The grey extinction distribution, δT(ẑ,t), for a slightly non-photometric night (2010-06-28). The top panel shows δT(ẑ,t) as a function of airmass while the bottom panel shows δT(ẑ,t) as a function of time. Since the night is non-photometric most of the scatter is due to clouds, but there is some scatter due to achromatic extraction errors. Recall that we normalize by the mean of δT(ẑ,t) rather than the largest (fewest clouds) value.