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Weather Generator variables

The UKCP09 Weather Generators produces daily and hourly time series of climate projections. You can access and use the Weather Generator through the UKCP09 User Interface.

The Weather Generator daily variables are:

  • Mean total daily precipitation rate (mm)
  • Minimum daily temperature (ºC)
  • Maximum daily temperature (ºC)
  • Vapour pressure (hPa)
  • Relative humidity (%)
  • Sunshine hours (hr)
  • Potential evapotranspiration (PET) (mm/day)
  • Direct irradiation (W/m2)
  • Downward diffuse irradiation )W/m2

The Weather Generator hourly variables are:

  • Mean total hourly precipitation rate (mm)
  • Mean hourly temperature (ºC)
  • Vapour pressure (hPa)
  • Relative humidity (%)
  • Sunshine hours (hr)
  • Direct radiation (W/m2)
  • Downward diffuse radiation (W/m2)

In detail

The Weather Generator uses a variety of methods to produce outputs, depending on the variable selected. Rainfall is the primary variable that is estimated by the Neyman-Scott Rectangular Pulses (NSRP) model, and all other variables that are generated from the Weather Generator depend on this rainfall data.

Inter-variable relationships (regression models developed between observed daily station data), are used to estimate daily mean temperature, daily temperature range, vapour pressure, and sunshine duration. Potential evapotranspiration is calculated from these variables, using a United Nations Food and Agriculture Organisation (UN-FAO) modified version of the Penman method.

Minimum and maximum daily temperature are calculated from mean daily temperature range. Relative humidity is estimated by combining vapour pressure with saturation vapour pressure at mean temperature. Direct radiation and diffuse radiation are calculated using formulae based on sunshine duration information.

The Weather Generator runs at a daily temporal resolution, and the hourly variables are derived from a temporal decomposition of the daily data, based on existing relationships in the observations. 

Find out more


  • An overview of the UKCP09 Weather Generator is given in  of the UKCIP09 Briefing Report. A small selection of pre-prepared results is given in .