Observations of entrainment into natural gravity-driven flows on sloping surfaces are described. It is shown that the laboratory-based entrainment law of Ellison & Turner (1959), which is often used for modelling of atmospheric and oceanic flows, underestimates the entrainment rates substantially, arguably due to the fact that the laboratory flows have been conducted at Reynolds numbers $(Re \lesssim 10^3)$ below what is required for mixing transition ($Re\sim10^3$–10^4) whereas natural flows occur at much higher Reynolds numbers $(Re\sim10^7)$. A new entrainment law of the form $E\sim Ri^{-3/4}$ is proposed for the atmospheric Richardson number range $0.15 \lt Ri \lt 1.5$. In contrast to the laboratory observation that entrainment ceases at $Ri = 0.8$, field observations show continuous entrainment over the entire Richardson number range.