e. alvarez, b. garca, r. green, s. kardel, r.m. ros, e ects of...

SpS17, Light Pollution: Protecting Astronomical Sites and Increas- ing Global Awareness through Education Proceedings IAU Symposium No. xxx, 2012 E. Alvarez, B. Garca, R. Green, S. Kardel, R.M. Ros, M. Stavinschi & C. Walker, eds. c 2012 International Astronomical Union DOI: 00.0000/X000000000000000X Effects of cloud amount on the night sky brightness Chu W. So 1 and Chun S. J. Pun 2 and Chung F. T. Wong 3 1 Department of Physics, The University of Hong Kong, Pokfulam, Hong Kong email: [email protected] 2 email: [email protected] 3 email: [email protected] Abstract. The cloud amount and the night sky brightness (NSB) were observed simultaneously and continuously from more than 320 hours of observations in an urban and a rural station in Hong Kong. The cloud amount strongly affect the NSB observed, with effects up to 3 mag in a single night. Night skies were generally brighter during overcast, with strong correlation observed in 63% / 45% of nights in urban / rural sites. This project is supported by the Environment and Conservation Fund of the Hong Kong SAR government (ECF 10/2009, ECF 1/2007). Keywords. Measurement: night sky brightness, light pollution, atmospheric effects, moon. Simultaneous cloud and NSB (taken with Sky Quality Meters - Lens Ethernet) observations were collected in two locations of the Hong Kong Night Sky Monitoring Network (Please refer to the contribution by Pun et al. in this volume), one urban (HKU ) and one rural (iObs ) since November 2010. The amount of cloud were estimated from a de- vice (Boltwood Cloud Sensor II) which measures the temperature difference T s-a (bigger T s-a represents larger cloud amount) between the ground and sky in the infrared. The NSB data taken on moonless nights with contribution from artificial lightings removed. On nights where a large variation of the cloud amount (blue ) was observed (Figure 1 left ), a similar trend was present for the NSB measurements (red ). The slope of the correlation was usually lower in the rural sites, indicating a smaller change in the NSB for a similar change in the cloud amount. Moreover, such correlation could be observed more frequently in the urban (63%) versus the rural site (45%). In the contrary, if the amount of cloud stayed steady, then the observed NSB was also nearly constant (Figure 1 right ). Figure 1. Variations of NSB (red ) and cloud amount (blue ) during two simultaneous observation runs, where large (left ) and no variation (right ) of cloud amount are observed. For details, please refer to the project website: http://nightsky.physics.hku.hk 1

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Page 1: E. Alvarez, B. Garca, R. Green, S. Kardel, R.M. Ros, E ects of …nightsky.physics.hku.hk/file/So_IAU2012_SpS17.pdfSpS17, Light Pollution: Protecting Astronomical Sites and Increas-ing

Effects of cloud amount on the night skybrightness

Chu W. So1 and Chun S. J. Pun2 and Chung F. T. Wong3

1Department of Physics, The University of Hong Kong, Pokfulam, Hong Kongemail: [email protected]

2email: [email protected]: [email protected]

Abstract. The cloud amount and the night sky brightness (NSB) were observed simultaneouslyand continuously from more than 320 hours of observations in an urban and a rural station inHong Kong. The cloud amount strongly affect the NSB observed, with effects up to 3 mag in asingle night. Night skies were generally brighter during overcast, with strong correlation observedin 63% / 45% of nights in urban / rural sites. This project is supported by the Environmentand Conservation Fund of the Hong Kong SAR government (ECF 10/2009, ECF 1/2007).

Keywords. Measurement: night sky brightness, light pollution, atmospheric effects, moon.

Simultaneous cloud and NSB (taken with Sky Quality Meters - Lens Ethernet) obser-vations were collected in two locations of the Hong Kong Night Sky Monitoring Network(Please refer to the contribution by Pun et al. in this volume), one urban (HKU ) andone rural (iObs) since November 2010. The amount of cloud were estimated from a de-vice (Boltwood Cloud Sensor II) which measures the temperature difference Ts−a (biggerTs−a represents larger cloud amount) between the ground and sky in the infrared. TheNSB data taken on moonless nights with contribution from artificial lightings removed.On nights where a large variation of the cloud amount (blue) was observed (Figure 1left), a similar trend was present for the NSB measurements (red). The slope of the cor-relation was usually lower in the rural sites, indicating a smaller change in the NSB for asimilar change in the cloud amount. Moreover, such correlation could be observed morefrequently in the urban (63%) versus the rural site (45%). In the contrary, if the amountof cloud stayed steady, then the observed NSB was also nearly constant (Figure 1 right).

Figure 1. Variations of NSB (red) and cloud amount (blue) during two simultaneousobservation runs, where large (left) and no variation (right) of cloud amount are observed.

For details, please refer to the project website: http://nightsky.physics.hku.hk

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