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How have Cooling Degree Days (CDD) and Heating Degree Days (HDD) been calculated in UKCP09?

The method for calculating degree days differs between the UKCP09 historical observation data and the method used by the UKCP09 weather generator and threshold detector. There are two ways of calculating CDD - a simple summation of the number of degrees Celsius the mean temperature is above a 22°C for each day and a weighted summation. There are two ways of calculate HDD - a simple summation of the number of degrees Celsius the mean temperature is below 15.5°C for each day and a weighted summation.

In detail

Cooling Degree Days

There are two ways of calculating CDD - a simple summation of the number of degrees Celsius the mean temperature is above a 22°C for each day and a weighted summation.

To derive CDD based on a simple summation (method used in the UKCP09 Weather Generator and its Threshold Detector), the number of degrees Celsius that the daily mean temperature is above 22°C is calculated for each day of the year (ignoring days when the mean temperature is below the threshold temperature of 22ºC) and summed.

To derive CDD based on a weighted summation (method used within the UKCP09 Observed trends report and by the Met Office in the UKCP09 historical observation data), the number of degrees Celsius and weighted increments using the following criteria are summed for all days of the year 

Daily situationCDD daily increment
Tmax ≤ Tbase0
Tmin > TbaseTmean - Tbase
Tmean > Tbase and
Tmin ≤ Tbase
0.5 (Tmax - Tbase) - 0.25 (Tbase - Tmin)
Tmean < Tbase and
Tmax > Tbase
0.25 (Tmax - Tbase)
Tmin = minimum daily temperature
Tmax = maximum daily temperature
Tmean = mean daily temperature = 0.5 (Tmin + Tmax)
Tbase = threshold temperature (22°C)

Heating Degree Days

There are two ways of calculate HDD - a simple summation of the number of degrees Celsius the mean temperature is below 15.5°C for each day and a weighted summation.

To derive HDD based on a simple summation (method used in the UKCP09 Weather Generator and its Threshold Detector), the number of degrees Celsius that the daily mean temperature is below 15.5°C is calculated for each day of the year (ignoring days when the mean temperature is above the threshold temperature) and summed.

To derive HDD based on a weighted summation (method used within the UKCP09 Observed trends report and by the Met Office in the UKCP09 historical observation data), the number of degrees Celsius and weighted increments using the following criteria are summed for all days of the year

Daily situationHDD daily increment
Tmin ≥ Tbase0
Tmax < Tbase Tbase - Tmean
Tmean < Tbase and
Tmax ≥ Tbase
0.5 (Tbase - Tmin) - 0.25 (Tmax - Tbase)
Tmean > Tbase and
Tmin < Tbase
0.25 (Tbase - Tmin)
Tmin = minimum daily temperature
Tmax = maximum daily temperature
Tmean = mean daily temperature = 0.5 (Tmin + Tmax)
Tbase = threshold temperature (15.5°C)