Frequently Asked Questions

• What is the definition of Running annual mean and Running 8hr mean.

  These are the definitions in the Defra Technical Guidance LAQM TG(09):

  Running 8-hour mean
  A running 8-hour mean is a mean which is calculated from hourly average concentrations. The running 8-hour mean for a particular hour is the mean of the hourly average concentrations for that hour and the preceding 7 hours. The averaging period is stepped forward by one hour for each value, so running mean values are given for the periods 00:00–07:59, 01:00–08:59 etc. There are, therefore, 24 possible 8–hour means in a day (calculated from hourly data). In order for a running average to be valid, 75% data capture is required i.e. 6 hourly averages out of every 8 must be valid. The maximum daily running 8–hour mean is the maximum 8–hour running mean measured on any one day, i.e. between 00:00 and 23:59 hours.

  Running annual mean
  A running annual mean is a mean which is calculated each hour from hourly average concentrations over a year. The running annual mean is the mean of the hourly average concentration for that hour and the preceding 8759 hours (365 days). For leap years annual means are based on 366 days.

  These terms are also defined in the Air Quality Regulations.