Below are a number of calculations that Harvest can calculate using various sensors attached to our monitoring systems. Check the list of sensors at the bottom of each description to see if you already have these sensors. If you do, it is as simple as calling and asking for the calculation to be added to your page. If you don't already have the sensors, give us a call and we can let you know if it will work with your unit and what the addition will cost.

Australian Apparent Temperature (Feels Like)

The Australian Bureau of Meteorology, using a model developed by the US Navy, researched and then developed their own formula, the Australian Apparent Temperature.

The apparent temperature, invented in the late 1970's, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980's to include the effect of sun and wind. 

The formula is:

{\displaystyle \mathrm {AT} =T_{\mathrm {a} }+0.33e-0.70v-4.00}


  • Ta = dry bulb temperature (°C)
  • e = water vapour pressure (hPa)
  • v = wind speed (m/s) at an elevation of 10 m

The vapour pressure can be calculated from the temperature and relative humidity using the equation:

Screenshot from 2019 02 27 162657

The Australian formula includes the important factor of humidity and is somewhat more involved than the simpler North American model. The North American formula was designed to be applied at low temperatures (as low as −46 °C), when humidity levels are also low.

Bacchus Botrytis Disease Model

Botrytis cinerea is a disease that affects soft fruits, mainly wine grapes, usually affecting vines that experience constant wet or humid conditions, or over irrigation. It can lead to the loss of produce.

To detect Botrytis risk we monitor leaf wetness using the Decagon leaf wetness sensor. For best results the sensor is recommended to be placed NEAR the crop but not in it to avoid it being sprayed. It is also recommended that the sensor be mounted at a 10 degree angle so it is almost horizontal. This will give the best data results.

By measuring the duration that leaves have been wet, and utilizing a temperature sensor we can apply these to the Botrytis disease risk model and provide an output in the form of an infection index value. When this index reaches 1.0 there is a high probability of infection and crops should be sprayed or irrigation reduced or stopped.

Data from the leaf wetness sensor is fed into a model developed in New Zealand by Dr Balasubramaniam (Bala).

The equation specifically is:

I= 84.37 - 7.238T + 0.1856T2

Where I is risk index and T is the mean air temperature during a wet hour. The I values were summed for the duration of each wet period. After four hours of "dry" the risk index is reset.

Botrytis Table

Required Sensors: Temperature SensorLeaf Wetness Sensor

Can I Spray Indicator

This calculation is currently in trial

The Can I Spray Indicator uses wind, humidity, temperature and pressure (optional) to indicate if it is possibly safe to spray or not.

Harvest uses the following checks when determining if it is possibly safe to spray:

  • Average wind speed in 3-15 km/h range
  • Avoid Calm conditions (below the 3km/h)
  • Avoid spraying above 28 degrees C
  • Spray when delta T (Dry Bulb Temp - Wet Bulb Temp) is between 2 and 8, and not greater than 10

We assume our temperature sensors in a shroud to be close to Dry Bulb Temp, and utilise temperature, humidity, and pressure (optional) for calculating Wet Bulb.

Please note that this is just and indicator, and further checks need to be made based on the type of spray you are using, as well as any other factors such as local Council rules. This calculation also doesn't utilise rain as a component, as such if your spray requires a period of dry weather before or after spraying please take the time to do extra checks.

The converter uses guidelines set out by the Bureau of Meteorology (BOM) 

Required Sensors: Temperature/Humidity SensorWind Speed/Direction Sensor

Chill Hours

The Chilling Hours Model (Chandler 1942) is the oldest method to quantify winter chill that is still widely used. It considers all hours with temperatures between 0 and 7.2°C as equally effective for winter chill accumulation.

The formula below is utilised to produce an accumulating total while outside of the Growing Season set for your Harvest Site:

300px ChillHours

Required Sensors: Temperature Sensor

Chill Portions

The Chill Portions Model (otherwise known as the Dynamic Model) will count the amount of time spent between 1.6°C to 12.7°C. Instead of only calculating the time spent in the 'cold', it will subtract from the number for warmer temperatures.

The formula below is utilised to produce an accumulating total while outside of the Growing Season set for your Harvest Site:

436px ChillPortionsCorrect2

Constants - The experimentally derived constants slp, tetmlt, a0, a1, e0, and e1, were set to 1.6, 277, 139,500, 2.567 × 1018, 4153.5, and 12,888.8, respectively (Erez et al., 1988). Tk is the measured hourly temperature in Kelvin, whereas t denotes the time during the season (in hours) with t0 being the starting point of chilling accumulation.

The Dynamic Model (Erez et al. 1990; Fishman et al. 1987a, b), is designed for warmer climates, where models such as the original RCU can end up with negative accumulation early on.

Required Sensors: Temperature Sensor 

Dew Point

The Dew Point is the temperature at which a pocket of air (in this case around the temperature/humidity sensor) would form dew.

To calculate the Dew Point temperature, the unit requires a temperature and humidity sensor to be attached.

