There is little doubt that increases in thermal load beyond the thermo-neutral state prove progressively stressful to all living organisms. Increasing temperatures across the globe represent in some locales, and especially for outdoors workers, a significant source of such chronic load increase. However, increases in thermal load affect cognition as well as physical work activities. Such human cognition has perennially been viewed as the primary conduit through which to solve many of the iatrogenic challenges we now face. Yet, thermal stress degrades the power to think. Here, we advance and refine the isothermal description of such cognitive decrements, founded upon a synthesis of extant empirical evidence. We report a series of mathematical functions which describe task-specific patterns of performance deterioration, linking such degrees of decrement to the time/temperature conditions in which they occur. Further, we provide a simple, free software tool to support such calculations so that adverse thermal loads can be monitored, assessed and (where possible) mitigated to preserve healthy cognitive functioning.