Automatic Weather Stations

Important Notice

Evapotranspiration figures quoted are Reference Crop Evapotranspiration figures and are calculated from climatic data using the Food and Agriculture Organisation of the United Nations Penman-Monteith equation published in FAO Irrigation and Drainage Paper 56: Crop evapotranspiration - Guidelines for computing crop water requirements.

These figures are intended only as a guide as actual Crop Evapotranspiration will vary depending on factors such as inter-row management. These figures are for use in ESTIMATING crop water requirement and should only be used with appropriate CROP COEFFICIENTS.

Crop Related Information

Evapotranspiration
Frost Hours
Daylight Hours
Degree Days
Chill Days

Data Records

Air Temperature

The air temperature sensor is housed in a protective enclosure, which shields it from rain and solar radiation, but allows free airflow.

Apparent Air Temperature

Apparent air temperature is a comfort indication. It is used to indicate the approximate temperature as felt by an adult in average clothing, standing in the sun. Our calculation uses the Steadman model, which is derived from air temperature, humidity, wind speed and radiation. Note that this differs from the calculation used on the Bureau of Meteorology website, which excludes the radiation component. The formula used here is:

AT = Ta + 0.348×e − 0.70×ws + 0.70×Q/(ws + 10) − 4.25

Dew Point

Dew point is the temperature at which moisture from the air condensates to form dew. It has a correlation to temperature and humidity. The dew point will be closer to the air temperature during higher humidity. The dew point calculation is pressure-corrected using the mean standard pressure at the station's altitude.

Relative Humidity

Relative humidity is the degree of saturation of water vapour in the air, at the given temperature.

Delta-T

Delta-T is the agricultural term for wet bulb depression. It is generally used as an indicator for evaporation, and droplet lifetime for spraying applications. The wet bulb temperature is not measured directly, but is approximated from the measured air temperature and relative humidity.

Wind Direction/Bearing

Average wind direction is calculated from the sine and cosine components of instantaneous wind direction, measured every five seconds. The direction is also displayed as a 16-point compass bearing, averaged over ten minutes. Wind speed and direction are measured 2 metres above the ground, whereas BoM weather stations typically measure wind velocity at a height of 10 metres.

Wind Speed

Average wind speed is calculated over a ten-minute interval. The maximum wind speed (gust) for this period is measured over a five-second interval. Wind speed and direction are measured 2 metres above the ground, as required for ETo formulae, whereas BoM weather stations typically measure wind velocity at a height of 10 metres.

Soil Temperature

The soil temperature is measured 100mm under the soil surface. Inter-station comparisons may differ markedly due to varying soil types and surface material or mulch.

Solar Radiation

The solar radiation measurement is made on the horizontal plane, and measures wide-spectrum incident radiation above the horizon. It is sometimes termed 'global solar radiation', as opposed to net radiation which subtracts radiation reflected from the earth's surface back into the atmosphere.

Rainfall

The 10-minute rainfall field contains the rainfall for that period only. The cumulative total for the current day, since 9 am CST, is displayed on the 'Summary' page.

Leaf Wetness

The leaf wetness is an indication of rain or dew formation. Readings between 0% and 100% generally indicate spot rain, or a drying surface, whereas 100% suggests dew or heavier rain. The sensor may be installed in a nearby crop canopy.

Grower and Irrigator Information

Evapotranspiration

Reference Evapotranspiration (ETo) is estimated from measured weather phenomena, using a complex calculation. A large number of factors contribute to the final result. The ETo figure aims to standardise evapotranspiration based on observed weather conditions. Note that Evapotranspiration is rather different from evaporation, which is normally measured using a Class A Evaporation Pan, and measures evaporation from an exposed body of water - it does not account for evaporation from a soil surface, and does not include plant transpiration. The crop evapotranspiration calculation has been published by the Food and Agriculture Organisation of the United Nations in the FAO "Irrigation and drainage paper 56".

Reference Evapotranspiration (ETo) figures are normally used to calculate Crop Evapotranspiration (ETc), using a crop coefficient (Kc). The recommended crop coefficient may change over the crop's various growth stages. Information for calculating Crop Evapotranspiration, including a list of crop coefficients, can be found here at the FAO document repository. Complete evapotranspiration documentation begins here.

Frost Hours

Frost hours are the period for which the temperature is below two degrees Celcius (2°C). The total duration is accumulated over 24 hours.

Daylight Hours

Similar to frost hours, this is the period for which solar radiation is apparent above low levels. The total is accumulated over 24 hours.

Degree Days

Degree-days is accumulated for all temperature observations above a reference. The calculation is the integral of the temperature above this reference over 24 hours. For example, if the temperature is 14 degrees for the whole day, 4 degree-day 'units' are accumulated. Similarly, if the temperature is 34 degrees for two hours, 22 degrees for 4 hours, and then falls below 10 degrees, this is also 4 degree days. In practice, the temperature is integrated on a much finer time-scale, namely every 5 seconds.

Cumulative degree day units over a growing period are generally used to predict crop ripening, though the reference may differ for different crops. Our data uses a reference of ten degrees (10°C).

Chill Days

The complement of degree days, this is, the integral of all temperatures below ten degrees (10°C)

Tipping Bucket Rain Gauges

The tipping rain gauges record daily rainfall. Rainfall data is updated periodically.

Sensors

The following phenomena are measured directly using the sensor listed. The remaining data fields are derived from calculations using data from one or more of these sensors.

Phenomenon	Accuracy	Unit	Sensor
Air Temperature	± 0.2	°C	Rotronics HC2-S3
Relative Humidity	± 2 typ.	%	Rotronics HC2-S3
Radiation	± 10	W/m²	LP02 pyranometer
Soil Temperature	± 0.2	°C	Buried thermistor
Wind Speed	± 0.3 / 2 %	m/s	PA2 wind sensor
Wind Direction	± 3	°	PRV wind sensor
Rainfall	1% ± 0.2	mm	TB6 tipping bucket rain gauge
Leaf Wetness	-	%	MEA2040 leaf wetness sensor

More Information

For more information on MEA Automatic Weather Stations and sensors, or related products and services, visit Green Brain.

References:

greenbrain.ag

www.bom.gov.au/lam

www.fao.org

www.wikipedia.org