“Water Stress Begins at 86F” according to 10- year-old research on corn in the Midwest. This is better stated: “At temperatures of 86F (30C) corn plants reach incipient water stress more than 50 percent of the time.” Naturally, this does not apply to irrigated crops and specifically it applies to non-irrigated corn in the Midwest. In a year with ideal moisture, a perfectly healthy corn plant in prime soil does not begin to expeience water stress until temperatures exceed 92F and perhaps not then if humidity is high. Still, 86F is the average and because the government tends to work with the average. the average was set as the upper bound for the U.S. Corn Growing Degree Day and as the lower bound for water stress. The number is rarely the exact switch-over point, but it is “good enough for government work.” Because we have the government’s critical point numbers, we can make use of them (while remaining keenly aware of the limitations).