The psychrometric constant

\gamma

relates the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...

of water in air to the air temperature. This lets one

interpolate In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has ...

actual vapor pressure from paired dry and wet

thermometer A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer ...

bulb temperature readings. ::

\gamma =\frac

\gamma =

psychrometric constant Pa °C⁻¹ : P = atmospheric pressure Pa :

\lambda_v =

latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be underst ...

of water vaporization, 2.45 J kg⁻¹ :

c_p =

specific heat In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...

of air at constant pressure, J kg⁻¹ °C⁻¹ :

MW_ =

ratio

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

of water vapor/dry air = 0.622. Both

\lambda_v

and

MW_

are constants.
Since

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

, P, depends upon altitude, so does

\gamma

.
At higher

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

water evaporates and boils at lower temperature. Although

\left( c_p \right)_

is constant, varied air composition results in varied

\left( c_p \right)_

. Thus on average, at a given location or altitude, the psychrometric constant is approximately constant. Still, it is worth remembering that weather impacts both atmospheric pressure and composition.

Vapor Pressure Estimation

Saturated vapor pressure,

e_s = e \left T_\right /math>
Actual vapor pressure, e_a = e_s - \gamma * \left( T_ - T_ \right) : here e is vapor pressure as a function of temperature, T.
::T

_dew = the

dewpoint The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will co ...

temperature at which water condenses. ::T_wet = the temperature of a wet thermometer bulb from which water can evaporate to air. ::T_dry = the temperature of a dry thermometer bulb in air.

References

{{authority control Chemical properties Gas laws Chemical engineering thermodynamics Physical chemistry Gases Underwater diving physics