Periods used in climate maps and statistics

You may have noticed that all of our climate maps and statistics have a reference to the period over which they were calculated. You may also have wondered why certain periods are common, and why some elements are calculated over different periods. The following is a discussion of the most common averaging periods used in our products.

1961 to 1990

This 30-year period is the most recent standard reference period as defined by the World Meteorological Organisation (WMO). The WMO describes "standard reference periods" for use by the international community in order to maintain consistency in the calculation of climate statistics across the world. This allows climate statistics from different countries to be compared and analysed. The standard reference period is commonly used in our climate maps, climate statistics and is the base period for most climate change studies. The next standard reference period will be 1991 to 2020.

1976 to 2005

This period is often used as a more recent 30 year period to calculate statistics that are more descriptive of our current climate. Many of our climate maps and statistics are calculated over this period.

Other periods

Some of our maps and statistics have been calculated over other periods, using different dates and varying lengths of time.

Deciles, percentiles, extreme values and variability in datasets are generally calculated using all years of record. This is because these measures need a longer period of record to be statistically valid and therefore useful to you. For example, how could you describe the "hottest day on record", if you had only used data from the last 20 years? In order to describe the "hottest day on record" you would need to look at data from all years in the record.

Sometimes, the data related limitations of certain elements or meteorological parameters will determine the period used for the analysis. For example, the solar exposure maps were created using satellite data. These specialised satellite data were not available prior to 1990, so the maps and statistics have been calculated using all the data available from 1990 onwards.