Actual weather data is received by National Grid from its weather data provider for six weather stations around Britain (Heathrow Bristol, Birmingham, Leeds, Manchester and Glasgow). This data is used to calculate the actual GB average of all measured temperatures as at 12:00 local time on the previous day. This calculated average temperature is the temperature used within National Grid as part of the electricity demand forecasting process. Three sets of reference temperatures are calculated, each with 365 values, one for each day of the year. The three sets are Normal, Low and High. Normal Temperature is the average GB temperature expected for average GB ambient temperature weather conditions and has a 50% chance of being exceeded. Low Temperature values have an 88% chance of being exceeded (1 in 8 chance that the temperature is lower) and High Temperature values have a 12% chance of being exceeded (1 in 8 chance that the temperature is higher, or 7 in 8 chance that it is lower). If data from a particular weather station is unavailable on a given day, the transmission company will temporarily substitute this with data from another station.