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The dry bulb temperature is simply the air temperature. That is, it is a measure of the kinetic energy of the molecules in the air. It is measured inside a Stevenson screen using a PT100 Resistance Temperature Dependent Thermometer.
The wet bulb temperature is the temperature air would have if its energy were used to evaporate an amount of water equal to the amount of water vapour it contains. It is measured inside a Stevenson screen using a PT100 Resistance Temperature Dependent Thermometer surrounded by a moist wick.
Pressure is a force per unit area. Atmospheric pressure at some point is equal to the weight of the vertical column of air above the point, per unit area. It is measured using a Pressure Transducer.
Vapour pressure is the pressure exerted by the motion of molecules of water vapour. It is dependent on the amount of water vapour in the air and the temperature of the air. The AWS derives vapour pressure from the wet and dry bulb temperatures.
The saturation vapour pressure is the vapour pressure the air would have if it were saturated. It is simply a function of temperature. The data logger derives the saturation vapour pressure from the dry bulb temperature.
Relative humidity is the ratio of vapour pressure to saturation vapour pressure, expressed as a percentage. It is 0% for completely dry air and 100% for saturated air. Relative humidity data provided by the AWS is derived from the wet and dry bulb temperatures by the data logger.
Precipitation is water, in liquid or solid form, that is falling upon the surface. Thus it includes rain, hail and snow. Typically, it is reported in units of liquid depth per unit time. For example, millimetres per hour. Precipitation is measured using a Tipping Bucket Rain-Gauge.
Wind direction is a measure of the direction of the horizontal air movement reported in degrees clockwise from true north. Wind Direction is measured using a Sonic Anemometer.
The standard deviation of wind direction provides an indication of the degree to which the wind direction is changing. It is derived by the data logger.
Sunshine duration is a measure of the percentage of bright sunshine observed. It is related to the duration and intensity of direct solar radiation as opposed to diffuse radiation and gives an indication of the presence of cloud. It is measured using a RS-4 Sunshine Duration Detector.
Global short wave radiation is the incident short wave radiation and comprises the direct and diffuse components. It is measured using a Middleton EPO7 Solarimeter.
Diffuse short wave radiation is the diffuse component of the incident short wave radiation. It is measured using a Kipp and Zonen CM5 Solarimeter. A shade ring is used to block out the direct short wave radiation.
Reflected short wave radiation is the short wave radiation coming from the surface of the earth. It is related directly to the global short wave radiation and the surface albedo. Reflected radiation is measured using a Middleton EPO7 Solarimeter.
Net all wave radiation is the downward radiation less the upward radiation. It includes both long and short wave radiation. Net radiation is measured using a REBS Net Radiometer.
UVB Radiation is part of the electromagnetic spectrum with a wavelength between 0.29 and 0.32 μm, which is shorter the the wavelength of visible light.
Longwave radiation is part of the electromagnetic spectrum with a wavelength between 0.77 and 1000 μm, which is longer than the wavelength of visible light.
Soil temperature is simply the temperature of the soil at various depths (1cm, 5cm, 10cm, 10cm, 50cm and 100cm) and is measured using omego thermistors encased in epoxy and housed within a stainless steel tube.
Soil heat flux is the rate at which heat is being conducted through the soil. It is measured using Huxeflux Soil Heat Flux Plates placed at depths of 5 cm and 50 cm.
Dry Bulb Temperature is measured using a PT100 Resistance Temperature Dependent Thermometer (RTD) which forms part of a modified Vector Instruments H301 Psychrometer. Wet Bulb Temperature is measured using the same instrument, except that the Wet Bulb RTD is wrapped in a wick kept saturated with distilled water. Ventilation of the instrument is by means of a motor and fan assembly. The psychrometer is mounted inside a Stevenson screen at a height of 1.2 m. The output voltage from the RTD is converted to temperature by the data logger.
The instrument has been modified as follows and is now referred to as a Nurse X302:
The original wick has been replaced by five strands of Bureau of Meteorology wick and muslin.
