VPD to maintain a healthy environment for plant growth.Ī common question when discussing vapor pressure deficit (VPD) is, why can't we just use relative humidity? One may reasonably argue that relative humidity is also a measure of moisture in the air, and it is much easier to measure than vapor pressure. While plants are able to regulate water loss to some extent through the opening and closing of the stomata in their leaves, the vapor pressure deficit still affects the rate of plant transpiration. High temperatures and a humid environment can be a dangerous mix for us, which we explain more thoroughly in the wet bulb calculator. If you've ever been in a dry climate, you may have realized that dry air causes us to lose our bodily moisture more quickly, whereas humid climates may cause us to feel sticky or sweaty due to slower evaporation rates. The vapor pressure deficit is a useful metric in horticulture because it roughly correlates to how quickly plants transpire (the loss of water through the leaf stomata). Vapor pressure deficit (VPD) is the difference between the amount of moisture in the air and the amount of moisture in the leaf of a crop, and we typically represent it in pressure units, such as kilopascals. ![]()
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