The Dew Point temperature is directly related to the temperature of the air in a given humidity. If the humidity is 100% then the Dew Point temperature will be equal to the temperature of the air. As the humidity decreases, so does the Dew Point temperature.

The Dew Point is helpful for predicting if and when dew will form on the vines. It is also important to note that if the Dew Point falls below freezing (0°c) then it is known as the frost point. This is a good indication that any dew forming will instead be in the form of a frost.

Required Sensors: Temperature/Humidity Sensor

Downy Mildew

Downy mildew is driven by the weather. The disease can devastate individual properties and in some seasons, affect production from the regions. It occurs sporadically according to the suitability of conditions for infection. Periods of high risk from Downy can be determined by monitoring the properties micro climate for factors such as temperature, rainfall, relative humidity (RH) and leaf wetness.

The rule of thumb for the primary infection of Downy Mildew is 10:10:24 which gives a guide to the conditions in which infection might occur, i.e. at least 10mm rainfall is needed while temperatures are at least 10ºC during a 24 hour period.

We have taken this rule of thumb and applied it to the weather station data to give a risk period for the primary infection of the disease.

Required Sensors: Temperature Sensor, Rain Gauge

Frost Potential

This calculation is currently in trial

If you have a Weather Forecast on your Harvest Site, you can now have a Frost Potential indicator added to it. This indicator will show the likelihood of Frost in your area (based on the forecast weather) with the following indices:

  • None
  • Low
  • Moderate
  • High

The system will provide a likelihood for Day (7am to 7pm) and Night (7pm to 7am).

Whilst the calculation of the Frost Potential is proprietary, the inputs utilised in determining the likelihood of Frost are: Dewpoint, Temperature, Wind Speed, Cloud Cover, and Precipitation.

Required Services: Weather Forecast

Growing Degree Days (GDD)

Growing degree days are used to estimate the maturity of crops during a growing season. We calculate GDD10 values (GDD50 for Fahrenheit), which means that the average daily temperature is accumulated only if it is above 10°C (50°F), and below 30°C (86°F). More specifically, we use the standard GDD formula:

GDD10 = max(0, Tavg−10)

GDD50 = max(0, Tavg−50)

where Tavg is a day's mean temperature. Each day's GDD value is then added to the current total for the period, and this summed result is displayed on the web page.

Required Sensors: Temperature Sensor

Potential Evapotranspiration (PET) 

Evapotranspiration is a term that describes the amount of water that travels to the air from sources of evaporation, such as water bodies, soil, and vine/tree canopies, and water lost through plant transpiration. 

Potential Evapotranspiration (PET) is defined as the amount of evaporation that would occur if a sufficient water source were available.

To calculate the Potential Evapotranspiration we require the weather station to have a temperature and humidity sensor, wind speed and direction sensor, and a solar radiation sensor. 

Data from the sensors are fed into a calculation based on an article (see the article here) by Jay M .Ham, Professor, Department of Agronomy, Kansas State University. The calculated value is recorded in mm/hr as well as total Evapotranspiration, in mm, experienced over the time period being viewed. 

By knowing how much water has been lost due to Evapotranspiration, a grower is able to confidently know how much irrigation will be needed to replace the water lost. 

Required Sensors: Temperature/Humidity SensorWind Speed/Direction SensorSolar Radiation Sensor

Potential Evapotranspiration Forecast

We are also trialling PET forecasting, this allows you to see the forecast potential evapotranspiration for the next 10 days based on the forecast temperature, humidity, wind, cloud cover, and geographical location.

Powdery Mildew (Gubler Model)

Powdery mildew is a fungal disease that affects many different plants. The damage is caused to both the leaves and fruit, and if left unchecked, will result in total crop loss as well as the potential death of the plant.

The calculation we use is based on the Conidial infections model developed by Doug Gubler from the University of California. It is an extremely complex and detailed calculation based upon the temperature we read from the desired sensor.

The optimal temperatures for Powdery Mildew Conidial development is between 21°C and 32°C. The greater the number of hours during the day that the temperature is within this range, the higher the risk for Powdery Mildew. 

  • Risk increases: With every day with equal or more than 6 hours of 21°C <= Temperature < 32°C ==> +20 Points
  • Risk decreases: With every day when temperature is 32°C or above or when 6 hours of at least 21°C are not reached ==> -10 Points

If the Powdery Mildew risk is less than 20 points the spraying interval can be extended. With 20 to 60 points the normal spraying interval is valid. If the risk is more than 60 points you should shorten the spraying interval.

You can read more about the control of Powdery Mildew using this model here.

