# Growing degree days, chill hours & VPD

URL: https://pro-weather.com/docs/features/agronomy

> 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](https://pro-weather.com/docs/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](https://pro-weather.com/docs/getting-started/import-weewx),
  [CumulusMX](https://pro-weather.com/docs/getting-started/import-cumulusmx) and
  [CSV](https://pro-weather.com/docs/getting-started/import-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.

## Next steps

- [Calibrate your sensors](https://pro-weather.com/docs/calibration)
- [Alerts on soil and leaf sensors](https://pro-weather.com/docs/alerts)
- [Supported hardware](https://pro-weather.com/docs/supported-hardware)