Growing degree days, chill hours & VPD
The grower numbers your site derives, and the settings that decide what they count.
The Agriculture tab turns your archive into the numbers a grower actually plans against. Most of them are not measurements: they are questions asked of measurements, and the answer depends on which question you asked.
What the tab shows
| Number | What it is |
|---|---|
| Growing degree days | Accumulated heat above a base temperature, this calendar year. |
| Chill hours | Hours in the cold band fruit trees need to break dormancy, this season. |
| Vapour pressure deficit | How much drier the air is than saturated, in hPa. |
| Evapotranspiration & water balance | ET against rainfall over 7 days. |
| Heating & cooling degree days | The last 30 days. |
| Soil moisture, soil temperature, leaf wetness | Live, plus a 7-day trend per channel. |
| Frost risk & irrigation guidance | Derived from the current conditions and the water balance. |
Why VPD and not humidity
Relative humidity moves when only the temperature changes, so a fixed humidity target is not a fixed anything. Vapour pressure deficit does not: it is how much more water the air could hold, so a VPD target is a fixed evaporative demand. That is why greenhouse growers manage on it.
The settings behind the numbers
Under Settings → Agronomy (shown when the Agriculture card is on):
Growing degree day base. Below this a day contributes nothing. 10 °C / 50 °F is the conventional field-crop base and the default.
Growing degree day cap. Optional. Growth does not keep accelerating in a heatwave, so a cap of 30 °C / 86 °F stops a hot summer overstating the total. The cap is applied to the day's high and low before they are averaged, not to the result - capping the average instead would let a 40 °C afternoon still pull the day upward.
Chill hours band. An hour counts when the temperature is inside this band. The 0-7 °C default is the model most published chill requirements use. To count the simpler "hours below 7 °C" instead, set the lower bound very low.
Chill season start. Chill accumulates from the 1st of this month through to spring, so a total is only comparable against a variety's requirement when both count from the same date. October in the northern hemisphere, April in the southern; your station's latitude picks the default.
Sunshine threshold. See below.
These re-answer your whole archive
Agronomy settings are applied when the page is READ, not when a reading arrives. Change the base from 10 °C to 8 °C and every day already in your archive is re-scored - nothing is rewritten, and nothing is lost. This is unlike calibration, which corrects readings as they arrive and cannot reach back.
Sunshine hours without a sunshine sensor
The Sun tab shows how many hours the sun was out today and yesterday, plus the share of possible daylight that represents.
There is no sunshine sensor involved. Every archived solar reading is compared against the clear-sky maximum for that exact moment - computed from your station's coordinates, altitude and the sun's position - and counts as sunshine when it beats a percentage of it. 75% is the default threshold, the same one CumulusMX uses; a floor of 30 W/m² stops the few minutes after sunrise passing the percentage test on almost no light at all.
Because this is derived from the archive rather than counted minute by minute:
- Imported weewx, CumulusMX and CSV history counts.
- A gap in collection costs resolution, not truth - an unrecorded gap is simply not claimed as sunshine.
- Changing the threshold re-scores every day, not just future ones.
Your station needs coordinates for this; without them there is no clear-sky maximum to measure against and the figure is hidden.
Heat stress (WBGT)
When conditions warrant it, the Temperature tab adds a WBGT (shade) reading - the wet bulb globe temperature that work-rest schedules are written against.
The full index weights a black-globe thermometer at 20-30%, and no consumer weather station has one. What is computable from temperature and humidity alone is the Australian Bureau of Meteorology's shade approximation, which is what is shown - so it reads the shade, and under-reads for anyone standing in direct sun. It appears only once heat stress is a real question; below that it would just repeat the wet bulb.