What is a degree day?
A degree day is a metric that records how hot or cold a location is by measuring each day’s duration, and deviation, above or below 65 degrees Fahrenheit. The SIMAP degree day data comes from NOAA’s National Weather Service Climate Prediction Center Degree Day Statistics. The degree day data is recorded and reported as an annual accumulation from July.
How are degree days calculated?
- Daily Degree Day = Average Daily Temperature - Base Temperature (65 degree F used by NOAA)
- Daily Heating Degree Days = Base Temperature - Daily Average Temperature
- Daily Cooling Degree Days = Daily Average Temperature - Base Temperature
Used in SIMAP:
- Annual Heating Degree Days = Sum of Fiscal Year's Daily Heating Degree Days
- Annual Cooling Degree Days = Sum of Fiscal Year's Daily Cooling Degree Days
- Annual Total Degree Days = Annual Heating Degree Days + Annual Cooling Degree Days
Example from NOAA: " Heating degree days are summations of negative differences between the mean daily temperature and the 65°F base; cooling degree days are summations of positive differences from the same base. For example, cooling degree days for a station with daily mean temperatures during a seven-day period of 67,65,70,74,78,65 and 68, are 2,0,5,9,13,0,and 3, for a total for the week of 32 cooling degree days."
What is the base temperature and how is it selected?
The base temperature in the SIMAP degree day calculation is 65 degrees Fahrenheit. This base temperature is traditionally used across the United States as it is considered to be the “heating and cooling point” for buildings.
Application:
Degree days are a useful metric that allow for heating and cooling energy consumption and emissions to be normalized over the weather data. Heating and cooling energy consumption with their associated emissions are assumed to be directly correlated to the prevailing weather. As such, improvements in building performance, changes in energy needs, and changes in heating and cooling emissions may be fairly evaluated with the weather in mind.
Degree day application example:
In order to understand the impact of an investment in building performance, like improved insulation, an analyst can normalize building energy consumption or emissions across degree days. If a facilities manager in the state of Alabama sets their energy consumption baseline in 2012, the annual heating degree days for Alabama was 1975. If the organization invests in higher grade insulation, they would be well advised to consider their building performance after normalizing the data over the annual heating degree days, because FY12 and 13 had HDD values of 2597 and 2928. In order to meet the heating demand caused by the increase in HDD’s, the fuel consumption and emissions will increase. Without considering fuel consumption and emissions in the context of the weather, an investment in building performance may be considered to ineffective, when, in fact, it may have been unseasonably cold.
What scopes and data are usually normalized across degree days?
Degree day data is most commonly used to normalize building fuel energy consumption and emissions within Scope 1. Within SIMAP, the normalization results can be restricted to just stationary fuels in order to properly use degree days for a building performance analysis.
Degree Day updated as of August 2021:
In August 2021, the Degree Days in SIMAP were update to correct for differences in the timeframe's of NOAA's Degree Day Data. NOAA tracks heating degree days on a calendar year basis, while heating degree days are tracked on a fiscal year (July to June) basis. Due to this, previous total degree days in SIMAP were not aligned, and were missing a portion of the cooling degree days. This has been corrected.
SIMAP degree day data source:
The SIMAP degree day data comes from NOAA’s National Weather Service Climate Prediction Center Degree Day Statistics. Annual degree day values are calculated as an annual average total from locations within each state. Because NOAA's Degree Days Statistics are accumulated differently (from NOAA: "The "heating year" during which heating degree days are accumulated extends from July 1st to June 30th and the "cooling year" during which cooling degree data are accumulated extends from January 1st to December 31st."), SIMAP has taken this data and done calculations to align the timeframe of both heating and cooling degree days. Heating, Cooling, and Total Degree Day data in SIMAP is presented on a fiscal year basis, from July 1st to June 30th.
Customized site specific degree day data may be found and used outside of SIMAP using…
BizEE Degree Days weather data: https://www.degreedays.net
NOAA National Weather Service Climate Prediction Center Degree Day Statistics: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/
Note: NOAA's degree day statistics are averaged accross entire states, and the climate divisions within any particular state are weighted by the proportion of population in that division. This can cause differences if an institution is using a degree day measurment from a single station close to their location
A full explanation of NOAA's degree day averageing can be found here: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/ddayexp.shtml
In the future SIMAP is considering adding the ability to customize an institution's degree days. If this feature would be useful for you, please let us know here.
Additional resources:
NOAA’s National Weather Service Climate Prediction Center: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/
Degree Days overview: