Data Methods: Heating & Cooling Degree Days
The steps below describe how the values shown in the tool were calculated, from raw climate model output to the final data product.
Input data
Climate model simulations
- Description
Ensemble of 4 bias-adjusted climate model simulations, dynamically downscaled using the WRF model for the state of California:
EC-Earth3-r1i1p1f1,MPI-ESM1-2-HR-r3i1p1f1,TaiESM1-r1i1p1f1,MIROC6-r1i1p1f1.- Variable
Average daily air temperature
- Spatial resolution
3 kilometers
- Temporal resolution
Daily
- Time period
1981-2099: the modeled historical period (1981-2014), followed by the SSP3-7.0 scenario (2015-2099). The model data partially covers 1980 and 2100, but HDD and CDD are only computed for complete calendar years.
- Download
Browse in the Cal-Adapt data catalog (STAC), or download directly from cloud storage (AWS).
Output data
Regional tables
- Description
Heating and cooling degree days for each year, as the mean, minimum, and maximum across simulations
- Shown in
- Format
One CSV table per region
- Download
Browse in the Cal-Adapt data catalog (STAC), or download directly from cloud storage (AWS).
Calculation steps
Data retrieval and preprocessing
Using Cal-Adapt's open source Python library climakitae, input data was retrieved. Climate model simulations were retrieved for the modeled historical period (1981-2014) and the future period under the SSP3-7.0 scenario (2015-2099). Inland water was masked from the data to discard artificially high values over water that are not representative of the variable.
Compute CDD/HDD values
Cooling degree days (CDDs) are calculated by subtracting the cooling threshold value (65 °F) from each day's average temperature, producing a measurement of how many degrees each day's temperature exceeds the threshold by. Days with an average temperature below the threshold are counted as zero.
Heating degree days are calculated in a similar way by measuring how many degrees each day's average temperature falls below the heating threshold (65 °F). Days with an average temperature above the threshold are counted as zero.
Annual sums
The daily values are summed over each calendar year to produce annual CDDs/HDDs.
Spatial aggregation
The gridded data is aggregated by spatial unit (e.g. county, city, etc) by taking a median of all the grid cells within the boundary. A median is used to capture the typical value of CDD/HDD within the area of interest without introducing bias from any anomalous grid cells.
Model mean and range
For each year of spatially aggregated data, the mean, minimum, and maximum values are taken across the four dynamically downscaled WRF simulations. The multi-model mean is used to plot the timeseries for CDD and HDD, with a shaded area indicating the range between the minimum and maximum model values.
Cal-Adapt. (2026). Data Methods: Heating & Cooling Degree Days. Cal-Adapt. https://cal-adapt.org/dashboard/data-methods/hdd-cdd