# 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 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.