All station information is drawn from the database of The Agromet Group of FAO.
All variables, except potential evapotranspiration, are direct observations or conversions of observations. Original data coming from a large number of meteorological stations as included in CLIMWAT, could not be uniform. For example, humidity and radiation can be expressed through different variables. With respect to humidity, data can be provided as relative humidity, dew point temperature or water vapour pressure. These three variables can be uniquely converted into each other if the mean temperature is known. However, if humidity is measured and provided in more than one of these variables, the actual numbers would not necessarily be in line. In this case it is necessary to decide which variable to use. When compiling CLIMWAT, it was decided to use water vapour pressure as a core variable and only where it is not available, use dew point temperature and relative humidity. However, there is a risk that the provided value of vapour pressure is higher than the one that is possible to obtain, given the mean temperature. The original databases were crosschecked for this possible inconsistency and one of the other variables was used in the few cases where it occurred.
The same problem arises with radiation. Instead of the solar energy flux at the surface often only sunshine hours or sunshine fraction are recorded, both of which though can be converted to radiation. In order to calculate evapotranspiration using the Penman-Monteith method, both radiation and sunshine fraction are necessary. To keep both these values in agreement the observed radiation was used as base variable and the sunshine fraction was estimated from it. When only the sunshine fraction (or hours) has been observed it was used to estimate radiation. If both (fraction and radiation) are observed radiation was preferred.
As a result, the provided relative humidity and sunshine hours are often deduced from observations of vapour pressure and radiation, even if the former are observed. The procedure, however, ensures that the different expressions are coherent.
In compiling the data, an effort was made to cover the period 1971 - 2000, but when data for this period were not available, any recent series that ends after 1975 and that has at least 15 years of data have been included. Some of the series are "broken", but they nevertheless have at least 15 years of data (e.g. 1961-70 and 1992-2000).
We prepared the dataset and the extraction software with great care and made every effort to provide reliable data. However, we cannot guarantee that all the observations that went into the procedure are free of errors.