Psychrometric Constant
Psychrometric constant, abbreviated as \(\gamma\), is an important factor in atmospheric science and meteorology, particularly in the study of evaporation and transpiration processes. It relates the rate of change of water vapor pressure with temperature and is used in equations to estimate evapotranspiration (the process where water is transferred from land to the atmosphere by evaporation and plant transpiration). The psychrometric constant helps quantify the relationship between air temperature and humidity in processes like evaporation and is crucial in climatological and agricultural studies.
Key Concepts
Definition - The psychrometric constant is the ratio of the specific heat of moist air at constant pressure to the latent heat of vaporization of water, multiplied by the atmospheric pressure. It reflects how the temperature of air changes as it gains or loses moisture.
Units - The psychrometric constant is typically expressed in units of \(kPa\;/\;C\).
Importance in Evapotranspiration - It is a key component in the Penman-Monteith equation, which is widely used to estimate reference evapotranspiration (\(ET_0\)). The constant helps account for the influence of temperature on the evaporation process in response to atmospheric conditions.
Dependence on Atmospheric Pressure - Since it depends on atmospheric pressure, the psychrometric constant will vary slightly depending on the elevation (altitude), as pressure changes with height above sea level.
Psychrometric Constant Formula |
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| \( \gamma \;=\; c_p \; p_a \;/\; \epsilon \; \lambda_v \;=\; 0.000665\;p_a \) | ||
| Symbol | English | Metric |
| \( \gamma \) (Greek symbol gamma) = Psychrometric Constant | - | \(kPa\;/\;C\) |
| \( c_p \) = Specific Heat Capacity at Constant Pressure | - | \(kJ\;/\;K \) |
| \( p_a \) = Atmospheric Pressure | - | \(kPa\) |
| \( \epsilon \) (Greek symbol epsilon) = Ratio Molecular Weight of Water Vapor / Dry Air (0.622) | - | \(dimensionless\) |
| \( \lambda_v \) (Greek symbol lambda) = Latent Heat of Vaporization | - | \(kJ\;/\;kg\) |
Tags: Hydrology