Weather Data Note 4: Annual variability

The distribution of yearly Irradiation values for several years (at least 10-15 years) is evaluated using a Gaussian distribution.

The year-to-year variability is the width (RMS) of this gaussian function.

Evaluation

• The year-to-year variability is now provided by the Meteonorm database.
• It is displayed in the "Geographic site" dialog, page "Monthly Weather Data", after importing the data from Meteonorm 7.2.
• If you get multi-years data from a weather data provider, you can evaluate the variability of these data using the tool "Databases > Compare Weather data". Here you can choose the years to be accumulated, and choose the "Histo and probabilities" option to see the corresponding plot.
• The variability of the weather data has been studied in detail by Pierre Ineichen for about 30 different sites, in a document available from our web site (cf Ineichen_2001¹)
  • He has collected data from different sources, and analyzed time series of 10 to 30 years for each of the 30 sites studied.
  • The main results are gathered on the table given below.
  • It appears that the variability is dependent on the annual irradiance: this may be explained by the fact that with sunny climates with many clear days, the possibility of middle or bad days is lower.
  • Therefore for use of these data in the evaluation of the P90 values, we have established an upper estimate which is used as default value.