Evaporation rate
How fast moisture leaves the surface of fresh concrete, driven by the combination of air temperature, concrete temperature, humidity, and wind speed.
What does evaporation rate mean?
Evaporation rate, not air temperature, is what actually predicts plastic shrinkage cracking, which is why a windy 65-degree day can be more dangerous than a still 95-degree one. Charts and apps estimate the rate from the four variables, and crews use them to decide whether windbreaks, fogging, or evaporation retarders are needed. Once the rate exceeds the concrete's bleeding rate, the surface starts drying and cracking.
In practice
A dry spring wind across an open slab is the classic setup for a crew getting surprised by cracks appearing while they are still finishing.