Solar Energy, 1972, Vol. 13, pp. 421-423. Pergamon Press. Printed in Great Britain

A Note on Dew Deposition on Pyrradiometers

E. I. MUKAMMAL*

(Received 8 January 1972)

DEw DEPOSITION over the polythene dome of a Funk-type pyrradiometer was found to result in an overestimation of incoming sky long-wave radiation but appeared to only slightly affect outgoing long-wave radiation. Experimental results are given for two pyrradiometers, each with its underneath surface enclosed within a cavity, one with a ventilation device to prevent dew deposition and the other without ventilation, The output of the non-ventilated pyrradiometer was at times as low as 50 per cent of the ventilated pyrradiometer.

1. DEW AND INCOMING SKY LONG-WAVE RADIATION Measurements of incoming and outgoing radiation were taken at about 4 m above the top of a

pine forest with two Funk (1959) type pyrradiometers. One measured net radiation and the other the downward component of net radiation. The underneath surface of the latter instrument was enclosed within a cavity, the temperature of which gave (black body) radiation falling on that side.

An unexpected gradual increase in incoming sky long-wave radiation (LD) followed by a sharp drop after sunrise was observed on nights with dew. Agents promoting an increase in Lo, such as clouds, the influx of a warmer, moister air mass, mist or fog do not appear to have been involved on such occasions. Figure la shows Lo on a typical night with dew when no cloud was observed.

The seemingly abnormal rise in measured LD was attributed to the deposition of dew, and the sharp drop after sunrise to the subsequent evaporation of dew on the polythene domes of the pyrradiometers. Dew on the domes would absorb LD and would radiate at its own temperature and emissivity. Thus, the upper surface of the pyrradiometer would receive higher radiation than had there been no dew present, since the effective temperature of atmospheric downward radiation on clear nights is much lower than the temperature of either the dew or the air above the pyrradiometer. The extent to which dew would obstruct LD would depend upon dew amounts and distribution and the size of the droplets. Of interest in Fig. l a is the slight gradual de- crease in measured LD between 0330 EST and sunrise, which is thought to be due to the evapora- tion of some of the dew as a result of relatively strengthening winds.

2. DEW AND OUTGOING LONG-WAVE RADIATION Upward long-wave radiation (Lv) evaluated from equation (3) given below would be only very

slightly affected by dew deposition.

Nl = S + L o - R - L u (1) N2 = S + L D - B (2)

Subtract (2) from (1) we get

Lv = B - R - ( N I - N z ) (3)

where (NO net radiation from a pyrradiometer with two polythene domes, ( N 2 ) net radiation from a pyrradiometer with the upper surface enclosed within a polythene dome and the under- neath surface enclosed within a cavity, the temperature of which gave (black body) radiation (B) falling on that side, (S) total solar radiation, and (R) reflected solar radiation.

*Canadian Meteorological Service. 421

422 E.I. MUKAMMAL

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Time (E.S.T.) 9 2 0 21 22 2~ 24 I 2 3 4 5 G

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Sept. f2,1967 Time (F.S.T.) Sept. 13,1967

Fig. I a. Incoming long-wave radiation above canopy on a typical night with dew.

Fig. lb. Ratio of radiation without to with dew on domes of a Funk-type pyrradiometer.

During the night R is zero and dew deposition over the upper polytbene domes of the two pyrradiometers can reasonably be assumed to be equal in amount and similar in distribution. Thus, Lu derived from equation (3) is in error to the extent of the difference between the energy emitted by the canopy and that emitted by the dew-covered underneath dome of the pyrradio- meter measuring N~. Dew temperatures over the dome are unlikely to be more than 1-2°C different from corresponding canopy temperature, and the surface emissivity is not believed to differ by more than 1 per cent for dew-covered needles, leaves and polythene dome. Thus, in the absence of flux divergence between the canopy surface and the level of the pyrradiometer, and ignoring the relatively small reflected long-wave radiation contribution, there is only a slight difference between radiation received by the underneath surface of the pyrradiometer from the canopy surface and that from the polythene dome covered with dew. In fact, this difference will be even smaller, since only very little dew is observed over the lower dome when the upper dome is completely covered with dew.

On a few nights with heavy dew hourly values of Lv were evaluated, converted to tem- perature by assuming an emissivity of 1 and compared with corresponding canopy air tem- perature. Quite realistic results were obtained, with air temperatures being generally higher by 1-2°C than radiation converted to temperature. Canopy temperature may actually have occasionally been lower than air temperature by a further 1-2°C and Lv overestimated according- ly, but by less than 3 per cent. Thus, it is not expected that dew formation on the lower dome would cause a large error in Lu as evaluated from Eq. (3).

During the night net radiation is not large. Consequently, the small error in Lu and the relatively larger error in Lo may result in a substantially magnified percentage error in observed net radiation from a pyrradiometer with dew-covered domes.

3. VERIFICATION BY EXPERIMENT To verify the effect on Lo of dew on the dome, two pyrradiometers similar to those mentioned

above but with the underneath surface of each enclosed within a cavity, were installed 25 cm above a grass surface in a forest clearing of about 300 m dia. One of the instruments was equipped with a ventilation device to prevent dew deposition on the polytbene dome and the other was unventilated. The device consisted of a plastic tube of about 1 cm dia. with 3 mm holes

Dew deposition on pyrradiometers 423

at 1 cm intervals encircling the periphery of the pyrradiometer. One end of the tube was blocked and the other connected to a blower with a container filled with a drying agent attached to the air intake. The air flow needed to prevent deposition of dew was about 3.7 x 104 cm 3 min-L The difference in temperature between the inner surfaces of the cavities enclosing the under- neath surfaces of the two pyrradiometers was less than 0.5°C, and was allowed for in the com- parison of the outputs of the two instruments. In a few tests made later using a different set of pyrradiometers the ventilation device did not apparently affect the instrument response. The ratios of the outputs of the pyrradiometer with, to the one without ventilation varied by about +_ 5 per cent with no bias in any particular direction.

Observations were taken on two nights with heavy dew, and the results are shown in Figure lb. It is evident that dew over the polythene dome was responsible for the underestimation of Lv. However, due to instrument error, the percentages given in Figure lb should perhaps be higher by about 6 per cent, since the ratio of the output of the two instruments prior to dew formation (1900-2000 EST) was about 94 per cent. Even with this correction there remains a substantial difference between the output of the two pyrradiometers, particularly during the latter part of the night when the pyrradiometer without ventilation was measuring only about 47 per cent that of the ventilated pyrradiometer.

Dew would interfere in radiation measurement, not only at night but also during the early hours of the morning, until such time as the dew has evaporated. In southern Ontario, for example, where the frequency of dew during the summer months is about three nights out of five, dew is known to remain on vegetation for about three to four hours after sunrise. Thus, it would seem imperative, at least in areas where the occurrence of dew is frequent, for Funk type radiometers to be equipped with a device to prevent dew deposition on domes.

REFERENCE Funk, J. P. Improved polythene-shielded net radiometer. J. scient. Instrum. 36, pp. 267-270 (1959).