A log-plot of the probability of a district crossing epidemic threshold generated by a differential equation-based model of disease propagation whose coefficients depended on relative humidity and other weather variables (weather-conditioned risk) and ignored all weather variables (unconditioned risk). The line for weather-conditioned risk shows an inflection point at about 40% relative humidity, with the probability of epidemic increasing significantly in districts when the relative humidity is less than 40%. For comparison, the unconditioned risk is about 0.018, which can be understood as the probability of epidemic predicted without knowing the relative humidity. Extracted from Hopson et al. (2014).