Accumulated temperature and seasonal shift
Heat accumulated in the current year, the date of a sustained transition through a temperature threshold, and the shift of the season relative to the multi-year mean.
Quantities
| Symbol | Quantity | Unit |
|---|---|---|
T_max, T_min | daily extremes of air temperature at 2 m | °C |
T̄ | daily mean temperature, T̄ = (T_max + T_min) / 2 | °C |
T_b | base temperature (threshold) | °C |
S | sum of active temperatures | °C·day |
Δ | seasonal shift relative to the multi-year mean | days |
Formulas
Sum of active temperatures
S = Σ T̄_i over all days i for which T̄_i ≥ T_b
Active temperatures are summed. The sum of effective temperatures is defined as Σ (T̄_i − T_b) and takes smaller values at the same threshold; comparison with published tables requires the definitions to agree.
Sustained transition through the threshold
the first day of a run of 5 consecutive days with T̄_i ≥ T_b
Seasonal shift
Δ = date S̄ is reached in the mean year − date S̄ is reached this year
S̄ is the sum accumulated by the current date averaged over the archive years.
Expected harvest date
start date + days to maturity − Δ
A calendar constant is shifted. The crop's heat requirement is not computed: the required degree-day sum is absent from the registries in use.
Thresholds
A threshold of 10 °C is used for the plot — the boundary of active vegetation under which agroclimatic sum maps are published. For a crop its own minimum growth temperature from ECOCROP is used (the absolute minimum temperature field): 7 °C for cabbage, 15 °C for watermelon. Where ECOCROP holds no record, the crop-level calculation is not performed.
Input data
Daily temperature extremes are taken from the ERA5 reanalysis archive through open-meteo for the plot's coordinates. The mean is computed over complete archive years; an incomplete year is excluded.
Applicability and limitations
- Air temperature at 2 m is used; soil temperature, which governs emergence, is lower in spring, so the air threshold is reached earlier.
- Protected cultivation is not represented: the microclimate under cover differs and is not measured.
- The mean is computed over five archive years; a climatic normal is defined over thirty.
- The reanalysis archive lags by several days; the most recent days are not included in the sum.
Sources
- McMaster, G.S. & Wilhelm, W.W., 1997. Growing degree-days: one equation, two interpretations. Agricultural and Forest Meteorology 87(4): 291–300 — abstract open, full text paywalled Open
- FAO. ECOCROP — crop environmental requirements database. Food and Agriculture Organization of the UN, CC BY 4.0 — full text open Open
- Open-Meteo. Weather forecast and historical weather API. open-meteo.com, CC BY 4.0 — full text open Open