Light pollution colour changes at MHAONB, from distant town conversions to blue-rich LED lighting, implications for rural UK skies

Abstract

The sky in the Malvern Hills Area of Outstanding Natural Beauty (MHAONB) has been monitored continually since 2012, when a dark sky survey of the area was carried out commissioned by Malvern Hills Conservators. Ever since then at F.C. Observatory Mathon, the sky brightness has been measured continually, in the last few years at minute intervals in all weathers. On the darkest of nights, a fisheye lensed camera was used at the same intervals. There is a trend in brightness distribution and colour changes on the sky, especially towards the horizon, with clearly separated bright sky domes. The sky quality meter (SQM) photometry data near zenith does not show any great change. In 2015, Malvern Hills Conservators commissioned the author for modelling of the effect on the MHAONB sky, of the ongoing blue rich LED re-lighting throughout Herefordshire.The SQM photometry shows the sky brightness are very weather dependant; and the camera shows colour changes from orange-pink to blue-rich LEDs. Besides the trend over recent years to blue white from orange, changes can occur over hours or even minutes, depending on cloud cover over individual towns on or beyond the horizon, and local humidity levels. It can vary from orange to blue and red. Clear skies can vary in overall colour from one night to another and brightness falls overnight. This is shown in isophotes and accurate profile curve fitting. The effect of 2020 lockdown is included. A measurement was made of the clear air back scatter ratio from a known luminance source which can be used to estimate the sky visibility degradation from any planned lighting scheme.For the rural UK, the Milky Way is only 20% contrast to background at zenith on the darkest nights, (MHAONB 21.10 mag.arcsec−2), doubling the road light level across Europe would make it invisible.