Required Sensors: Temperature Sensor

Richardson Chill Units (RCU)

Plants need a certain amount of cold weather during the winter in order to mature properly later on. Straight chill units simply count the number of hours below 7°C, but we use the Richardson Chill Units to provide a more accurate model for orchards and vineyards. First we calculate the average temperature for each hour, and then use the table below to accumulate the RCU outside of the Growing Season set for your Harvest Site:

Temperature (°C)Temperature (°F)RCU (per hour)
T < 1.5 T < 34.7 +0.0
1.5 ≤ T < 2.5 34.7 ≤ T < 36.5 +0.5
2.5 ≤ T < 9.2 36.5 ≤ T < 48.6 +1.0
9.2 ≤ T < 12.5 48.6 ≤ T < 54.5 +0.5
12.5 ≤ T < 16.0 54.5 ≤ T < 60.8 +0.0
16.0 ≤ T < 18.0 60.8 ≤ T < 64.4 −0.5
T ≥ 18.0 T ≥ 64.4 −1.0

Required Sensors: Temperature Sensor

Soil Moisture Measurement using the Topp Equation

In June of 1980, G. Clarke Topp and his team members, J.L Davis and P. Annan, published a watershed paper which included what would come to be known as the Topp equation. The equation is so commonly used that some people don’t even know they’re using it. The Topp Equation makes it possible for us to measure an electrical property of the soil (the dielectric permittivity) and correlate that electrical property with the water content in the soil. 

It has been shown that the relationship between volumetric water content (θ) and dielectric water constant (Ka - the ratio of the absolute permittivity of a substance to the absolute permittivity of free space.) is essentially independent of soil texture, porosity, and salt content. The equation below was developed by Topp et al. (1980) for conversion of Ka to volumetric water content:

θ = -5.3 x 10-2 + 2.92 x 10-2 Ka – 5.5 x 10-4 Ka2 + 4.3 x 10-6Ka3


True Time Domain probes like the Acclima measure dielectric permittivity and report that value as well as calculating and reporting the Volumetric Water Content (VWC) using the Topp equation.

Click on this link to see Acclima True TDR sensors: Soil Moisture Sensor

Sunshine Hours

Sunshine hours or sunshine duration is a climatological indicator, measuring the duration of sunshine in a given period, usually over a day or a year for a given location on Earth. The number of sunshine hours achieved over a period can be considered as an indicator for the cloudiness of a location. 

To calculate sunshine hours for a given location with a Solar Radiation sensor (instead of an expensive pyrheliometer which would use the standard threshold of 120 Watts/m²), Harvest utilises a formula developed by Campbell Scientific. This formula uses the definition of sunshine hours as being "when the measured global radiation is greater than 0.4 times the potential solar radiation outside the earth’s atmosphere on a horizontal surface".

The Technical Note released by Campbell scientific can be viewed here.

Required Sensors: Solar Radiation Sensor 

Weather Forecasting

We offer a localised forecast provided by IBM - The Weather Company for any Harvest system. The forecast is specific to your location as a 4 km square section in the IBM grid - each square having it's own forecast. The forecast can be displayed on the Harvest Data Portal or Web App and we offer a 1-month free trial of this service.

IBM Forecast Screenshot2

Wet Bulb

Wet bulb is a calculation that refers to the lowest temperature on the vine that will be experienced by the evaporation of water alone.

To calculate the Wet Bulb temperature, the unit needs to have a humidity and temperature sensor attached.

Using the air temperature and the dew point (calculated with humidity and air temperature), the Wet Bulb temperature is able to be calculated.

The application of the Wet Bulb temperature is primarily in regards to frost protection. It is used to give an indication of when to start/stop misters or over-vine sprinklers to release latent heat and increase air temperature.

Required Sensors: Temperature/Humidity Sensor 

Wind Chill

Wind chill is the lowering of the body temperature due to the passing flow of lower temperature air. This is because the wind strips away the thin layer of warm air above your skin. The stronger the wind, the more heat lost from your body, and the colder it will feel. When the winds are light, it will feel closer to the actual air temperature.

Wind Chill Temperature is only defined for temperatures at or below 10°C and wind speeds above 4.8 kph. 

The equation specifically is:

Wind Chill (°F) = 35.74 + 0.6215T - 35.75(V0.16) + 0.4275T(V0.16)

Where T= Air Temperature (°F) and V= Wind Speed (mph)

We can then display this data in either °F or °C with our built in converters

Required Sensors: Temperature SensorWind Speed/Direction Sensor

Wind Measurements (Speed, Gust and Direction)

The World Meteorological Organisation (WMO) recommends the method that should be used to report wind measurements.

All Harvest wind measurements use these recommendations

Average Wind Speed is the 10 min average. 

Gust is the maximum 3 sec average wind speed during the same ten minute interval as the average speed.

The compass direction typically graphed is the scalar vector average of direction over the same 10 minute interval as the average speed.

If you wish to see the raw wind vane direction in degrees at each log then this can be plotted separately.

Some systems report logs more frequently than every 10 minutes. To ensure that these logs are not distorted due to a sampling interval of less than 10 minutes (or delayed by ten minutes) at Harvest we use a moving 10 minute average ie every minute the 10 minute window moves forward by one minute .