Structure noise N^str(θ) in selected fields. The upper frame shows N^str(θ = 40′′) for the 250 μm surface brightness (blue dots), compared to the surface-brightness dependence of Helou & Beichman (1990; dashed line). The lower frame shows N^str(θ) in individualfields as a function of the angular scale θ. The black dashed line indicates the median relation. The coloured lines highlight particular fields discussed in the text (G141.25+34.37 as the bottom blue dashed line, G358.96+36.75 as the red solid line, and the fields G345.39–3.97, G198.58–9.10, and G157.08–8.68 as the uppermost dashed green lines).