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Heating Degree Days (HDD)
A conservative indicator of relative heating demand over time.
Heating Degree Days summarize how much a station’s daily average temperature falls below a fixed 65°F base. They describe a relative heating-demand signal derived from temperature, not measured building energy usage or HVAC consumption.
Daily temperature signal
How cold is the station day?
Relative heating demand
How much daily deficit is indicated?
One day, step by step
Calculate the average of the day’s local daily low and high. Compare that temperature to the 65°F (18.3°C) base used for HDD. A day contributes heating degree days only when the mean is below that reference; the result is the positive difference.
Daily low
40°F
Daily high
58°F
Average of low and high 49°F
Subtract from the 65°F base
16 HDD
16°F·days of heating demand, or about 8.9°C·days below the 18.3°C equivalent.
How Corydal calculates HDD
Corydal uses the station-local daily low and high summary, then computes a daily mean and compares it to the 65°F base. Daily heating degree days are the positive difference between the base and the calculated daily mean.
HDD = max(0,Tbase − ((Tmin + Tmax) / 2))
- Tmin
- The lowest valid temperature from the station’s daily summary.
- Tmax
- The highest valid temperature from the station’s daily summary.
- Tbase
- The 65°F (18.3°C) HDD base used by Corydal.
If the daily average is 71°F, the daily HDD contribution is zero. The model does not estimate a building’s heat load or an annual energy bill; it only shows that the station temperature relationship is warmer than the selected heating-demand threshold.
Why Corydal uses 65°F as the fixed base
Corydal uses the 65°F (18.3°C) convention because it separates HDD from GDD and matches the shared degree-day semantics used across the station’s local daily temperature summaries. The conversion is exactly 65°F = 18.3°C. An equivalent Celsius degree-day equation is based on the same 18.3°C threshold.
HDD base65°F / 18.3°C
A different base temperature would describe a different model. Corydal’s published formula uses one conservative, common reference point for the HDD interpretation and keeps the Celsius equivalent explicit.
Small daily deficits add up
Heating Degree Days accumulate a series of daily positive heating-demand deficits. A three-day total should be read as the sum of those station-local daily positive deficits, not as a promise of energy or heat-loss performance.
| Day | Daily HDD | Cumulative HDD |
|---|---|---|
| Day 1 | 4.2 | 4.2 |
| Day 2 | 6.1 | 10.3 |
| Day 3 | 0.0 | 10.3 |
A day with a daily mean above 65°F contributes 0 HDD. Missing readings remain unknown and incomplete coverage can make the total provisional. Daily values are added only when the station-local daily temperature summary yields a positive heating-deficit relationship.
When comparing periods, keep the base, local daily summary method and coverage consistent. A cumulative total should be interpreted as a relative demand signal rather than an energy estimate.
What can HDD help explain?
Heating season context
HDD provides a comparative temperature relationship over time, helping show how cold conditions accumulate across a local calendar period.
Relative demand signal
Large totals suggest a stronger heating-demand relationship in the daily mean-versus-base comparison, but not the energy draw of a specific building.
Longer-period comparisons
Comparing equivalent months or seasons can show how the station’s observed temperature relationship changes between periods.
Limited interpretation
HDD does not include occupancy, insulation, solar gain, wind, humidity or actual system efficiency. Keep the number as a weather-based indicator.
Good observations matter
HDD is derived from the station’s local daily min/max summaries. Missing lows or highs or incomplete daily coverage can make a period provisional. The station daily summary is prior to any daily degree-day calculation; missing days never become a measured zero.
- Complete
- The station summary provides enough valid daily temperature evidence for the full observed period.
- Provisional
- Coverage is incomplete. The cumulative total still reflects available station evidence, but the period is not complete.
- Missing data
- A day without a usable station daily summary remains unknown and never contributes a implied zero HDD.
Corydal’s methodology
Corydal uses the station’s local daily min/max summaries and converts the daily average to Celsius internally when needed. The fixed 65°F base is 18.3°C, and daily HDD values are produced when the daily mean is less than that threshold.
The calculation uses the station’s local calendar and timezone for the period. It starts from the day’s valid station low and high and does not interpolate missing readings. Negative daily totals are treated as zero daily heating demand; missing daily totals remain unknown.
Coverage is expressed across the expected five-minute temperature sampling intervals from the station’s raw data. Incomplete or missing daily summaries do not make the model estimate building consumption or operating cost. They describe the available daily evidence and the limitations of that evidence.
References & learn more
Background on degree-day conventions, daily temperature summaries and the difference between climate indicators and actual energy calculations.
For warmth accumulation, learn about Growing Degree Days. For cooling demand, learn about Cooling Degree Days. For the full station picture, return to Insights.
From understanding to observation
See station-based heating demand through weather data
Corydal turns saved weather-station observations into accumulated Heating Degree Days and other environmental Insights. Explore the current station’s heating-demand signal, daily windows and data coverage.
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