Weather extremes impact agriculture over both short and long time scales. Seasonal extreme event impacts are shown in US historical yields of major grain crops. Important ‘flash’ extremes that do not appear in monthly averages are shown. Short-term extreme events that had impacts: a) expressed during subsequent ‘normal’ conditions or b) sustained long after the event occurs are presented. When an extreme happens can be more important than the extreme value. Plants and animals respond (cold adaptation, dormancy, following year buds, etc.) to conditions over time. Plant sensitivity varies with crop development stage which in turn depends on time-integrative metrics like: chilling hours (CH), growing degree hours (GDH), growing degree days (GDD) whose timing is crucial. Plants tend to be most sensitive from just prior to flowering until after nascent fruit set (often 1-2 months), and near harvest. Duration (including sub-daily time scales) below or above a threshold matters. Impacts include: mortality, morbidity, and marketability, each with different thresholds.

It would be convenient for climate modelers if major agricultural commodities had well-defined thresholds of model variables: extreme values beyond which crop losses could be pegged. But, such thresholds are imprecise due to variations among cultivars, timing, management practices, and prior conditions the plant experiences. Yet, threshold temperatures near freezing and near 35-40C are common. The impact can be amplified by the moisture present, either too much or too little humidity. When considering high temperatures, heat index (HI) and temperature-humidity index (THI) include moisture and are simple metrics to track, but one might emphasize more specialized (e.g. stem water potential, SWP) or comprehensive metrics like comprehensive climate index (CCI) or wet-bulb globe temperature (WBGT) which incorporate wind and radiative factors. There are precipitation and wind thresholds as well.

After a synthesis of some broadly applicable thresholds and timing, historical trends over the CONUS of summer maximum temperatures will be contrasted with trends of maximum THI.