Growing degree days, chill hours & VPD

The grower numbers your site derives, and the settings that decide what they count.

The Garden & agriculture tab turns your archive into the numbers a grower plans against. Turn it on or off under Homepage → Sections and tabs → Garden and agriculture. It is part of Pro and the free trial.

What the tab shows

Number	What it is
Growing degree days	Heat above a base temperature, accumulated this calendar year, with an optional second base alongside
Temperature sums	Three uncapped running totals of the daily mean above a base
Chill hours	Hours inside the cold band fruit trees need, this season
Vapour pressure deficit	How much drier the air is than saturated, in hPa
Evapotranspiration	ET today and over the last 7 days
Heating & cooling degree days	The last 30 days
Soil moisture, soil temperature, soil conductivity, leaf wetness	Live, plus a 7-day trend per channel
Frost risk	From the current temperature and dew point
Irrigation guidance	From soil moisture or the 7-day water balance

The settings

Under Advanced → Weather calculations → Agronomy, shown while the Garden and agriculture tab is on. Temperatures are in your site's units. Changing any of them re-scores your whole archive, past days included, without rewriting a reading.

Setting	Default	What it does
Growing degree day base	10 °C / 50 °F	Below this a day adds nothing
Growing degree day cap	None	Applied to the day's high and low before averaging, so a 40 °C afternoon cannot inflate the total. 30 °C / 86 °F is the usual crop cap
Second degree day base	None	Charts a second curve alongside the first, with the same cap
Temperature sum 0, 1, 2 base	0, 5, 10 °C	One base per sum
Chill hours from / to	0-7 °C	An hour counts when the temperature is inside this band. Set the lower bound very low to count "hours below 7 °C"
Chill season starts	October (April south of the equator)	Chill accumulates from the 1st of this month
Sunshine threshold	75%	See sunshine hours

Field sensors

Every soil moisture, soil temperature, soil conductivity and leaf wetness channel your hardware supplies gets a live reading and a 7-day trend: up to 16 soil and 8 leaf channels from Ecowitt WH51, WN34, WH51-EC and WN35 sensors through the upload URL, or a Davis soil/leaf station through WeatherLink. Alerts can watch every channel at once; see accessory sensor channels.

Irrigation guidance

The irrigation line reads water now, consider watering or no watering needed from either of two inputs, and insufficient data with neither:

Soil moisture, averaged over your channels: below 25% means water now, below 40% consider it (on tension sensors, 60 and 35 cb and above).
The 7-day water balance: evapotranspiration minus rainfall over the last seven days. A deficit of 20 mm means water now, 8 mm consider it.

The water balance needs evapotranspiration in the archive, which comes only from a WeatherLink Pro or Pro+ plan (on a Davis station with a solar radiation sensor) or an imported weewx archive. Stations on the upload URL or WeatherLink Basic need a soil moisture sensor for irrigation guidance.

Everything else on the tab needs only temperature and humidity.

Sunshine hours without a sunshine sensor

The Sun tab shows the hours of sunshine today and yesterday, and the share of possible daylight that represents.

Each archived solar reading is compared with the clear-sky maximum for that moment, computed from your station's coordinates and altitude. It counts as sunshine when it beats the threshold (75% by default, as in CumulusMX) and is at least 30 W/m².

Because it is worked out from the archive, imported history counts too, a collection gap is never counted as sunshine, and changing the threshold re-scores past days. Your station needs coordinates; without them the figure is hidden.

Heat stress (WBGT)

From a WBGT of 25 °C upward, the Temperature tab adds a WBGT (shade) reading, the wet bulb globe temperature that work-rest schedules use. No consumer station has the black-globe thermometer the full index needs, so this is the Australian Bureau of Meteorology's shade approximation from temperature and humidity. It under-reads for anyone in direct sun.