Typical Weather Years
TRY Data
Test Reference Year (TRY) data is composed of 12 separate months of data, each representing an "average" month as derived from the collected data. TRY data files are created by compiling yearly weather datasets through a filtering procedure that discards years with extremely high or low air temperatures to arrive finally at the most average months. Because extremes are discarded and the data represents average conditions, the TRY process tends to result in a particularly "mild" year which does not fully represent the long-term weather variability for a given site.
TMY Data
TMY weather data is composed of 12 separate months of data, each chosen to be the most "typical" month from the total years of data – which can vary depending upon data availability. In Climate Analytics there are two year ranges to choose from: 1970/1980-2010 and 1995-2015.
TMY data selection is carried out using the Sandia method which is an empirical approach that selects individual months from different years of the period of record. For example, when the 1970/1980-2010 option is selected, there are 40 years of data to be analysed. In this case all 40 Januarys are examined and the one judged "most typical" is selected to be included in the TMY. The other months of the year are treated in a similar manner, and then the 12 selected typical months are concatenated to form a complete year.
The Sandia method selects a "typical" month based on nine daily indices consisting of the maximum, minimum, and means dry bulb and dew point temperatures; the maximum and mean wind velocity; and the total global horizontal solar radiation. For each month of the calendar year, five candidate months with cumulative distribution functions (CDFs) for the daily indices that are the closest to the long-term CDFs are selected. The CDF gives the proportion of values that are less than or equal to a specified value of an index. Candidate monthly CDFs are compared to the long-term CDFs by using the Finkelstein-Schafer (FS) statistics for each index. Adjacent months in the TMY are likely to be selected from different years and so discontinuities at the month interfaces are smoothed for 6 hours on each side.
Because the Sandia TMY method includes consideration of maximum and minimum values and CDF distributions, it will tend to result in data with more typical variability than the equivalent TRY data which is based purely on averages.