Ceilometer backscatter imagery and pollution layer depth estimates. Example ceilometer backscatter imagery from (a) 00 UTC 4 December 2015–00 UTC 5 December 2015 and (b) 00 UTC 3 January 2016–00 UTC 4 January 2016. The clean air, gradient zone, and polluted air are denoted. The red circles represent the visually estimated pollution depth determined between 2000 and 2300 UTC. The average values of these estimates are provided below (a) and (b). The brown and black lines indicate automated algorithm estimates of the height of the pollution layer depth used in the daily height estimate calculations and the red line indicates a shallower embedded pollution layer. DOI: https://doi.org/10.1525/elementa.362.f2