toip_logo_sc_tr_200

WEATHER STATIONS

 

IT’S ALL ABOUT THE WEATHER

 

Weather has a huge impact on our lives and on  the crops we grow. It makes a lot  of sense not just to monitor t the current weather, but also to keep a record of historic data which in turn becomes the basis for statistical analysis and inter and intra seasonal comparison.
 

 

 

 

What a weather station will look like will depend on the purpose for which it is used. In agriculture stations are normally 2m tall whereas for odour monitoring, sensors may also be placed at 10m. Most stations are designed ed for permanent deployment and will utilise a post set into the ground although portable stations, which employ a folding tripod base are also available. These have the advantage of being able to be quickly deployed at a site, then packed up and moved to another location.
 

 

 

 

Siting is an important consideration as it has a considerable influence on the quality of data a station returns. Typically you want a representative site: one which best matches the typical conditions on the site. If you are interested in monitoring for the highest winds or coolest temperatures then you are better advised to install a smaller station at these sites rather than compromising the integrity of the data from the  main station. As buildings and other structures create wind turbulence you need to keep your station well clear of them. A good guide is 10 times the height clear of upwind obstructions and 5 times the height for downwind obstructions.
 

 

SENSORS

 

Here is a list of the sensors you will most often find on a weather station-
 

 

TBSHT03_100

 

Air temperature: typically measured with a sensor based on a thermistor (made up of a junction of two dissimilar metals, the resistance of which changes in a predictable fashion). The sensor is normally installed in a radiation screen which protects it from the cooling influence of the wind and the warming influence of the sun. Screens need to be big enough to afford the sensor with sufficient protection although smaller screens can be used for in-canopy measurement. As temperature increases, plants have to increase their rate of transpiration to cool the cells in their leaves.
 

 

TBSHT03_100

 

Relative humidity: originally measured by first calculating the difference in temperature between a thermometer in the air (dry bulb) and a second thermometer cooled by a wick placed in water (wet bulb). The drier the air the more the  temperature difference between the two as more more moisture can be taken  up by the air. Electronic sensors have now become the norm for humidity measurement. Relative humidity (as distinct from absolute humidity) expresses the amount of moisture in the  air as a percentage of the total that could be held at that temperature and barometric pressure. The higher the humidity the less comfortable we feel at a given temperature because less cooling moisture can be  evaporated from our skin. The same goes for plants: as humidity increases, the level of transpiration possible reduces.
 

 

tbsws1_100

 

Wind speed: traditionally measured with a cup anemometer: the faster the wind blows, the faster the cups turn. A sender on the base of the unit typically outputs one or more pulses per revolution. The pulses are counted and converted to a measure of wind speed. Ultrasonic wind sensors are now available which remove the moving parts needed in cup anemometers. High winds can cause damage to crops and low winds are often a marker for the cold, low humidity conditions which precede a frost. As wind speed increases, a plant’s transpiration level increases: the more air molecules moving past the leaf surface, the more water molecules can be removed. As a consequence, many plant water use models make use of the “wind run” the number of metres of km the air has moved past the sensor on a given day

 

tbswv1_100

 

Wind direction sensors, or wind vanes, use a mechanical arm whose orientation follows the wind. A potentiometer (variable resistor) on the base of the sensor, converts the position to a resistance which is measured by applying a voltage across the resistor and measuring the voltage at the moving arm. A single resistor has a dead zone at north and to avoid this, some sensors use a pair of resistors mounted 90 degrees apart. One is designated the SIN and the other the COS and the two are combined to give the wind direction. Ultrasonic sensors return both wind speed and direction. They do this by sending ultrasonic pulses between 3 pairs of electrodes. Depending on  the speed and direction of the wind, different combinations will show higher or lower speeds. This information is then processed to derive WS & WD. Wind direction is very important  in  determining where spray or aerosol will travel when picked up by the wind. A wind rose is commonly used to view wind direction data: it shows the percentage of time the  wind has blown from each direction and the relative strength.
 

 

SDI-SR_120

 

Solar radiation is a measure of  the intensity of  the level of sunshine reaching the site. This varies according to the angle of the sun which changes not just according to the time of day, but also with the time of year. As sites go further north or south,  the  sun’s intensity falls . In most parts of Australia, midday solar radiation will peak at over 1000 W/m2 and in winter at 200 to 300 W/m2  Sunshine is the primary driver of crop growth and hence transpiration. Whilst applications such as evapo-transpiration modeling rely on full spectrum sensors, plant growth studies use  narrow band sensors which measure the level of energy in the frequencies relevant to transpiration (PAR or photosynthetically active radiation). Radiation sensors may be built with cheap phototransistors or more expensive thermopiles. Sensors should also include a dome which focuses the light ensuring that the same level of radiation is received regardless of the sensor to the sun. Values from a sensor  without a lens can only be compared at the same time each day

 

tbsmp02_100

 

Soil temperature is important to plant emergence, with most seeds only becoming active once the soil temperature reaches a threshold. Similarly many nutrients are not available to plants until the temperature gets above a set level. Although for many years, single level soil temperature sensors were the norm, most single level and profiling soil moisture probes now include temperature measurement.
 

 

rg-pr85

 

Rainfall is recorded to measure the water reaching the crop. In irrigated applications, it must be added to the water applied as irrigation to determine the total water use. The standard measurement device for rainfall is the  tipping bucket rain gauge. Water is collected in a standard sized funnel (typically 200cm2) and flows through a filter and siphon in to a pair of buckets. When the first bucket fills, the assembly tips, placing the  second bucket in the flow and emptying the first bucket.   A magnet fitted to the bucket activates a reed switch as the bucket tips, giving a pulse for every activation. t. For a given funnel size, the bucket size can be set to correspond to a set volume of rain: with one tip per 0.2mm and 1 tip per 0.1mm being the most common. The simpler tipping spoon gauges employ a single spoon which tips when full.