pvlib.atmosphere.gueymard94_pw

pvlib.atmosphere.gueymard94_pw(temp_air, relative_humidity)

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 [2]:

\[Pw = 0.1 H_v \rho_v\]

using Eq. 2 in [2]

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

\(Pw\) is the precipitable water (cm), \(H_v\) is the apparent water vapor scale height (km) and \(\rho_v\) is the surface water vapor density (g/m^3). . The expression for \(H_v\) is Eq. 4 in [2]:

\[H_v = 0.4976 + 1.5265 \frac{T}{273.15} + \exp \left(13.6897 \frac{T}{273.15} - 14.9188 \left( \frac{T}{273.15} \right)^3 \right)\]

In the expression for \(\rho_v\), \(e_s\) is the saturation water vapor pressure (millibar). The expression for \(e_s\) is Eq. 1 in [3]

\[e_s = \exp \left(22.330 - 49.140 \frac{100}{T} - 10.922 \left(\frac{100}{T}\right)^2 - 0.39015 \frac{T}{100} \right)\]

Parameters:

• temp_air (numeric) – ambient air temperature \(T\) at the surface. [C]
• relative_humidity (numeric) – relative humidity \(R_H\) at the surface. [%]

Returns:

pw (numeric) – precipitable water. [cm]

References

[1]	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)

[2]	(1, 2, 3) 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.

[3]	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.