The motor and fan assembly has been modified with a larger fan (12V) and the fan blades are now at the top of the air duct instead of the middle for easier cleaning etc.
The pressure transducer is mounted inside the datalogger enclosure however the door is not sufficiently air-tight to prevent a correct pressure reading.
The weight of the water in the bucket makes it tip, a reed switch is actuated by a magnet attached to the bucket, and the other bucket (there are two) moves into position. One bucket tip occurs each 0.2mm of rain. The contact closure of the reed switch is counted by the logger. Thus the number of times the buckets tip multiplied by 0.2 gives the amount of rain in mm.
Some possible errors:
if rain is flowing into a bucket while it is tipping and is spilled;
if the rain collected in a bucket is not sufficient to tip it and the water evaporates;
partial loss can occur if the funnel is blocked by hail or snow and overflow occurs or late data can be recorded when the hail and snow melt;
debris can cause funnel blockage and complete loss of data
spider webs can hinder and prevent bucket tipping.
Specifications
Accuracy: ±2% for intensities from 25 to 600 mm/hr
Wind speed and direction are measured with a Met One sonic anemometer. This sensor sends and recieves sonic pulses (200 KHz) to determine wind speed in the U and V axes (ie N-S and E-W) and to compute wind direction.
Since it has no moving parts and does not require periodic calibration it requires substantially less maintenance than the more traditional cup and vane anemometers.
Specifications (general)
Maximum operating range: 0 to 85 m/s
Operating Temperature: -30 to +55 ºC
Specifications (Wind Speed)
Accuracy: ±1% of reading or 0.5 m/s whichever is greater
The RS-6 comprises 6 pairs of bi-metallic blackened elements, equally spaced under an acrylic dome. Each element pair has a pre-set contact gap. The inner and outer elements of each pair deflect by the same amount as the ambient temperature changes so their contact clearance remains unchanged. In direct sunlight the outer elements absorb more radiant energy than the shielded inner elements and therefore deflect more and make contact with the inner element completing a circuit and sending a voltage to indicate to the datalogger that the sensor is in direct sun.
Incident radiation causes a thermopile in the EP07 to produce an output voltage which the data logger converts to W/m². The thermopile's warm junctions are attached to a black surface housed within a double glass dome and its cold junctions are attached to the base of the instrument.
Global Short Wave Radiation is measured using a Middleton EP07 Solarimeter mounted pointing upwards at 1.2m above the ground.
Reflected Short Wave Radiation is measured using a Middleton EP07 Solarimeter mounted pointing downwards at 1.2m above the ground.
Like the Middleton EP07, Incident radiation causes a thermopile in the CM5 to produce an output voltage which the data logger converts to W/m². The thermopile's warm junctions are attached to a black surface housed within a double glass dome and its cold junctions are attached to the base of the instrument.
Diffuse Short Wave Radiation is measured at 1 m above the ground using a Kipp and Zonen CM5 Solarimeter fitted with a shade bar. The shade bar is 76 mm wide and is set at an angle for latitude -33 degrees so that it blocks direct radiation reception on the glass dome of the solarimeter. The bar must be adjusted north and south weekly for the angle of the ecliptic above the horizon.
Incident radiation from the sky (mainly shortwave) is absorbed by the upper sensor disc, resulting in an elevation of it's temperature. Incident radiation from the ground (mainly longwave) is absorbed by the lower sensor disc, resulting in an elevation of ite temperature. A temperature gradient form between the upper and lower junctions of the thermopile resulting in a linear voltage output which is proportional to the differential magnitude of the upper and lower irradiance. The sign convention is downwards is positive.
The UVR1-B is a precision filter pyranometer for measuring solar global ultraviolet irradiance. The UVR1-B is suitable for biological and human erythema (sunburn) monitoring.
The MS-201 measures longwave radiation (>3.0 μm). A specially coated silicon dome blocks out radiation < 3.0 μm.The output of the thermopile sensor is added automatically to the output of a built-in temperature compensation circuit that incorporates a thermistor to produce the correct electrical signal corresponding to the incident infrared radiation.
