pvlib.atmosphere.gueymard94_pw

pvlib.atmosphere.gueymard94_pw(temp_air, relative_humidity)[source]

Calculates precipitable water (cm) from ambient air temperature (C) and relatively humidity (%) using an empirical model. The accuracy of this method is approximately 20% for moderate PW (1-3 cm) and less accurate otherwise.

The model was developed by expanding Eq. 1 in [R19]:

\[w = 0.1 H_v \rho_v\]

using Eq. 2 in [R19]

\[\rho_v = 216.7 R_H e_s /T\]

\(H_v\) is the apparant water vapor scale height (km). The expression for \(H_v\) is Eq. 4 in [R19]:

\[H_v = 0.4976 + 1.5265*T/273.15 + \exp(13.6897*T/273.15 - 14.9188*(T/273.15)^3)\]

\(\rho_v\) is the surface water vapor density (g/m^3). In the expression \(\rho_v\), \(e_s\) is the saturation water vapor pressure (millibar). The expression for \(e_s\) is Eq. 1 in [R20]

\[e_s = \exp(22.330 - 49.140*(100/T) - 10.922*(100/T)^2 - 0.39015*T/100)\]
Parameters:

temp_air : numeric

ambient air temperature at the surface (C)

relative_humidity : numeric

relative humidity at the surface (%)

Returns:

pw : numeric

precipitable water (cm)

References

[R18]W. M. Keogh and A. W. Blakers, Accurate Measurement, Using Natural Sunlight, of Silicon Solar Cells, Prog. in Photovoltaics: Res. and Appl. 2004, vol 12, pp. 1-19 (DOI: 10.1002/pip.517)
[R19](1, 2, 3, 4) C. Gueymard, Analysis of Monthly Average Atmospheric Precipitable Water and Turbidity in Canada and Northern United States, Solar Energy vol 53(1), pp. 57-71, 1994.
[R20](1, 2) C. Gueymard, Assessment of the Accuracy and Computing Speed of simplified saturation vapor equations using a new reference dataset, J. of Applied Meteorology 1993, vol. 32(7), pp. 1294-1300.