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  • Author or Editor: S. M. McGinn x
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S. M. McGinn
H. D. J. McLean


The automation of weather stations necessitates an alternative approach to the traditional manual measure of free-water evaporation made using a class A pan. This study compared commercially available water-level sensing transducers mounted on class A pans to manual measurements using a class A pan. Measurements of free-water evaporation with two automated transducers over a 24-h period resulted in mean differences of 0.23 and 0.98 mm. Hourly measurements for free-water evaporation allowed examination of the correlation between principal weather elements and evaporation. Evaporation from the pan was highly correlated with wind speed at night (r = 0.86) and with air temperature during the day (r = 0.75). In addition, it was found that during the summer some 33% of the daily free-water evaporation occurred at night. For a 24-h period, accumulated free-water evaporation was highly correlated with air temperature (r = 0.85), net radiation (r = 0.81), incoming solar radiation (r = 0.80), and wind speed (r = 0.69).

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S. M. McGinn
K. M. King
, and
G. W. Thurtell


The objective of this study was to improve the accuracy of net radiation (R n ) measurements under conditions conducive to dew or frost deposition. Two nonventilated net pyrradiometers were mounted over grass during November and December 1986. A heating element was located on the supporting arm of each radiometer so that heal would be conducted to the sensing head. Heat was applied to one radiometer for a few days, followed by a period during which no heat was applied. The procedure was repeated, alternating between radiometers throughout the experiment. Heating the radiometers successfully averted the deposition of dew and frost on the domes, which produced errors in R n as high as 54 W m−2. The effect of heating alone was slightly asymmetric and resulted in a significant decrease in the mean R n of 8 W m−2 relative to the unheated radiometer. This effect can be compensated for in the calibration of the radiometer.

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