Understand your weather
Cooling Degree Days (CDD)
A conservative indicator of relative cooling demand over time.
Cooling Degree Days summarize how much a station’s daily average temperature rises above a fixed 65°F base. They describe a relative cooling-demand signal derived from temperature, not measured building energy usage or HVAC consumption.
Daily temperature signal
How hot is the station day?
Relative cooling demand
How much daily surplus 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 CDD. A day contributes cooling degree days only when the mean is above that reference; the result is the positive difference.
Daily low
70°F
Daily high
88°F
Average of low and high 79°F
Subtract from the 65°F base
14 CDD
14°F·days of cooling demand, or about 7.8°C·days above the 18.3°C equivalent.
How Corydal calculates CDD
Corydal uses the station-local daily low and high summary, then computes a daily mean and compares it to the 65°F base. Daily cooling degree days are the positive difference between the calculated daily mean and the base.
CDD = max(0,((Tmin + Tmax) / 2) − Tbase)
- 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) CDD base used by Corydal.
If the daily average is 60°F, the daily CDD contribution is zero. The model does not estimate a building’s cooling load or a monthly energy bill; it only shows that the station temperature relationship is cooler than the selected cooling-demand threshold.
Why Corydal uses 65°F as the fixed base
Corydal uses the 65°F (18.3°C) convention because it separates CDD 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.
CDD 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 CDD interpretation and keeps the Celsius equivalent explicit.
Small daily surpluses add up
Cooling Degree Days accumulate a series of daily positive cooling-demand surpluses. A three-day total should be read as the sum of those station-local daily positive surpluses, not as a promise of energy or cooling-load performance.
| Day | Daily CDD | Cumulative CDD |
|---|---|---|
| Day 1 | 4.2 | 4.2 |
| Day 2 | 6.1 | 10.3 |
| Day 3 | 0.0 | 10.3 |
A day with a daily mean below 65°F contributes 0 CDD. 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 cooling-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 CDD help explain?
Cooling season context
CDD provides a comparative temperature relationship over time, helping show how warm conditions accumulate across a local calendar period.
Relative demand signal
Large totals suggest a stronger cooling-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
CDD does not include occupancy, insulation, solar gain, wind, humidity or actual system efficiency. Keep the number as a weather-based indicator.
Good observations matter
CDD 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 an implied zero CDD.
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 CDD values are produced when the daily mean is greater 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 cooling 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 heating demand, learn about Heating Degree Days. For the full station picture, return to Insights.
From understanding to observation
See station-based cooling demand through weather data
Corydal turns saved weather-station observations into accumulated Cooling Degree Days and other environmental Insights. Explore the current station’s cooling-demand signal, daily windows and data coverage.
Exploring station Insights →