EPA gets weather data for Portfolio Manager from the National Oceanic and Atmospheric Agency's (NOAA) Global Surface Summary of the Day (GSOD) dataset:
This dataset includes data from over 9,000 weather stations around the world. Portfolio Manager uses over 750 weather stations in the United States. We only use a weather station if it has complete weather data going back to the year 2000.
Portfolio Manager gets this GSOD data every month (usually within the first two weeks of the month for the previous month). It is used to compute:
•Heating Degree Days (HDD)
•Cooling Degree Days (CDD)