Taconite Inlet Project



3. Equipment/calibration & images

Over the three year period of the project, the number of variables being measured was expanded. Consistency was maintained from year to year, however, in terms of station location, types of sensors used at each site, and in the heights of sensors above the ground.

This section presents details on the meteorological sensors and their installations. The measurements themselves, numerical processing by the datalogger, and the subsequent adjustment and quality control of data are discussed in subsequent sections.



a. Air temperature

Air temperature measurements were all made with the same type of sensor, a Fenwal Electronics UUT51J1 thermistor incorporated into a Campbell Scientific, Inc. (CSI) model 207 probe, and housed in a naturally ventilated 12 Plate Gill Radiation Shield. (IMAGE: sensor in use) The resolution of this sensor, over a measurement range of -20° C to +20° C, with a 12 bit conversion, is 0.035° C (Tanner, 1990). The overall probe accuracy, in a worst case additive error situation, is specified by CSI as +/- 0.4° C, which is probably of the same magnitude, or less than, the error associated with radiation shield limitations (Tanner, 1990). The sensor height was consistent from year to year at 180 to 190 cm above the ground.

The 1992 air temperature values have been adjusted slightly, based on a four day post-field season intercomparison of the sensors used at Delta and Echo. Using an average difference in the comparison period values of 0.16° C, Delta values were adjusted up 0.08° C, and Echo values down 0.08° C.

Time Plots of ECHO Weather Data index page

Weather Station ECHO index page


b. Atmospheric water vapor

Water vapor measurements were of relative humidity (RH), which was measured directly, and vapor pressure, which was calculated from the RH and temperature measurements (1992 only). Relative humidity was measured by a CSI model 207 probe, comprised of a Phys-Chemical Research Corp. model PCRC-11 RH sensor, which detects vapor adsorption onto a sulfonated polystyrene surface. The probe was housed, with the air temperature sensor, in a naturally ventilated 12 Plate Gill Radiation Shield. (IMAGE: sensor in use) Because the model PCRC-11 sensor loses stability through time, the RH chip was either purchased or replaced within one month of field use, in both years. The manufacturer's stated accuracy of the sensor is +/- 1 percent, although operational accuracy has proven to be more like +/- 5 percent (Tanner, 1990). A limited number of comparisons at Taconite Inlet, between relative humidity measured by the RH sensor and handheld psychrometers, revealed acceptably small differences. The sensor height on the weather stations was consistent from year to year at 180 to 190 cm above the ground.

The 1992 RH values have been adjusted slightly, based on a four-day post-field season intercomparison of the sensors used at Delta and Echo. Based on an average difference in the values of 1.2 percent, Delta values were adjusted down 0.6 percent, and Echo values up 0.6 percent.

Time Plots of ECHO Weather Data index page

Weather Station ECHO index page


c. Wind

The Echo station used a Met One 014A cup anemometer, and a Met One 024A wind vane. (IMAGE: sensor in use) The accuracy of the anemometer was within 0.1 ms^-1 or 1.5 percent of full range (0-45 m^s-1), with a starting threshold of 0.45 m^s-1. Wind vane accuracy was +/- 5° , with a threshold of 0.45 ms^-1. Although the accuracy and starting thresholds of this sensor and that at Delta appear to be very similar, Tanner (1990) points out that comparison of sensor performance from manufacturer's specifications is difficult. No further attempt will be made here. When the Echo sensors were installed, the height of a horizontal axis through each instrument was 240-250 cm.

Time Plots of ECHO Weather Data index page

Weather Station ECHO index page


d. Atmospheric pressure

Not measured at Echo weather station.


e. Solar radiation

Incoming solar radiation (K-down) was measured throughout the study period as the total short-wave flux on a horizontal surface. This flux is typically termed global radiation, integrating both direct and diffuse components. All measurements were made with Eppley Laboratory, Inc. Black and White Pyranometers, model 8-48, which are thermopile-type detectors. The domes on all pyranometers were Schott WG7 glass, with a manufacturers stated transparency range of 0.28 to 2.8 µ m. Eppley states the instrument has a linearity of +/- 1 percent from 0 to 1400 Wm^-2, and the following cosine response: +/- 2 percent from 0-70° zenith angle, and +/- 5 percent from 70-80° zenith angle. The instruments at all installations were kept level, at between 1.05 and 1.5 m above the snow-free ground surface.

Reflected solar radiation (K-up) was measured in 1992, also with an Eppley model 8-48 pyranometer. Each instrument was mounted in an inverted position beneath the K-down radiometer, at one end of a horizontal support. The other end of the support was attached to a vertical shaft. (IMAGE: sensor in use) The radiometers were situated 3 to 4 m from the tripod.

