Understand your weather
Growing Degree Days (GDD)
A simple way to measure accumulated warmth over time.
Growing Degree Days combine temperature over successive days to estimate warmth relevant to biological development. They can help put the progress of plants and insects into context.
Today’s temperature
How warm is it right now?
Accumulated GDD
How much warmth has built up?
One day, step by step
Take the average of the day’s observed low and high. Subtract a chosen base temperature: the threshold above which this model counts warmth. Keep the result only if it is positive.
Daily low
12°C
Daily high
28°C
Average of low and high 20°C
Subtract the 10°C base
10 GDD
10 degrees above the base, for one day.
With Celsius inputs, the unit is °C·days.
How Corydal calculates GDD
Average the extremes, subtract the base, then use zero if the result is negative.
GDD = max(0,((Tmin + Tmax) / 2)− Tbase)
- Tmin
- The lowest valid temperature observed that day.
- Tmax
- The highest valid temperature observed that day.
- Tbase
- The base temperature chosen for the calculation.
If that average is 8°C and the base is 10°C, the result is 0 GDD, not −2. A cool day adds no warmth in this model; it does not undo warmth already accumulated. The average here is the midpoint of the low and high, not an average of every reading.
The base temperature changes the meaning
Different organisms and development stages respond to different thermal thresholds. The base represents the lower threshold for development in the chosen model; it is not a universal setting for all living things.
Base 5°C15 GDD
Base 10°C10 GDD
Base 15°C5 GDD
These bases illustrate the arithmetic, not recommendations for a crop or species. Corydal currently defaults to 10°C. Interpret any GDD value with its base, calculation method and temperature units: models with temperature cutoffs can give different results for the same weather.
Small daily amounts add up
A single day describes one contribution. Cumulative GDD adds those contributions from a defined start date, helping you follow thermal exposure over a longer period.
| Day | Daily GDD | Cumulative GDD |
|---|---|---|
| Day 1 | 4.2 | 4.2 |
| Day 2 | 6.1 | 10.3 |
| Day 3 | 0.0 | 10.3 |
Day 3 has a calculated zero, so the total stays level. A missing day is different: its contribution is unknown. In Corydal’s cumulative Insights chart, missing days create gaps and the available total remains provisional. Each displayed range starts its own accumulation.
When comparing periods, keep the base, method, units and start-date convention consistent, and check their data coverage.
What can GDD help explain?
Plant development
Track crop phenology — the timing of life stages — and put flowering or maturity into context when an appropriate model is available.
Insect development
Species-specific models can help estimate the timing of insect life stages, including those of pests.
Seasonal patterns
Compare warmth accumulated across equivalent periods to understand how thermal conditions differ between seasons.
Context, not certainty
GDD estimates thermal accumulation, not actual growth. Water, day length and other conditions also matter. Pair it with observations of the organism itself.
Good observations matter
Missed readings can hide a day’s true low or high. Coverage tells you how much temperature evidence is available; it does not guarantee sensor accuracy.
- Complete
- Valid temperature readings cover every expected sampling interval in the period.
- Provisional
- Some expected temperature evidence is absent. Available daily estimates still contribute, but the total is incomplete.
- Missing data
- No usable daily temperature result is available. That day is unknown, never silently counted as a measured zero.
Corydal’s methodology
Corydal uses persisted weather-station observations and derives daily Tmin and Tmax from valid Celsius readings. Days follow the station’s local calendar and timezone. Insights uses completed local days and the configured base temperature, currently 10°C.
The calculation uses the observed extremes directly, with no upper or lower temperature cutoff. Only a negative final daily GDD result is set to zero. Missing temperatures are excluded, with no interpolation or extrapolation.
Temperature coverage tracks expected five-minute sampling intervals. Available incomplete days contribute their calculated GDD without scaling by coverage. Missing or unusable daily results remain unknown and make the period provisional; a total with no analyzed days does not establish zero accumulation.
References & learn more
Background on degree days, biological timing and model choice. Corydal’s calculation is explained above; external tools may use different methods or units.
For the cold side of seasonal development, learn about Chill Hours.
From understanding to observation
See what weather-station data can tell you
Corydal turns saved weather-station observations into accumulated GDD and other environmental Insights. Explore the current station’s results, cumulative chart and data coverage.
Explore station Insights →