@BeginModule
@NewPage
@Name      = EcaHd
@Title     = Heating degree days per time period
@Section   = Climate indices
@Class     = Climate index
@Arguments = ifile ofile
@Operators = eca_hd

@BeginDescription
Let @file{ifile} be a time series of the daily mean temperature TG, then the heating degree days 
are defined as the sum of @math{T1} - TG, where only values TG < @math{T2} are considered. 
If @math{T1} and @math{T2} are omitted, a temperature of 17@celsius is used for both parameters. 
If only @math{T1} is given, @math{T2} is set to @math{T1}. Note that TG have to be given in units 
of kelvin, whereas @math{T1} and @math{T2} have to be given in degrees Celsius.
The date information of a timestep in @file{ofile} is the date of the last contributing timestep in @file{ifile}.
@EndDescription
@EndModule


@BeginOperator_eca_hd
@Title     = Heating degree days per time period
@Parameter = [T1] [T2]
@EndOperator


@BeginParameter
@Item = T1
FLOAT   Temperature limit (unit: @celsius; default: T1 = 17@celsius)
@Item = T2
FLOAT   Temperature limit (unit: @celsius; default: T2 = T1)
@EndParameter


@BeginExample
To compute the heating degree days of a time series of daily mean temperatures use:
@BeginVerbatim
   cdo eca_hd tgfile ofile
@EndVerbatim
@EndExample
