Table 1.

Calculation of Heating Degree-Days (HDD) and Cooling Degree-Days (CDD) according to the UK Met Office for a day defined with a daily mean (Tmean), minimum (Tmin) and maximum (Tmax) surface air temperature.

Condition on temperature	HDD UK calculation (Tbase = 15.5 °C)	Interpretation
	CDD UK calculation (Tbase = 22 °C)
Tmax ≤ Tbase	HDD = Tbase − Tmean	Cold day
	CDD = 0	Cold day
Tmean ≤ Tbase < Tmax	HDD = (Tbase − Tmin)/2 − (Tmax − Tbase)/4	Mostly cold day
	CDD = (Tmax − Tbase)/4	Mostly cold day
Tmin < Tbase < Tmean	HDD = (Tbase − Tmin)/4	Mostly warm day
	CDD = (Tmax − Tbase)/2 − (Tbase − Tmin)/4	Mostly warm day
Tmin ≥ Tbase	HDD = 0	Warm day
	CDD = Tmean − Tbase	Warm day

HDD is calculated with a base temperature (Tbase) of 15.5 °C, and CDD with a base temperature of 22 °C.