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Global distribution of Vapour-pressure deficit averaged over the years 1981-2010 from the CHELSA-BIOCLIM+ data set [1] Vapour pressure-deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated, water will condense to form clouds ...
This is illustrated in the vapor pressure chart (see right) that shows graphs of the vapor pressures versus temperatures for a variety of liquids. [7] At the normal boiling point of a liquid, the vapor pressure is equal to the standard atmospheric pressure defined as 1 atmosphere, [1] 760 Torr, 101.325 kPa, or 14.69595 psi.
Vapour-pressure deficit, a measure of the difference between air humidity and saturation; Computing. Virtual private database, masks data in a larger database;
δe = vapor pressure deficit (Pa) g a = Conductivity of air, atmospheric conductance (m s −1) g s = Conductivity of stoma, surface or stomatal conductance (m s −1) γ = Psychrometric constant (γ ≈ 66 Pa K −1) Note: Often, resistances are used rather than conductivities.
The Penman equation describes evaporation (E) from an open water surface, and was developed by Howard Penman in 1948. Penman's equation requires daily mean temperature, wind speed, air pressure, and solar radiation to predict E. Simpler Hydrometeorological equations continue to be used where obtaining such data is impractical, to give comparable results within specific contexts, e.g. humid vs ...
δe = vapor pressure deficit (kPa) γ = Psychrometric constant (γ ≈ 66 Pa K −1) N.B.: The coefficients 0.408 and 900 are not unitless but account for the conversion from energy values to equivalent water depths: radiation [mm day −1] = 0.408 radiation [MJ m −2 day −1].
where temperature T is in degrees Celsius (°C) and saturation vapor pressure P is in kilopascals (kPa). According to Monteith and Unsworth, "Values of saturation vapour pressure from Tetens' formula are within 1 Pa of exact values up to 35 °C." Murray (1967) provides Tetens' equation for temperatures below 0 °C: [3]
A typical vapor phase osmometer consists of: (1) two thermistors, one with a polymer-solvent solution droplet adhered to it and another with a pure solvent droplet adhered to it; (2) a thermostatted chamber with an interior saturated with solvent vapor; (3) a liquid solvent vessel in the chamber; and (4) an electric circuit to measure the bridge output imbalance difference between the two ...