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M. Fuchs
and
C. B. Tanner

Abstract

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M. Fuchs
and
C. B. Tanner

Abstract

A combination formula for evaporation that uses the surface temperature measured by infrared thermometers as a boundary condition is successfully tested against the evaporation obtained from a detailed energy balance and Bowen ratio measurements. The transfer coefficients used in the combination formula are obtained from aerodynamic similarity and include the effect of the diabatic turbulence. Two simple resistive models which attempt to account for the reduction of evaporation due to the water vapor desaturation of the soil surface are analyzed, but fail to describe correctly the transfer processes through the dry upper layer of the soil.

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M. Fuchs
and
C. B. Tanner

Abstract

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D. H. Sargeant
and
C. B. Tanner

Abstract

A simple apparatus has been devised which automatically reverses the vertical positions of two pairs of shielded and aspirated wet- and dry-bulb diodes. Analog voltage signals proportional to the dry and wet bulb temperatures T and Tw and to the differences ΔT and ΔTw are provided. The diodes present a low source impedance, a linear response over normal ranges, a sensitivity of about 2.3 mV(°C)−1, and as used here, a time constant of about one minute. From these measurements the Bowen ratio can be determined conveniently.

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G. W. Thurtell
and
C. B. Tanner

Abstract

A transistorized, battery-operated integrator suitable for obtaining the time average of fluctuating electrical analogue signals in the field with relative errors of about 0.1 to 0.2 per cent is described.

A solid state operational amplifier is used in an R-C integrator circuit. Continuous integration is accomplished by reversing the polarity of the input signal each time a fixed time-integral has occurred. The addition of a resistor to the ordinary R-C integrating circuit reduces the error resulting from the finite switching time. The integral is obtained by counting the input polarity reversals which are registered on a counter (mechanical or electronic) to be automatically recorded at desired intervals.

The use of transistor converters allows the complete circuit as well as additional equipment to be operated several days from one 12V automobile battery.

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S. B. Idso
,
M. Fuchs
, and
C. B. Tanner

Abstract

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J. P. Kerr
,
G. W. Thurtell
, and
C. B. Tanner

Abstract

The silicon photovoltaic solar cell has made possible the construction of simple pyranometers of reasonable accuracy. Cell response is linear, temperature sensitivity is low, and spectral response does not cause serious error, provided the cell is used in open sunlight.

The solar cell has been mounted beneath a special diffusing unit to obtain a rugged pyranometer with excellent cosine response. This pyranometer has been coupled with a solid state integrator developed for this purpose. The integral is recorded with either visual or printing counters.

Tests were made during September 1965 through February 1966 when low solar altitude and severe operating conditions would cause greatest error; and again during March 1966 through July 1966 when solar radiation intensities were high. For the first period the standard error of estimate and the solar radiation means were, respectively, 84 and 2200 Wh m−2 day−1. For the second period the corresponding values were 158 and 5630 Wh m−2 day−1.

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J. P. KERR
,
G. W. THURTELL
, and
C. B. TANNER

Abstract

The time and space variability of global radiation have been studied using data collected from a mesoscale network of integrating pyranometers established in Wisconsin, for the period December 1966 through June 1967. The data have been normalized so that they are expressed as a percent of the clear day global radiation. The atmospheric transmission coefficient over the State changes from about 0.75 in winter to 0.60 in summer. For a typical month, the standard deviations of the State daily mean varied from a few percent up to 50 percent of the State mean. Mean day-to-day changes of approximately ± 18 percent-radiation were recorded. From use of records for any one site in the State, the global radiation elsewhere in the State can be estimated with an approximate standard error of ±25 percent or less of the clear day radiation on a daily basis, ± 15 percent or less on a 5-day basis, and ± 10 percent or less on a monthly basis. Alternatively, if the network data from the sites surrounding the unknown point can be used for interpolation, the global radiation anywhere in the State can be estimated with an approximate standard error of ± 20 percent or less of the clear day radiation on a daily basis, ± 10 percent or less on a 5-day basis, and ± 6 percent or less on a monthly basis.

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M. L. Wesely
,
G. W. Thurtell
, and
C. B. Tanner

Abstract

A three-dimensional pressure-sphere anemometer and fast thermometer system (P.S.A.T.) was used to measure vertical heat flux density in the atmospheric surface layer at 1–4 m above alta fescue and snap beans. Good agreement with independent measurements was obtained, which shows the the P.S.A.T. is sufficiently small and has adequate high-frequency response and accuracy for eddy correlation measurements within 1 m of the surface. Also obtained with the P.S.A.T. were (uT )/( wT ), r u,T , r w,T and σ T /T* , and their dependence upon stability. When the atmosphere was thermally stable, slow wave motions frequently increased σ T even though turbulent mixing was lacking.

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G. W. Thurtell
,
C. B. Tanner
, and
M. L. Wesely

Abstract

A rugged and stable pressure-sphere anemometer system is described which Provides an accurate measurement of wind velocity and direction within a meter of the ground. The horizontal wind velocity, (u 2 + v 2)½, agreed very closely with cup anemometer measurements, indicating good accuracy in the measurement of the dominant term u. Eddy correlation measurements of shear stress with the pressure sphere agreed very well with Davis shear-stress meter measurements and satisfactory agreement was found with data obtained from wind velocity profiles and from wind measurements using a drag coefficient. Ratios of σ w /u * during neutral periods were found to be in excellent agreement with values derived by Panofsky and Lettau, providing further indication of the accuracy obtainable with the pressure-sphere system.

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