Psychrometric constant

Psychrometric constant is a physical constant used in calculations of evapotranspiration. It is derived from the ideal gas law, and its value is approximately 0.0665 kPa/°C. The psychrometric constant is used in the Penman equation and the Penman-Monteith equation, which are both used to estimate evapotranspiration.

Definition

The psychrometric constant (γ) is defined as the ratio of the heat capacity at constant pressure (Cp) of the air to the latent heat of vaporization (λ) of water, times the ratio of the atmospheric pressure (P) to the gas constant for water vapor (Rw). This can be expressed mathematically as:

γ = (Cp * P) / (λ * Rw)

The units of the psychrometric constant are kilopascals per degree Celsius (kPa/°C).

Calculation

The value of the psychrometric constant depends on the atmospheric pressure and the temperature. It can be calculated using the following formula:

γ = 0.000646 * P * (1 + 0.000422 * T)

where:

• P is the atmospheric pressure in kilopascals (kPa),
• T is the temperature in degrees Celsius (°C).

Applications

The psychrometric constant is used in the calculation of evapotranspiration, which is the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere. It is used in the Penman equation and the Penman-Monteith equation, which are both used to estimate evapotranspiration.

See also

• Evapotranspiration
• Penman equation
• Penman-Monteith equation
• Ideal gas law