Seasons and sky
Heating Degree Day
Direct answer
Heating Degree Day: A Heating Degree Day is a temperature-demand index accumulated when a day mean temperature falls below a selected base.
Heating Degree Days estimate cold-weather heating demand below a base temperature.
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At a glance
- Found in
- Seasons & Earth Almanac
- Related
- Growing Degree Day, Cooling Degree Day, Astronomical Season, Equinox
Heating Degree Days estimate cold-weather heating demand below a base temperature.
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The story behind Heating Degree Day
Utilities and climatologists adopted degree-day summaries to relate outdoor temperature to fuel demand.
Why Heating Degree Day matters
They support energy planning, climate comparison, and seasonal building-performance analysis.
Heating Degree Day in practice: an example
With a 65-degree-Fahrenheit base, a daily mean of 50 contributes 15 Heating Degree Days under the simple method.
Where readings of Heating Degree Day go wrong
The usual misreading is to predict an individual building energy use from degree days alone; insulation, occupancy, wind, and solar gain matter.
Sources and editorial method
Research references used for this entry include:
- NOAA NCEI: U.S. Climate Normals - Official methodology and data for climate normals, degree days, growing seasons, and freeze dates.
- Source
- The Great Almanac Dictionary
- Category
- Seasons & Earth Almanac
- Last updated
- 2026-09-26
Compare seasonal heating demand
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Common questions
What is a Heating Degree Day?
A Heating Degree Day is a temperature-demand index accumulated when a day mean temperature falls below a selected base.
What do people get wrong about a Heating Degree Day?
The usual misreading is to predict an individual building energy use from degree days alone; insulation, occupancy, wind, and solar gain matter.
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