| Climate / Weather Monitoring |
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| 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 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. |
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| What an automatic weather station (AWS) will look like will depend on the purpose for which it is used. In agriculture, stations are normally 2m tall, whereas for odour or inversion monitoring, sensors may also be placed at 10m. Most stations are designed 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. |
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| AWS 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. |
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| Sensors | Here is a list of the sensors you will most often find on a weather station. Check our Products section for details on the range we carry. |
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| Air Temperature | Air temperature is 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 must be 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. In very cold conditions, the cells can freeze and die. View our Temperature Sensors range. |
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| Relative Humidity | 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. Relative humidity was 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. This difference - the wet bulb depression or Delta-T, was then used to calculate the relative humidity. Electronic sensors have now become the norm for humidity measurement. There is also a lot you can do once you have air temperature and relative humidity: you can combine them using formulae in order to derive: wet bulb temperature, dew point, vapour pressure, vapour pressure deficit etc. Because relative humidity is dependent on the temperature, temperature and humidity sensors are usually combined into the one instrument. View our Relative Humidity Sensor range. |
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| Wind Speed | Wind speed has long been 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. Sensor manufcturers can design aneometers with precisely the characteristics they need: with price increasing as the accuracy increase, the starting threshold falls and the life increases. 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. View our Wind Speed Sensor range. |
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| Wind Direction | 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, higher quality sensors use a pair of variable 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. Replacing the variable resistor with a digital position sensor avoids the dead zone problem and allows for a sensor with longer life and no dead zone. 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, the different signal paths 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. View our Wind Direction Sensor range. |
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| Solar Radiation | Solar radiation is a measure of the intensity or 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 and the latitude. 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 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). Solar Radiation sensors may be built with cheap phototransistors or more expensive thermopiles. The latter are called pyranometers and have a much wider spectral response and higher accuracy than the photo-transistor based devices. 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. Sensors which rely on a diffuser rather than a dome lens, will have higher levels of variation in output due to latitude and changes in the sun's position. View our Radiation Sensor range. |
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| Soil Temperature | 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 soil moisture probes now include temperature measurement as an output. View our Soil Temperature Sensor range. |
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| Rainfall (Precipitation) | Rainfall is expressed as the depth of water in mm which reaches theground - showing the volume of water reaching the crop (1mm of water over 1 m2 of area = 1 litre of water). In irrigated applications, rainfall must be added to the water applied as irrigation to determine the total water received by the crop. 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 into 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. For a given funnel size, one tip of the buekct will correspond to a set volume of rain: with one tip per 0.2mm and 0.25mm being the most common. The simpler tipping spoon gauges employ a single spoon which tips when full. They are cheaper to produce but are not as accurate in high rainfall conditions. View our Rain Gauge range. |
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| Barometric Pressure | Air pressure is a measure of the pressure exerted by the air around us. The reference or standard air pressure of 1 bar, is taken at sea level at a temperature of 25 degrees. Air pressure is usually shown in "mBar" or milli-bars, where 1000 mBar = 1 Bar. 1000 mBar is also equal to 100 kPa (1 Bar = 10m of pressure head = 100 kPa). As the temperature of a parcel increases, so too will its pressure (the air molecules vibrate more strongly against one another as temperature rises. Air pressure will also rise with falling humidity: the less moisture there is in the air, the more it will be heated by a given amount of sunshine. Air pressure is measured with a pressure transducer and the typical range is 900 to 1100 mBar. Barometric pressure sensors are available as a stand alone sensor (YDOC Baro expansion board) but it is often more economical to include it in a temperature - humidity sensor (e.g. ERHTP20, HTP06 and HTP05). View our Barometric Pressure sensor range. |
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| Weather Summaries and Indeces | Once you have collected basic weather data, there is a lot that you can do with it in order to add value. The simplest form is to provide time based summaries e.g. daily average, minimum and maximum temperature, daily rain. Users may also like to see the current values displayed on a "virtual instrument". There are a range of wind calculations such as wind gust (highest wind speed maintained for a period of 3 seconds) and standard deviation of the wind speed (a vector calculation of the variability of the wind direction over a time period). |
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| But there are also a wide range of calculations which combine data from several sensors in order to produce a new parameter or index. Examples of these are: - fire danger index (air temp, RH, wind speed) - evapotranspiration (ET from air temp, RH, wind speed and solar radiation) - black bulb globe temperature - heat stress & chill for animals - disease models (botrytis, powdery mildew, downy mildew etc.) - vertical temperature difference - frost and inversion risk. |
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