Terms and definitions 
(Water) vapor pressure is the pressure
exerted by a (water) vapor that is in equilibrium with its nonvapor
phases. When the water vapor is in a mixture with a carrier gas
(select it from the dropdown list in parameters settings), its vapor pressure can also be
regarded as a partial pressure, having its stable maximum value at an
equilibrium vapor pressure. The equilibrium vapor pressure
(also referred to as a saturation vapor pressure),
increases exponentially with the gas temperature according to relation,
originally described by ClausiusClapeyron equation. The humidity
calculator / moisture calculator uses more accurate equations, which can be
selected from the dropdown list in parameters settings.

In the presence of a carrier gas, the
equilibrium vapor pressure for pure water is slightly increased by
an enhancement factor, which depends upon the type of the carrier
gas, the overall absolute gas pressure and the dewpoint
temperature. If checked, the humidity calculator / moisture
calculator takes the enhancement factor into account for
dry air and nitrogen. For other selected gases, the
enhancement factor is disregarded. For gases other than those listed
in the parameters settings, please provide also the molar
mass. 

Relative humidity is the ratio of the
actual vapor pressure (equilibrium vapor pressure at dew
point or frost point) to the equilibrium (saturation) vapor pressure at the gas
temperature, expressed as a percentage. At gas temperatures below
freezing, where possibility of supercooling exists, the humidity
calculator calculates relative
humidity in either standard (%rh) or WMO
(World Meteorological Organization) way. Since the
supercooling phenomenon does occur in clouds and the upper
atmosphere, the WMO has decided that the relative humidity be
calculated by computing the maximum possible water content over
water rather than ice regardless of the temperature.

Dew point is the temperature at which dew,
or condensation, forms, on cooling a gas. It is also the
temperature, at which water vapor is in equilibrium with liquid
water. The humidity calculator calculates the dew point from the
inverse water vapor pressure formulae, selected by user.

Frost point is the temperature at which
frost forms, on cooling a gas. It is also the temperature, at which
water vapor is in equilibrium with ice. The humidity calculator
calculates the frost point from the inverse water vapor pressure
formulae, selected by user.

Parts per million by volume (PPMv) is the
ratio of the water vapor volume to the volume of the dry carrier
gas, multiplied by 10^{6}.

Parts per million by weight (PPMw) is the
ratio of the water vapor mass to the mass of the dry carrier gas,
multiplied by 10^{6}.

Wetbulb temperature is the temperature
indicated by a thermometer sheathed in a wet wicking, and influenced
by a rate of evaporation from the wicking. The humidity calculator calculator
finds the water vapor pressure
e in the gas from the wetbulb temperature t_{w
}, the gas temperature t, the gas pressure P and
equilibrium vapor pressure e' at t_{w}
using the following equation: 
e = e'  A.P.(t  t_{w}).(1+B.t_{w})
, 
where A and B are the psychrometric
constants, which can be set by user in the input boxes below. 
A:
°C^{1} B:
°C^{1
} 
Absolute humidity is the mass of the water
vapor present in unit volume of moist gas of a given temperature and
pressure.

Specific humidity is the mass of water
vapor per unit mass of moist gas.

Mixing (humidity) ratio is the mass of
water vapor per unit mass of dry carrier gas with which it is
associated.

Moist gas density is the mass of the moist
gas in unit volume of moist gas of a given temperature and pressure.

Enthalpy is a measure of the total energy
in a humid gas. Enthalpy of a gas can be defined as the sum of
sensible and latent heat for each component in the gas.
Values of enthalpy are conventionally expressed relative to a datum
point (for a dry gas, this is normally the heat content at 0 °C).
The humidity calculator calculates the enthalpy for different carrier gasses and gas
temperatures, based on the following constants (if
left in auto mode, the calculator will use the mean value of
c_{p} with respect to the gas temperature,
except for SF_{6} , for which a manual input value shall be used):

Vapor pressure deficit it is the difference between the saturation vapor pressure and the actual vapour pressure at given temperature.

References
 Hardy R., ITS90 Formulations for Vapor
Pressure, Frost point Temperature, Dew point Temperature, and
Enhancement Factors in the range –100 to +100 C, Papers and
Abstracts of the Third International Symposium on Humidity and
Moisture, Vol. 1, pp. 214222, April 1998
 Sonntag D., Advancements in the field of
hygrometry. (Review Article) Meteorologische Zeitschrift,
N.F., 3, pp. 5166, 1994
 Wexler A., Vapor Pressure Formulation for
Water in the Range 0 to 100°C. A Revision, Journal of
Research of the National Bureau of Standards, Physics and
Chemistry, Vol. 80A, Nos. 5 and 6, pp.
775785, 1976
 Wexler A., Vapor Pressure Formulation for
Ice, Journal of Research of the National Bureau of Standards
–
A. Physics and Chemistry, Vol. 81A, No.
1, pp. 519, 1977
 Buck Research Manual
(1996); updated equation from Buck A. L., New equations for
computing vapor pressure and enhancement factor, J. Appl.
Meteorol., 20, pp. 15271532, 1981
 Greenspan L., Functional Equations for the
Enhancement Factors of CO2Free Moist Air, Journal of
Research of the National Bureau of Standards, Physics and
Chemistry, Vol. 30A, No. 1, pp. 4144, 1976
 A Guide to the Measurement of Humidity,
ISBN 0904457249, The Institute of Measurement and Control,
London, 1996
 tables for specific heat capacities at constant
pressure  http://www.engineeringtoolbox.com/

The information contained in the humidity calculator
/ moisture calculator is subject to change without notice.
The author of the humidity calculator / moisture
calculator
makes no warranties, either express or implied, regarding the
calculations, or the fitness of these procedures or program for a
particular purpose. The humidity calculator / moisture calculator is made available solely on an
"as is" basis, and the entire risk as to its quality and performance
rests with the user. The author shall not be liable for any
incidental or consequential damages in connection with or arising
out of the furnishing, use, or performance of the humidity calculator
/ moisture calculator.

