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Precipitable water or ‘PWAT’ in short, is the depth of water in a column of the atmosphere if all the water in that column were precipitated as rain. PWAT, expressed in mm or kg/m2, should not be confused with precipitation. Whilst PWAT represents how much precipitation would occur in a column of air (all the way up from ground level) if it were completely squeezed out, precipitation refers to the actual water content that falls on the Earth’s surface. Values of precipitable water are generally high near the Equator.
The weather parameters that have the greatest influence on the performance of a PV system are irradiation and temperature. All widely-used PV simulation software make use of mathematical models that account for the impact on the performance of modules when irradiation and temperature vary from STC conditions (1000 W/m2 and 25°C). Less known is the fact that PV module performance also depends on spectral irradiation distribution.
Pyranometer measurements and solar radiation databases such as Solargis provide broadband radiation. However, the data do not specify the spectral distribution. Because of the high cost of spectroradiometers, data on spectral distribution is not easily available. In the absence of measured spectral distribution data, models have been proposed to characterize spectral shift based on available atmospheric data. Models that consider precipitable water have shown relatively good performance. Such models are being increasingly incorporated into PV energy modeling software available on the market.
In response to a growing number of request for PWAT data from our customer base, we have now included precipitable water as a default data parameter in our data product ‘Time Series – extended’. PWAT data is also included in the TMY files generated from ‘Time Series – extended’.
The source of the data is the NOAA NCEP Climate Forecast System: CFSR (past to 2010) and CFSv2 (2011 to present). The CFS model incorporates observations from satellites fitted with microwave sensors, that can detect precipitable water. PWAT data are available in hourly time step from Solargis.
We hope that the inclusion of precipitable water to Solargis datasets will help the solar industry improve the accuracy of PV energy predictions.
