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 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

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


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

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:

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. The beauty of the equation is that it is pretty accurate in most typical soils, saving us the job of doing custom calibrations.

The equation specifically is: 

T (m3/m3) = 4.3 X 10-6 * e3 - 5.5 X 10-4 * e2 + 2.92 X 10-2 * e -5.3 X 10-2

Topp and his team showed that the volumetric water content of soil can be determined from the apparent dielectric constant of the soils, independent of soil type, soil temperature, and soluble salt content.

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.


Required 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. 

Sunshine hours are defined as the total number of hours in which the direct solar irradiance exceeds a threshold of 120 Watts/m²

The formula that we use to determine the sunshine hours for a given location was developed by Campbell Scientific.

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