Fig. 2: Observational domains within two dimensions.

a–d, Kernel density estimates of observational densities within the domains defined by: resolution and interval (of temporally replicated observations) (a), duration and interval (of temporally replicated observations) (b), resolution and extent (c), and duration and extent (d). Density estimates were applied to the log-transformed values of each observational dimension, and density estimates were rescaled to represent percentages. The letters in the plots denote the median values of different observational methods (a, automated sensing; f, field observations; p, palaeo-observations; r, remote sensing). The grey shaded areas represent physically impossible domains (intervals greater than duration and resolutions greater than extent). Density values below the lower third percentile fall within the darkest portion of the colour gradient. The grey vertical line in d indicates that the majority (>95%) of observations of ≤1 day duration were temporally unreplicated.