Calibration of the short-wave pyranometers was done by Eppley, with respect to the World Radiation Reference (+/- 2 percent for model 8-48). The specific instrument used in 1991 to measure K-down had most recently been recalibrated in May 1982, because prior to this field season, the instrument only had an estimated six months of cumulative use following the 1982 calibration. In April 1992, two additional 8-48 pyranometers were recalibrated by Eppley Labs for use during the 1992 season. These two radiometers were used to conduct a clear-sky comparison with the instrument used to measure K-down in 1991. Measurements were made on the roof of Morrill Sciences building at the University of Massachusetts. Measurements both before and after solar noon indicated that the older instrument was within 0.5 percent of one of those recently recalibrated, closer than the recalibrated pyranometers were to one another. Therefore, a pair of "matched" instruments was used together during the 1992 field season (i.e. for K-down and K-up). The instruments recalibrated in April were done again after the 1992 field season, and not found to have changed significantly. The significance of this calibration discussion is that solar radiation measurements over the two years were both as accurate, and as precise, as possible using 8-48 pyranometers.

Time Plots of ECHO Weather Data index page

Weather Station ECHO index page


f. UV-B radiation

Not measured at Echo weather station.


g. Thermal radiation

Thermal radiation was measured through the 1992 field season only. Thermal irradiance from the atmosphere, or incoming long-wave (L-down), was measured directly, while thermal exitance, or outgoing long-wave (L-up), was computed from remote measurements of surface temperature.

Incoming thermal irradiance (L-down) was determined using an Eppley Laboratory, Inc. model PIR pyrgeometer. The PIR measures 4-50 µ m wavelength radiation transmitted through a coated silicon hemisphere. Manufacturer's stated linearity is +/- 1 percent, and for a diffuse radiation source there is an insignificant cosine response. Field measurements were made of the pyrgeometer thermopile signal (mV output), rather than through the thermistor-battery-resistance circuit incorporated into the PIR.

Field measurements of pyrgeometer case thermistor resistance were also made, using a DC half bridge (YSI model 44031 precision thermistor). These resistances were used to determine the pyrgeometer case temperature, allowing a correction to be entered into the L-down computation accounting for changes in ambient temperature.

The pyrgeometers at both sites were mounted at the end of vertical rods at 1.6 m, adjacent to the tripod. (IMAGE: sensor in use) The pyrgeometers were not ventilated, due to logistical considerations.

Calibration of the pyrgeometers was done both before and after the 1992 field season. One instrument was calibrated at the Eppley Laboratory (in April), and all other calibrations were conducted at the National Atmospheric Radiation Centre (NARC) of Environment Canada. The use of calibration factors is discussed further in the quality control section.

Thermal exitance from the surface (L-up) at each station in 1992 was computed from remote surface temperature measurement by an infrared temperature transducer, or staring radiometer. The instruments used were Everest Industries, Inc. model 4000A, mounted at 1.7 m, on or near the weather station tripod. (IMAGE: sensor in use) The field of view is 15° , resulting in a temperature measurement from 1.7 m of approximately a 900 cm^2 area. The instruments use filters to achieve a spectral band pass of 8 to 14 µ m wavelength. The manufacturers stated accuracy is +/- 0.5° C, with a repeatability of +/- 0.1° C.

Surface temperature values measured by the staring radiometers were uniformly adjusted, based on calibration at 0° C. The adjustment, or offset, was determined by plotting snow surface temperatures during a period of warm temperatures and uniform snow cover, when the snow was melting. The interval used was a 38-hour period from the afternoon of June 20 to early on June 22, when snow surface temperature was very consistent. Measured values indicated an offset of 1.51° C (+/- 0.13 S.D.) (38 hours at 60 measurements per hour yields n = 2280; deviation calculated from n = 38). This field calibration was supported by a laboratory experiment using an ice-water bath, which confirmed the sense and relative magnitude of the offset, over a shorter averaging period.

Time Plots of ECHO Weather Data index page

Weather Station ECHO index page


h. Net all-wave radiation

During the 1992 field season, net radiation was computed from measurement of the four components. In 1991 an instrument was used to directly measure net all-wave irradiance. For comparison, the two different sets of instruments were operated simultaneously for 113 hours in 1992 over snow-free tundra. The net irradiance values were highly correlated (r = 0.97), and mean values for the period differed by only 3.4 percent. The two methods thus appear comparable as a measure of net radiation, at least over snow-free tundra.

The 1991 measurement of net all-wave radiation was done with a Micromet Instruments miniature net radiometer, also referred to as a Fritschen type. This is a temperature compensated device that measures the difference between incoming and outgoing radiation. The manufacturers stated spectral response is from 0.3 to 60 µ m. The instrument had new domes installed, and was calibrated by the manufacturer, in April of 1991. The instrument was mounted approximately 1 m horizontally from the weather station tripod, at a height above the snow-free surface of 0.8 m. Radiometer domes were only irregularly cleared of all moisture following precipitation. No adjustment has been made to net radiation data to compensate for precipitation wetness effects and/or riming.

Time Plots of ECHO Weather Data

Weather Station ECHO index page


i. Precipitation

Precipitation measurement at Echo was unsuccessful, due to frequent snowfall events, high wind, and infrequent inspection of the gauge (i.e. once every 3 to 5 days).

Time Plots of ECHO Weather Data

Weather Station ECHO index page