The calculations for normal values found in the Period Of Record (POR) reports are based on data from 1971-2000. Because many weather stations have an incomplete set of data throughout this period it was necessary to adopt a method whereby the normal calculation would give the best representation of how much data was available from the 30-year normal period for a particular station. The letters "A" through "G" and "M" are attached to each normal value according to how much data was available for the calculation of the normal.
The WMO establishes that normal values should be arithmetic means calculated for each month of the year from the daily data. To qualify, temperature data, soil temperatures and evapotranspiration must fit the following rule: "if more than 3 consecutive daily values are missing or more than 5 daily values in total in a given month are missing, the monthly mean should not be computed and the year-month mean should be considered missing." This is referred to as the "3/5" rule. For total precipitation, degree-days and "days with" calculations no missing days are allowed.
Once the year-month values that qualify are determined, a similar "3/5" rule is also applied to each month of the year over the 30-year normal period. This is done to determine if the normal value for a given month can be designated with a normal code of "A". For instance, to meet the WMO standard, a value of a monthly element, such as normal mean temperature for May, with a normal code of "A" can have no more than 3 consecutive, or 5 in total, midding mean temperature values for any of the months of May in between 1971 and 2000.
If a month does not meet the "3/5" rule standard it is not given a normal code of "A". Rather, it is given a normal code of "B" through "G" or "M" based simply on the number of valid year-month values found for each month over the 30-year normal period. The following chart illustrates the standards for the assignment of normal codes:
|Normal Code||# of Valid Year-Month Values Over the 30-year Normal Period|
|A||"3/5" Rule - No more than 3 consecutive or 5 total missing values over the 30-year period.|
Considering the 1971-2000 normal period, if, for example, the month of January were missing for the years of 1981, 1985, 1988, 1989, and 1993, the normal code of "A" would be assigned because no more than 3 consecutive and 5 total years are missing. However, if the month of January were missing for the years of 1981, 1985, 1986, 1987, and 1988, the normal code of "B" would be assigned because more than 3 consecutive years are missing but no more than 5 years are missing (i.e. there are still 25 valid years available for calculation of the normal.)
The annual normal values are calculated based on the final normal values given for each month. The normal code assigned to the annual value is equal to the monthly normal code representing the least degree of completeness. For example, if the months of January-November all had a normal code of "C" but December had a normal code of "D" then the annual normal value would be given a normal code of "D".